In cyanobacteria, the transcriptional activator NtcA is involved in global nitrogen control and, in the absence of ammonium, regulates the expression of genes involved in the assimilation of alternative nitrogen sources. The oceanic picocyanobacterium Synechococcus sp. strain WH 8103 harbors a copy of ntcA, but in the present study, we show that unlike other marine cyanobacteria that have been investigated, this strain is capable of coassimilating nitrite when grown in the presence of ammonium. Transcript levels for the genes encoding the nitrate/nitrite-bispecific permease NrtP and nitrate reductase (NarB) were substantially down-regulated by ammonium, whereas the abundances of nitrite reductase (NirA) transcripts were similar in nitrite- and ammonium-grown cells. The growth of Synechococcus sp. strain WH 8103 in medium containing both ammonium and nitrite resulted in only minor changes in the expression profile in comparison to that of nitrite-grown cells with the exception that the gene encoding the high-affinity ammonium transporter Amt1 was down-regulated to the levels seen in ammonium-grown cells. Whereas the expression of nrtP, narB, and amt1 appears to be NtcA dependent in this marine cyanobacterium, the transcription and expression of nirA appear not to be. The ability to coassimilate nitrite and reduced-nitrogen sources like ammonium may be an adaptive trait that enables oceanic strains like Synechococcus sp. strain WH 8103 to exploit the low nitrite concentrations found in oceanic surface waters that are not available to their principal and more numerous competitor, Prochlorococcus.
The genes encoding the structural components of the nitrate/nitrite assimilation system of the oceanic cyanobacterium Synechococcus sp. strain WH 8103 were cloned and characterized. The genes encoding nitrate reductase (narB) and nitrite reductase (nirA) are clustered on the chromosome but are organized in separate transcriptional units. Upstream of narB is a homologue of nrtP that encodes a nitrate/nitrite-bispecific permease rather than the components of an ABC-type nitrate transporter found in freshwater cyanobacteria. Unusually, neither nirA nor ntcA (encoding a positive transcription factor of genes subject to nitrogen control) were found to be tightly regulated by ammonium. Furthermore, transcription of glnA (encoding glutamine synthetase) is up-regulated in ammonium-grown cells, highlighting significant differences in nitrogen control in this cyanobacterium. Nitrogen depletion led to the transient up-regulation of ntcA, nirA, nrtP, narB, and glnA in what appears to be an NtcA-dependent manner. The NtcA-like promoters found upstream of nirA, nrtP, and narB all differ in sequence from the canonical NtcA promoter established for other cyanobacteria, and in the case of nirA, the NtcA-like promoter was functional only in cells deprived of combined nitrogen. The ecological implications of these findings are discussed in the context of the oligotrophic nature of oceanic surface waters in which Synechococcus spp. thrive.
The marine cyanobacteria Prochlorococcus have been considered photoautotrophic microorganisms, although the utilization of exogenous sugars has never been specifically addressed in them. We studied glucose uptake in different high irradiance- and low irradiance-adapted Prochlorococcus strains, as well as the effect of glucose addition on the expression of several glucose-related genes. Glucose uptake was measured by adding radiolabelled glucose to Prochlorococcus cultures, followed by flow cytometry coupled with cell sorting in order to separate Prochlorococcus cells from bacterial contaminants. Sorted cells were recovered by filtration and their radioactivity measured. The expression, after glucose addition, of several genes (involved in glucose metabolism, and in nitrogen assimilation and its regulation) was determined in the low irradiance-adapted Prochlorococcus SS120 strain by semi-quantitative real time RT-PCR, using the rnpB gene as internal control. Our results demonstrate for the first time that the Prochlorococcus strains studied in this work take up glucose at significant rates even at concentrations close to those found in the oceans, and also exclude the possibility of this uptake being carried out by eventual bacterial contaminants, since only Prochlorococcus cells were used for radioactivity measurements. Besides, we show that the expression of a number of genes involved in glucose utilization (namely zwf, gnd and dld, encoding glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and lactate dehydrogenase, respectively) is strongly increased upon glucose addition to cultures of the SS120 strain. This fact, taken together with the magnitude of the glucose uptake, clearly indicates the physiological importance of the phenomenon. Given the significant contribution of Prochlorococcus to the global primary production, these findings have strong implications for the understanding of the phytoplankton role in the carbon cycle in nature. Besides, the ability of assimilating carbon molecules could provide additional hints to comprehend the ecological success of Prochlorococcus.
