Microbial genomic sequence analyses have indicated widespread horizontal gene transfer (HGT). However, an adequate mechanism accounting for the ubiquity of HGT has been lacking. Recently, high frequencies of interspecific gene transfer have been documented, catalyzed by Gene Transfer Agents (GTAs) of marine α-Proteobacteria. It has been proposed that the presence of bacterial genes in highly purified viral metagenomes may be due to GTAs. However, factors influencing GTA-mediated gene transfer in the environment have not yet been determined. Several genomically sequenced strains containing complete GTA sequences similar to Rhodobacter capsulatus (RcGTA, type strain) were screened to ascertain if they produced putative GTAs, and at what abundance. Five of nine marine strains screened to date spontaneously produced virus-like particles (VLP's) in stationary phase. Three of these strains have demonstrated gene transfer activity, two of which were documented by this lab. These two strains Roseovarius nubinhibens ISM and Nitratireductor 44B9s, were utilized to produce GTAs designated RnGTA and NrGTA and gene transfer activity was verified in culture. Cell-free preparations of purified RnGTA and NrGTA particles from marked donor strains were incubated with natural microbial assemblages to determine the level of GTA-mediated gene transfer. In conjunction, several ambient environmental parameters were measured including lysogeny indicated by prophage induction. GTA production in culture systems indicated that approximately half of the strains produced GTA-like particles and maximal GTA counts ranged from 10–30% of host abundance. Modeling of GTA-mediated gene transfer frequencies in natural samples, along with other measured environmental variables, indicated a strong relationship between GTA mediated gene transfer and the combined factors of salinity, multiplicity of infection (MOI) and ambient bacterial abundance. These results indicate that GTA-mediated HGT in the marine environment with the strains examined is favored during times of elevated bacterial and GTA abundance as well as in areas of higher salinity.
Culture-independent studies have indicated that there is significant diversity in the ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) enzymes used by marine, freshwater, and terrestrial autotrophic bacteria. Surprisingly, little is known about the catalytic properties of many environmentally significant RubisCO enzymes. Because one of the goals of RubisCO research is to somehow modify or select for RubisCO molecules with improved kinetic properties, a facile means to isolate functional and novel RubisCO molecules directly from the environment was developed. In this report, we describe the first example of functional RubisCO proteins obtained from genes cloned and characterized from metagenomic libraries derived from DNA isolated from environmental samples. Two form IA marine RubisCO genes were cloned, and each gene supported both photoheterotrophic and photoautotrophic growth of a RubisCO deletion strain of Rhodobacter capsulatus, strain SBI/II−, indicating that catalytically active recombinant RubisCO was synthesized. The catalytic properties of the metagenomic RubisCO molecules were further characterized. These experiments demonstrated the feasibility of studying the functional diversity and enzymatic properties of RubisCO enzymes without first cultivating the host organisms. Further, this “proof of concept” experiment opens the way for development of a simple functional screen to examine the properties of diverse RubisCO genes isolated from any environment, and subsequent further bioselection may be possible if the growth conditions of complemented R. capsulatus strain SBI/II− are varied.
In the United States, total maximum daily load standards for bodies of water that do not meet bacterial water quality standards are set by each state. The presence of human polyomaviruses (HPyVs) can be used as an indicator of human-associated sewage pollution in these waters. We have developed and optimized a TaqMan quantitative PCR (QPCR) assay based on the conserved T antigen to both quantify and simultaneously detect two HPyVs; JC virus and BK virus. The QPCR assay was able to consistently quantify ≥10 gene copies per reaction and is linear over 5 orders of magnitude. HPyVs were consistently detected in human waste samples (57 of 64) and environmental waters with known human fecal contamination (5 of 5) and were not amplified in DNA extracted from 127 animal waste samples from 14 species. HPyV concentrations in sewage decreased 81.2 and 84.2% over 28 days incubation at 25 and 35°C, respectively. HPyVs results were compared to Escherichia coli, fecal coliform, and enterococci concentrations and the presence of three other human-associated microbes: Bacteroidetes, Methanobrevibacter smithii, and adenovirus. HPyVs were the most frequently detected of these in human and contaminated environmental samples and were more human specific than the Bacteroidetes (HF183) or M. smithii. HPyVs and M. smithii more closely mimicked the persistence of adenovirus in sewage than the other microbes. The use of this rapid and quantitative assay in water quality research could help regulatory agencies to identify sources of water pollution for improved remediation of contaminated waters and ultimately protect humans from exposure to pathogens.