Our view of marine microbes is transforming, as culture-independent methods facilitate rapid characterization of microbial diversity. It is difficult to assimilate this information into our understanding of marine microbe ecology and evolution, because their distributions, traits, and genomes are shaped by forces that are complex and dynamic. Here we incorporate diverse forces—physical, biogeochemical, ecological, and mutational—into a global ocean model to study selective pressures on a simple trait in a widely distributed lineage of picophytoplankton: the nitrogen use abilities of Synechococcus and Prochlorococcus cyanobacteria. Some Prochlorococcus ecotypes have lost the ability to use nitrate, whereas their close relatives, marine Synechococcus, typically retain it. We impose mutations for the loss of nitrogen use abilities in modeled picophytoplankton, and ask: in which parts of the ocean are mutants most disadvantaged by losing the ability to use nitrate, and in which parts are they least disadvantaged? Our model predicts that this selective disadvantage is smallest for picophytoplankton that live in tropical regions where Prochlorococcus are abundant in the real ocean. Conversely, the selective disadvantage of losing the ability to use nitrate is larger for modeled picophytoplankton that live at higher latitudes, where Synechococcus are abundant. In regions where we expect Prochlorococcus and Synechococcus populations to cycle seasonally in the real ocean, we find that model ecotypes with seasonal population dynamics similar to Prochlorococcus are less disadvantaged by losing the ability to use nitrate than model ecotypes with seasonal population dynamics similar to Synechococcus. The model predictions for the selective advantage associated with nitrate use are broadly consistent with the distribution of this ability among marine picocyanobacteria, and at finer scales, can provide insights into interactions between temporally varying ocean processes and selective pressures that may be difficult or impossible to study by other means. More generally, and perhaps more importantly, this study introduces an approach for testing hypotheses about the processes that underlie genetic variation among marine microbes, embedded in the dynamic physical, chemical, and biological forces that generate and shape this diversity.
Prochlorococcus, an extremely small cyanobacterium that is very abundant in the world's oceans, has a very streamlined genome. On average, these cells have about 2,000 genes and very few regulatory proteins. The limited capability of regulation is thought to be a result of selection imposed by a relatively stable environment in combination with a very small genome. Furthermore, only ten non-coding RNAs (ncRNAs), which play crucial regulatory roles in all forms of life, have been described in Prochlorococcus. Most strains also lack the RNA chaperone Hfq, raising the question of how important this mode of regulation is for these cells. To explore this question, we examined the transcription of intergenic regions of Prochlorococcus MED4 cells subjected to a number of different stress conditions: changes in light qualities and quantities, phage infection, or phosphorus starvation. Analysis of Affymetrix microarray expression data from intergenic regions revealed 276 novel transcriptional units. Among these were 12 new ncRNAs, 24 antisense RNAs (asRNAs), as well as 113 short mRNAs. Two additional ncRNAs were identified by homology, and all 14 new ncRNAs were independently verified by Northern hybridization and 5′RACE. Unlike its reduced suite of regulatory proteins, the number of ncRNAs relative to genome size in Prochlorococcus is comparable to that found in other bacteria, suggesting that RNA regulators likely play a major role in regulation in this group. Moreover, the ncRNAs are concentrated in previously identified genomic islands, which carry genes of significance to the ecology of this organism, many of which are not of cyanobacterial origin. Expression profiles of some of these ncRNAs suggest involvement in light stress adaptation and/or the response to phage infection consistent with their location in the hypervariable genomic islands.
Prochlorococcus is the most abundant phototroph in the vast, nutrient-poor areas of the ocean. It plays an important role in the ocean carbon cycle, and is a key component of the base of the food web. All cells share a core set of about 1,200 genes, augmented with a variable number of “flexible” genes. Many of the latter are located in genomic islands—hypervariable regions of the genome that encode functions important in differentiating the niches of “ecotypes.” Of major interest is how cells with such a small genome regulate cellular processes, as they lack many of the regulatory proteins commonly found in bacteria. We show here that contrary to the regulatory proteins, ncRNAs are present at levels typical of bacteria, revealing that they might have a disproportional regulatory role in Prochlorococcus—likely an adaptation to the extremely low-nutrient conditions of the open oceans, combined with the constraints of a small genome. Some of the ncRNAs were differentially expressed under stress conditions, and a high number of them were found to be associated with genomic islands, suggesting functional links between these RNAs and the response of Prochlorococcus to particular environmental challenges.
The phytoplankton community in the oligotrophic open ocean is numerically dominated by the cyanobacterium Prochlorococcus, accounting for approximately half of all photosynthesis. In the illuminated euphotic zone where Prochlorococcus grows, reactive oxygen species are continuously generated via photochemical reactions with dissolved organic matter. However, Prochlorococcus genomes lack catalase and additional protective mechanisms common in other aerobes, and this genus is highly susceptible to oxidative damage from hydrogen peroxide (HOOH). In this study we showed that the extant microbial community plays a vital, previously unrecognized role in cross-protecting Prochlorococcus from oxidative damage in the surface mixed layer of the oligotrophic ocean. Microbes are the primary HOOH sink in marine systems, and in the absence of the microbial community, surface waters in the Atlantic and Pacific Ocean accumulated HOOH to concentrations that were lethal for Prochlorococcus cultures. In laboratory experiments with the marine heterotroph Alteromonas sp., serving as a proxy for the natural community of HOOH-degrading microbes, bacterial depletion of HOOH from the extracellular milieu prevented oxidative damage to the cell envelope and photosystems of co-cultured Prochlorococcus, and facilitated the growth of Prochlorococcus at ecologically-relevant cell concentrations. Curiously, the more recently evolved lineages of Prochlorococcus that exploit the surface mixed layer niche were also the most sensitive to HOOH. The genomic streamlining of these evolved lineages during adaptation to the high-light exposed upper euphotic zone thus appears to be coincident with an acquired dependency on the extant HOOH-consuming community. These results underscore the importance of (indirect) biotic interactions in establishing niche boundaries, and highlight the impacts that community-level responses to stress may have in the ecological and evolutionary outcomes for co-existing species.
Nitrogen (N) often limits biological productivity in the oceanic gyres where Prochlorococcus is the most abundant photosynthetic organism. The Prochlorococcus community is composed of strains, such as MED4 and MIT9313, that have different N utilization capabilities and that belong to ecotypes with different depth distributions. An interstrain comparison of how Prochlorococcus responds to changes in ambient nitrogen is thus central to understanding its ecology. We quantified changes in MED4 and MIT9313 global mRNA expression, chlorophyll fluorescence, and photosystem II photochemical efficiency (Fv/Fm) along a time series of increasing N starvation. In addition, the global expression of both strains growing in ammonium-replete medium was compared to expression during growth on alternative N sources. There were interstrain similarities in N regulation such as the activation of a putative NtcA regulon during N stress. There were also important differences between the strains such as in the expression patterns of carbon metabolism genes, suggesting that the two strains integrate N and C metabolism in fundamentally different ways.
cyanobacteria; interstrain; nitrogen; Prochlorococcus; transcription
Cyanobacteria of the genera Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms on earth, occupying a key position at the base of marine food webs. The cynS gene that encodes cyanase was identified among bacterial, fungal, and plant sequences in public databases, and the gene was particularly prevalent among cyanobacteria, including numerous Prochlorococcus and Synechococcus strains. Phylogenetic analysis of cynS sequences retrieved from the Global Ocean Survey database identified >60% as belonging to unicellular marine cyanobacteria, suggesting an important role for cyanase in their nitrogen metabolism. We demonstrate here that marine cyanobacteria have a functionally active cyanase, the transcriptional regulation of which varies among strains and reflects the genomic context of cynS. In Prochlorococcus sp. strain MED4, cynS was presumably transcribed as part of the cynABDS operon, implying cyanase involvement in cyanate utilization. In Synechococcus sp. strain WH8102, expression was not related to nitrogen stress responses and here cyanase presumably serves in the detoxification of cyanate resulting from intracellular urea and/or carbamoyl phosphate decomposition. Lastly, we report on a cyanase activity encoded by cynH, a novel gene found in marine cyanobacteria only. The presence of dual cyanase genes in the genomes of seven marine Synechococcus strains and their respective roles in nitrogen metabolism remain to be clarified.
Horizontal or lateral transfer of genetic material between distantly related prokaryotes has been shown to play a major role in the evolution of bacterial and archaeal genomes, but exchange of genes between prokaryotes and eukaryotes is not as well understood. In particular, gene flow from eukaryotes to prokaryotes is rarely documented with strong support, which is unusual since prokaryotic genomes appear to readily accept foreign genes.
Here, we show that abundant marine cyanobacteria in the related genera Synechococcus and Prochlorococcus acquired a key Calvin cycle/glycolytic enzyme from a eukaryote. Two non-homologous forms of fructose bisphosphate aldolase (FBA) are characteristic of eukaryotes and prokaryotes respectively. However, a eukaryotic gene has been inserted immediately upstream of the ancestral prokaryotic gene in several strains (ecotypes) of Synechococcus and Prochlorococcus. In one lineage this new gene has replaced the ancestral gene altogether. The eukaryotic gene is most closely related to the plastid-targeted FBA from red algae. This eukaryotic-type FBA once replaced the plastid/cyanobacterial type in photosynthetic eukaryotes, hinting at a possible functional advantage in Calvin cycle reactions. The strains that now possess this eukaryotic FBA are scattered across the tree of Synechococcus and Prochlorococcus, perhaps because the gene has been transferred multiple times among cyanobacteria, or more likely because it has been selectively retained only in certain lineages.
A gene for plastid-targeted FBA has been transferred from red algae to cyanobacteria, where it has inserted itself beside its non-homologous, functional analogue. Its current distribution in Prochlorococcus and Synechococcus is punctate, suggesting a complex history since its introduction to this group.
Cultured isolates of the marine cyanobacteria Prochlorococcus and Synechococcus vary widely in their pigment compositions and growth responses to light and nutrients, yet show greater than 96% identity in their 16S ribosomal DNA (rDNA) sequences. In order to better define the genetic variation that accompanies their physiological diversity, sequences for the 16S-23S rDNA internal transcribed spacer (ITS) region were determined in 32 Prochlorococcus isolates and 25 Synechococcus isolates from around the globe. Each strain examined yielded one ITS sequence that contained two tRNA genes. Dramatic variations in the length and G+C content of the spacer were observed among the strains, particularly among Prochlorococcus strains. Secondary-structure models of the ITS were predicted in order to facilitate alignment of the sequences for phylogenetic analyses. The previously observed division of Prochlorococcus into two ecotypes (called high and low-B/A after their differences in chlorophyll content) were supported, as was the subdivision of the high-B/A ecotype into four genetically distinct clades. ITS-based phylogenies partitioned marine cluster A Synechococcus into six clades, three of which can be associated with a particular phenotype (motility, chromatic adaptation, and lack of phycourobilin). The pattern of sequence divergence within and between clades is suggestive of a mode of evolution driven by adaptive sweeps and implies that each clade represents an ecologically distinct population. Furthermore, many of the clades consist of strains isolated from disparate regions of the world's oceans, implying that they are geographically widely distributed. These results provide further evidence that natural populations of Prochlorococcus and Synechococcus consist of multiple coexisting ecotypes, genetically closely related but physiologically distinct, which may vary in relative abundance with changing environmental conditions.
The transcription factor of the cyclic AMP receptor protein/FNR family, NtcA, and the PII signaling protein play central roles in global nitrogen control in cyanobacteria. A dependence on PII for NtcA-regulated transcription, however, has not been observed. In the present investigation, we examined alterations in gene expression following nitrogen deprivation in Synechococcus elongatus strain PCC 7942 and specifically the roles of NtcA and PII. Global changes in de novo protein synthesis following combined-nitrogen deprivation were visualized by in vivo [35S]methionine labeling and two-dimensional polyacrylamide gel electrophoresis analysis. Nearly all proteins whose synthesis responded specifically to combined-nitrogen deprivation in wild-type cells of S. elongatus failed to respond in PII- and NtcA-deficient mutants. One of the proteins whose synthesis was down-regulated in a PII- and NtcA-dependent manner was RbcS, the small subunit of RubisCO. Quantification of its mRNA revealed that the abundance of the rbcLS transcript following combined-nitrogen deprivation rapidly declined in wild-type cells but not in PII and NtcA mutant cells. To investigate further the relationship between PII and NtcA, fusions of the promotorless luxAB reporter genes to the NtcA-regulated glnB gene were constructed and these constructs were used to transform wild-type cells and PII− and NtcA− mutants. Determination of bioluminescence under different growth conditions showed that NtcA represses gene expression in the presence of ammonium in a PII-independent manner. By contrast, NtcA-dependent activation of glnB expression following combined-nitrogen deprivation was impaired in the absence of PII. Together, these results suggest that under conditions of combined-nitrogen deprivation, the regulation of NtcA-dependent gene expression requires the PII signal transduction protein.
The smallest genomes of any photosynthetic organisms are found in a group of free-living marine cyanobacteria, Prochlorococcus. To determine the underlying evolutionary mechanisms, we developed a new method to reconstruct the steps leading to the Prochlorococcus genome reduction using 12 Prochlorococcus and 6 marine Synechococcus genomes. Our results reveal that small genome sizes within Prochlorococcus were largely determined shortly after the split of Prochlorococcus and Synechococcus (an early big shrink) and thus for the first time decouple the genome reduction from Prochlorococcus diversification. A maximum likelihood approach was then used to estimate changes of nucleotide substitution rate and selection strength along Prochlorococcus evolution in a phylogenetic framework. Strong genome wide purifying selection was associated with the loss of many genes in the early evolutionary stage. The deleted genes were distributed around the genome, participated in many different functional categories and in general had been under relaxed selection pressure. We propose that shortly after Prochlorococcus diverged from its common ancestor with marine Synechococcus, its population size increased quickly thus increasing efficacy of selection. Due to limited nutrients and a relatively constant environment, selection favored a streamlined genome for maximum economy. Strong genome wide selection subsequently caused the loss of genes with small functional effect including the loss of some DNA repair genes. In summary, genome reduction in Prochlorococcus resulted in genome features that are similar to symbiotic bacteria and pathogens, however, the small genome sizes resulted from an increase in genome wide selection rather than a consequence of a reduced ecological niche or relaxed selection due to genetic drift.
The minute photosynthetic prokaryote Prochlorococcus, which was discovered about 10 years ago, has proven exceptional from several standpoints. Its tiny size (0.5 to 0.7 μm in diameter) makes it the smallest known photosynthetic organism. Its ubiquity within the 40°S to 40°N latitudinal band of oceans and its occurrence at high density from the surface down to depths of 200 m make it presumably the most abundant photosynthetic organism on Earth. Prochlorococcus typically divides once a day in the subsurface layer of oligotrophic areas, where it dominates the photosynthetic biomass. It also possesses a remarkable pigment complement which includes divinyl derivatives of chlorophyll a (Chl a) and Chl b, the so-called Chl a2 and Chl b2, and, in some strains, small amounts of a new type of phycoerythrin. Phylogenetically, Prochlorococcus has also proven fascinating. Recent studies suggest that it evolved from an ancestral cyanobacterium by reducing its cell and genome sizes and by recruiting a protein originally synthesized under conditions of iron depletion to build a reduced antenna system as a replacement for large phycobilisomes. Environmental constraints clearly played a predominant role in Prochlorococcus evolution. Its tiny size is an advantage for its adaptation to nutrient-deprived environments. Furthermore, genetically distinct ecotypes, with different antenna systems and ecophysiological characteristics, are present at depth and in surface waters. This vertical species variation has allowed Prochlorococcus to adapt to the natural light gradient occurring in the upper layer of oceans. The present review critically assesses the basic knowledge acquired about Prochlorococcus both in the ocean and in the laboratory.
Summary: Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45°N to 40°S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.
Nitrogen nutrition in cyanobacteria is regulated by NtcA, a transcriptional activator that is subject to negative control by ammonium. Using Synechococcus sp. strain WH7803 as a model organism, we show that ntcA expression was induced when cells were exposed to nitrogen stress but not when they were subjected to phosphorus or iron deprivation. Transcript levels accumulated in cells grown on a variety of inorganic and organic nitrogen sources, with the sole exception of ammonium. ntcA transcription was induced when ammonium levels dropped below 1 μM and reached maximal levels within 2 h. Furthermore, the addition of more than 1 μM ammonium led to a rapid decline in ntcA mRNA. The negative effect of ammonium was prevented by the addition of l-methionine-d,l-sulfoximine (MSX) and azaserine, inhibitors of ammonium assimilation. Thus, basal ntcA transcript levels are indicative of ammonium utilization. Conversely, the highest ntcA transcript levels were found in cells lacking a nitrogen source capable of supporting growth. Therefore, maximal ntcA expression would indicate nitrogen deprivation. This state of nitrogen deprivation was induced by a 1-h incubation with MSX. The rapid response of ntcA gene expression to the addition of ammonium and MSX was used to design a protocol for assessing relative ntcA transcript levels in field populations of cyanobacteria, from which their nitrogen status can be inferred. ntcA was basally expressed in Synechococcus at a nutrient-enriched site at the northern tip of the Gulf of Aqaba, Red Sea. Therefore, these cyanobacteria were not nitrogen stressed, and their nitrogen requirements were met by regenerated nitrogen in the form of ammonium.
Prochlorococcus and Synechococcus, which numerically dominate vast oceanic areas, are the two most abundant oxygenic phototrophs on Earth. Although they require solar energy for photosynthesis, excess light and associated high UV radiations can induce high levels of oxidative stress that may have deleterious effects on their growth and productivity. Here, we compared the photophysiologies of the model strains Prochlorococcus marinus PCC 9511 and Synechococcus sp. WH7803 grown under a bell-shaped light/dark cycle of high visible light supplemented or not with UV. Prochlorococcus exhibited a higher sensitivity to photoinactivation than Synechococcus under both conditions, as shown by a larger drop of photosystem II (PSII) quantum yield at noon and different diel patterns of the D1 protein pool. In the presence of UV, the PSII repair rate was significantly depressed at noon in Prochlorococcus compared to Synechococcus. Additionally, Prochlorococcus was more sensitive than Synechococcus to oxidative stress, as shown by the different degrees of PSII photoinactivation after addition of hydrogen peroxide. A transcriptional analysis also revealed dramatic discrepancies between the two organisms in the diel expression patterns of several genes involved notably in the biosynthesis and/or repair of photosystems, light-harvesting complexes, CO2 fixation as well as protection mechanisms against light, UV, and oxidative stress, which likely translate profound differences in their light-controlled regulation. Altogether our results suggest that while Synechococcus has developed efficient ways to cope with light and UV stress, Prochlorococcus cells seemingly survive stressful hours of the day by launching a minimal set of protection mechanisms and by temporarily bringing down several key metabolic processes. This study provides unprecedented insights into understanding the distinct depth distributions and dynamics of these two picocyanobacteria in the field.
marine cyanobacteria; Synechococcus; Prochlorococcus; light/dark cycle; light stress; UV radiations; oxidative stress; photophysiology
The phylogeny and taxonomy of cyanobacteria is currently poorly understood due to paucity of reliable markers for identification and circumscription of its major clades.
A combination of phylogenomic and protein signature based approaches was used to characterize the major clades of cyanobacteria. Phylogenetic trees were constructed for 44 cyanobacteria based on 44 conserved proteins. In parallel, Blastp searches were carried out on each ORF in the genomes of Synechococcus WH8102, Synechocystis PCC6803, Nostoc PCC7120, Synechococcus JA-3-3Ab, Prochlorococcus MIT9215 and Prochlor. marinus subsp. marinus CCMP1375 to identify proteins that are specific for various main clades of cyanobacteria. These studies have identified 39 proteins that are specific for all (or most) cyanobacteria and large numbers of proteins for other cyanobacterial clades. The identified signature proteins include: (i) 14 proteins for a deep branching clade (Clade A) of Gloebacter violaceus and two diazotrophic Synechococcus strains (JA-3-3Ab and JA2-3-B'a); (ii) 5 proteins that are present in all other cyanobacteria except those from Clade A; (iii) 60 proteins that are specific for a clade (Clade C) consisting of various marine unicellular cyanobacteria (viz. Synechococcus and Prochlorococcus); (iv) 14 and 19 signature proteins that are specific for the Clade C Synechococcus and Prochlorococcus strains, respectively; (v) 67 proteins that are specific for the Low B/A ecotype Prochlorococcus strains, containing lower ratio of chl b/a2 and adapted to growth at high light intensities; (vi) 65 and 8 proteins that are specific for the Nostocales and Chroococcales orders, respectively; and (vii) 22 and 9 proteins that are uniquely shared by various Nostocales and Oscillatoriales orders, or by these two orders and the Chroococcales, respectively. We also describe 3 conserved indels in flavoprotein, heme oxygenase and protochlorophyllide oxidoreductase proteins that are specific for either Clade C cyanobacteria or for various subclades of Prochlorococcus. Many other conserved indels for cyanobacterial clades have been described recently.
These signature proteins and indels provide novel means for circumscription of various cyanobacterial clades in clear molecular terms. Their functional studies should lead to discovery of novel properties that are unique to these groups of cyanobacteria.
Prochlorococcus is a genus of marine cyanobacteria characterized by small cell and genome size, an evolutionary trend toward low GC content, the possession of chlorophyll b, and the absence of phycobilisomes. Whereas many shared derived characters define Prochlorococcus as a clade, many genome-based analyses recover them as paraphyletic, with some low-light adapted Prochlorococcus spp. grouping with marine Synechococcus. Here, we use 18 Prochlorococcus and marine Synechococcus genomes to analyze gene flow within and between these taxa. We introduce embedded quartet scatter plots as a tool to screen for genes whose phylogeny agrees or conflicts with the plurality phylogenetic signal, with accepted taxonomy and naming, with GC content, and with the ecological adaptation to high and low light intensities. We find that most gene families support high-light adapted Prochlorococcus spp. as a monophyletic clade and low-light adapted Prochlorococcus sp. as a paraphyletic group. But we also detect 16 gene families that were transferred between high-light adapted and low-light adapted Prochlorococcus sp. and 495 gene families, including 19 ribosomal proteins, that do not cluster designated Prochlorococcus and Synechococcus strains in the expected manner. To explain the observed data, we propose that frequent gene transfer between marine Synechococcus spp. and low-light adapted Prochlorococcus spp. has created a “highway of gene sharing” (Beiko RG, Harlow TJ, Ragan MA. 2005. Highways of gene sharing in prokaryotes. Proc Natl Acad Sci USA. 102:14332–14337) that tends to erode genus boundaries without erasing the Prochlorococcus-specific ecological adaptations.
marine cyanobacteria; horizontal gene transfer; introgression; quartet decomposition; supertree; genome evolution
Prochlorococcus is a marine cyanobacterium that numerically dominates the mid-latitude oceans and is the smallest known oxygenic phototroph. Numerous isolates from diverse areas of the world's oceans have been studied and shown to be physiologically and genetically distinct. All isolates described thus far can be assigned to either a tightly clustered high-light (HL)-adapted clade, or a more divergent low-light (LL)-adapted group. The 16S rRNA sequences of the entire Prochlorococcus group differ by at most 3%, and the four initially published genomes revealed patterns of genetic differentiation that help explain physiological differences among the isolates. Here we describe the genomes of eight newly sequenced isolates and combine them with the first four genomes for a comprehensive analysis of the core (shared by all isolates) and flexible genes of the Prochlorococcus group, and the patterns of loss and gain of the flexible genes over the course of evolution. There are 1,273 genes that represent the core shared by all 12 genomes. They are apparently sufficient, according to metabolic reconstruction, to encode a functional cell. We describe a phylogeny for all 12 isolates by subjecting their complete proteomes to three different phylogenetic analyses. For each non-core gene, we used a maximum parsimony method to estimate which ancestor likely first acquired or lost each gene. Many of the genetic differences among isolates, especially for genes involved in outer membrane synthesis and nutrient transport, are found within the same clade. Nevertheless, we identified some genes defining HL and LL ecotypes, and clades within these broad ecotypes, helping to demonstrate the basis of HL and LL adaptations in Prochlorococcus. Furthermore, our estimates of gene gain events allow us to identify highly variable genomic islands that are not apparent through simple pairwise comparisons. These results emphasize the functional roles, especially those connected to outer membrane synthesis and transport that dominate the flexible genome and set it apart from the core. Besides identifying islands and demonstrating their role throughout the history of Prochlorococcus, reconstruction of past gene gains and losses shows that much of the variability exists at the “leaves of the tree,” between the most closely related strains. Finally, the identification of core and flexible genes from this 12-genome comparison is largely consistent with the relative frequency of Prochlorococcus genes found in global ocean metagenomic databases, further closing the gap between our understanding of these organisms in the lab and the wild.
Prochlorococcus—the most abundant photosynthetic microbe living in the vast, nutrient-poor areas of the ocean—is a major contributor to the global carbon cycle. Prochlorococcus is composed of closely related, physiologically distinct lineages whose differences enable the group as a whole to proliferate over a broad range of environmental conditions. We compare the genomes of 12 strains of Prochlorococcus representing its major lineages in order to identify genetic differences affecting the ecology of different lineages and their evolutionary origin. First, we identify the core genome: the 1,273 genes shared among all strains. This core set of genes encodes the essentials of a functional cell, enabling it to make living matter out of sunlight and carbon dioxide. We then create a genomic tree that maps the gain and loss of non-core genes in individual strains, showing that a striking number of genes are gained or lost even among the most closely related strains. We find that lost and gained genes commonly cluster in highly variable regions called genomic islands. The level of diversity among the non-core genes, and the number of new genes added with each new genome sequenced, suggest far more diversity to be discovered.
PCR was used to amplify DNA-dependent RNA polymerase gene sequences specifically from the cyanobacterial population in a seawater sample from the Sargasso Sea. Sequencing and analysis of the cloned fragments suggest that the population in the sample consisted of two distinct clusters of Prochlorococcus-like cyanobacteria and four clusters of Synechococcus-like cyanobacteria. The diversity within these clusters was significantly different, however. Clones within each Synechococcus-like cluster were 99 to 100% identical, while each Prochlorococcus-like cluster was only 91% identical at the nucleotide level. One Prochlorococcus-like cluster was significantly more closely related to a Mediterranean Sea (surface) Prochlorococcus isolate than to the other cluster, showing the highly divergent nature of this group even in one sample. The approach described here can be used as a general method for examining cyanobacterial diversity, while an oligotrophic ocean ecosystem such as the Sargasso Sea may be an ideal model for examining diversity in relation to environmental parameters.
Picocyanobacteria represented by Prochlorococcus and Synechococcus have an important role in oceanic carbon fixation and nutrient cycling. In this study, we compared the community composition of picocyanobacteria from diverse marine ecosystems ranging from estuary to open oceans, tropical to polar oceans and surface to deep water, based on the sequences of 16S-23S rRNA internal transcribed spacer (ITS). A total of 1339 ITS sequences recovered from 20 samples unveiled diverse and several previously unknown clades of Prochlorococcus and Synechococcus. Six high-light (HL)-adapted Prochlorococcus clades were identified, among which clade HLVI had not been described previously. Prochlorococcus clades HLIII, HLIV and HLV, detected in the Equatorial Pacific samples, could be related to the HNLC clades recently found in the high-nutrient, low-chlorophyll (HNLC), iron-depleted tropical oceans. At least four novel Synechococcus clades (out of six clades in total) in subcluster 5.3 were found in subtropical open oceans and the South China Sea. A niche partitioning with depth was observed in the Synechococcus subcluster 5.3. Members of Synechococcus subcluster 5.2 were dominant in the high-latitude waters (northern Bering Sea and Chukchi Sea), suggesting a possible cold-adaptation of some marine Synechococcus in this subcluster. A distinct shift of the picocyanobacterial community was observed from the Bering Sea to the Chukchi Sea, which reflected the change of water temperature. Our study demonstrates that oceanic systems contain a large pool of diverse picocyanobacteria, and further suggest that new genotypes or ecotypes of picocyanobacteria will continue to emerge, as microbial consortia are explored with advanced sequencing technology.
cyanobacteria; Prochlorococcus; Synechococcus; diversity; global ocean; 16S-23S rRNA ITS
The physiological regulation of glutamine synthetase (GS; EC 220.127.116.11) in the axenic Prochlorococcus sp. strain PCC 9511 was studied. GS activity and antigen concentration were measured using the transferase and biosynthetic assays and the electroimmunoassay, respectively. GS activity decreased when cells were subjected to nitrogen starvation or cultured with oxidized nitrogen sources, which proved to be nonusable for Prochlorococcus growth. The GS activity in cultures subjected to long-term phosphorus starvation was lower than that in equivalent nitrogen-starved cultures. Azaserine, an inhibitor of glutamate synthase, provoked an increase in enzymatic activity, suggesting that glutamine is not involved in GS regulation. Darkness did not affect GS activity significantly, while the addition of diuron provoked GS inactivation. GS protein determination showed that azaserine induces an increase in the concentration of the enzyme. The unusual responses to darkness and nitrogen starvation could reflect adaptation mechanisms of Prochlorococcus for coping with a light- and nutrient-limited environment.
Nitrogen is one of the major nutrients limiting microbial productivity in the ocean, and as a result, most marine microorganisms have evolved systems for responding to nitrogen stress. The highly abundant alphaproteobacterium “Candidatus Pelagibacter ubique,” a cultured member of the order Pelagibacterales (SAR11), lacks the canonical GlnB, GlnD, GlnK, and NtrB/NtrC genes for regulating nitrogen assimilation, raising questions about how these organisms respond to nitrogen limitation. A survey of 266 Alphaproteobacteria genomes found these five regulatory genes nearly universally conserved, absent only in intracellular parasites and members of the order Pelagibacterales, including “Ca. Pelagibacter ubique.” Global differences in mRNA and protein expression between nitrogen-limited and nitrogen-replete cultures were measured to identify nitrogen stress responses in “Ca. Pelagibacter ubique” strain HTCC1062. Transporters for ammonium (AmtB), taurine (TauA), amino acids (YhdW), and opines (OccT) were all elevated in nitrogen-limited cells, indicating that they devote increased resources to the assimilation of nitrogenous organic compounds. Enzymes for assimilating amine into glutamine (GlnA), glutamate (GltBD), and glycine (AspC) were similarly upregulated. Differential regulation of the transcriptional regulator NtrX in the two-component signaling system NtrY/NtrX was also observed, implicating it in control of the nitrogen starvation response. Comparisons of the transcriptome and proteome supported previous observations of uncoupling between transcription and translation in nutrient-deprived “Ca. Pelagibacter ubique” cells. Overall, these data reveal a streamlined, PII-independent response to nitrogen stress in “Ca. Pelagibacter ubique,” and likely other Pelagibacterales, and show that they respond to nitrogen stress by allocating more resources to the assimilation of nitrogen-rich organic compounds.
Pelagibacterales are extraordinarily abundant and play a pivotal role in marine geochemical cycles, as one of the major recyclers of labile dissolved organic matter. They are also models for understanding how streamlining selection can reshape chemoheterotroph metabolism. Streamlining and its broad importance to environmental microbiology are emerging slowly from studies that reveal the complete genomes of uncultured organisms. Here, we report another remarkable example of streamlined metabolism in Pelagibacterales, this time in systems that control nitrogen assimilation. Pelagibacterales are major contributors to metatranscriptomes and metaproteomes from ocean systems, where patterns of gene expression are used to gain insight into ocean conditions and geochemical cycles. The data presented here supply background that is essential to interpreting data from field studies.
Large swaths of the nutrient-poor surface ocean are dominated numerically by cyanobacteria (Prochlorococcus), cyanobacterial viruses (cyanophage), and alphaproteobacteria (SAR11). How these groups thrive in the diverse physicochemical environments of different oceanic regions remains poorly understood. Comparative metagenomics can reveal adaptive responses linked to ecosystem-specific selective pressures. The Red Sea is well-suited for studying adaptation of pelagic-microbes, with salinities, temperatures, and light levels at the extreme end for the surface ocean, and low nutrient concentrations, yet no metagenomic studies have been done there. The Red Sea (high salinity, high light, low N and P) compares favorably with the Mediterranean Sea (high salinity, low P), Sargasso Sea (low P), and North Pacific Subtropical Gyre (high light, low N). We quantified the relative abundance of genetic functions among Prochlorococcus, cyanophage, and SAR11 from these four regions. Gene frequencies indicate selection for phosphorus acquisition (Mediterranean/Sargasso), DNA repair and high-light responses (Red Sea/Pacific Prochlorococcus), and osmolyte C1 oxidation (Red Sea/Mediterranean SAR11). The unexpected connection between salinity-dependent osmolyte production and SAR11 C1 metabolism represents a potentially major coevolutionary adaptation and biogeochemical flux. Among Prochlorococcus and cyanophage, genes enriched in specific environments had ecotype distributions similar to nonenriched genes, suggesting that inter-ecotype gene transfer is not a major source of environment-specific adaptation. Clustering of metagenomes using gene frequencies shows similarities in populations (Red Sea with Pacific, Mediterranean with Sargasso) that belie their geographic distances. Taken together, the genetic functions enriched in specific environments indicate competitive strategies for maintaining carrying capacity in the face of physical stressors and low nutrient availability.
Cyanophage; metagenomics; osmolyte; Pelagibacter; population genomics; Prochlorococcus; SAR11
NtcA is a transcriptional activator involved in global nitrogen control in cyanobacteria. In the absence of ammonium it regulates the transcription of a series of genes encoding proteins required for the uptake and assimilation of alternative nitrogen sources (I. Luque, E. Flores, and A. Herrero, EMBO J. 13:2862–2869, 1994). ntcA, present in a single copy in the marine Synechococcus sp. strain WH 7803, was cloned and sequenced. The putative amino acid sequence shows a high degree of identity to NtcA from freshwater cyanobacteria in two functional domains. The expression of ntcA was negatively regulated by ammonium from a putative transcription start point located downstream of an NtcA consensus recognition sequence. Addition of either rifampin or ammonium led to a rapid decline in ntcA transcript levels with half-lives of less than 2 min in both cases. Nitrate-grown cells showed high ntcA transcript levels, as well as the capacity for active nitrite uptake. However, ammonium-grown cells showed low levels of the ntcA transcript and did not utilize nitrite. The addition of ammonium to nitrite uptake-active cells resulted in a gradual decline in the rate of uptake over a 24-h period. Active nitrite uptake was not induced in cells transferred to medium lacking a nitrogen source despite evidence of elevated expression of ntcA, indicating that ntcA expression is not sufficient for uptake capacity to develop. Nitrate and nitrite addition led to the development of nitrite uptake, whereas the addition of leucine did not. Furthermore, nitrite addition triggered the de novo protein synthesis required for uptake capacity to develop. These data suggest that nitrite and nitrate act as specific inducers for the synthesis of proteins required for nitrite uptake.