A myovirus-like temperate phage, ΦHAP-1, was induced with mitomycin C from a Halomonas aquamarina strain isolated from surface waters in the Gulf of Mexico. The induced cultures produced significantly more virus-like particles (VLPs) (3.73 × 1010 VLP ml−1) than control cultures (3.83 × 107 VLP ml−1) when observed with epifluorescence microscopy. The induced phage was sequenced by using linker-amplified shotgun libraries and contained a genome 39,245 nucleotides in length with a G+C content of 59%. The ΦHAP-1 genome contained 46 putative open reading frames (ORFs), with 76% sharing significant similarity (E value of <10−3) at the protein level with other sequences in GenBank. Putative functional gene assignments included small and large terminase subunits, capsid and tail genes, an N6-DNA adenine methyltransferase, and lysogeny-related genes. Although no integrase was found, the ΦHAP-1 genome contained ORFs similar to protelomerase and parA genes found in linear plasmid-like phages with telomeric ends. Southern probing and PCR analysis of host genomic, plasmid, and ΦHAP-1 DNA indicated a lack of integration of the prophage with the host chromosome and a difference in genome arrangement between the prophage and virion forms. The linear plasmid prophage form of ΦHAP-1 begins with the protelomerase gene, presumably due to the activity of the protelomerase, while the induced phage particle has a circularly permuted genome that begins with the terminase genes. The ΦHAP-1 genome shares synteny and gene similarity with coliphage N15 and vibriophages VP882 and VHML, suggesting an evolutionary heritage from an N15-like linear plasmid prophage ancestor.
Phage integrase genes often play a role in the establishment of lysogeny in temperate phage by catalyzing the integration of the phage into one of the host's replicons. To investigate temperate phage gene expression, an induced viral metagenome from Tampa Bay was sequenced by 454/Pyrosequencing. The sequencing yielded 294,068 reads with 6.6% identifiable. One hundred-three sequences had significant similarity to integrases by BLASTX analysis (e≤0.001). Four sequences with strongest amino-acid level similarity to integrases were selected and real-time PCR primers and probes were designed. Initial testing with microbial fraction DNA from Tampa Bay revealed 1.9×107, and 1300 gene copies of Vibrio-like integrase and Oceanicola-like integrase L−1 respectively. The other two integrases were not detected. The integrase assay was then tested on microbial fraction RNA extracted from 200 ml of Tampa Bay water sampled biweekly over a 12 month time series. Vibrio-like integrase gene expression was detected in three samples, with estimated copy numbers of 2.4-1280 L−1. Clostridium-like integrase gene expression was detected in 6 samples, with estimated copy numbers of 37 to 265 L−1. In all cases, detection of integrase gene expression corresponded to the occurrence of lysogeny as detected by prophage induction. Investigation of the environmental distribution of the two expressed integrases in the Global Ocean Survey Database found the Vibrio-like integrase was present in genome equivalents of 3.14% of microbial libraries and all four viral metagenomes. There were two similar genes in the library from British Columbia and one similar gene was detected in both the Gulf of Mexico and Sargasso Sea libraries. In contrast, in the Arctic library eleven similar genes were observed. The Clostridium-like integrase was less prevalent, being found in 0.58% of the microbial and none of the viral libraries. These results underscore the value of metagenomic data in discovering signature genes that play important roles in the environment through their expression, as demonstrated by integrases in lysogeny.
The genome for the marine pseudotemperate member of the Siphoviridae φHSIC has been sequenced using a combination of linker amplification library construction, restriction digest library construction, and primer walking. φHSIC enters into a pseudolysogenic relationship with its host, Listonella pelagia, characterized by sigmoidal growth curves producing >109 cells/ml and >1011 phage/ml. The genome (37,966 bp; G+C content, 44%) contained 47 putative open reading frames (ORFs), 17 of which had significant BLASTP hits in GenBank, including a β subunit of DNA polymerase III, a helicase, a helicase-like subunit of a resolvasome complex, a terminase, a tail tape measure protein, several phage-like structural proteins, and 1 ORF that may assist in host pathogenicity (an ADP ribosyltransferase). The genome was circularly permuted, with no physical ends detected by sequencing or restriction enzyme digestion analysis, and lacked a cos site. This evidence is consistent with a headful packaging mechanism similar to that of Salmonella phage P22 and Shigella phage Sf6. Because none of the phage-like ORFs were closely related to any existing phage sequences in GenBank (i.e., none more than 62% identical and most <25% identical at the amino acid level), φHSIC is unique among phages that have been sequenced to date. These results further emphasize the need to sequence phages from the marine environment, perhaps the largest reservoir of untapped genetic information.
A pilot field experiment to assess the relationship between traditional biogeochemical rate measurements and transcriptional activity of microbial populations was carried out at the LEO 15 site off Tuckerton, N.J. Here, we report the relationship between photosynthetic capacity of autotrophic plankton and transcriptional activity of the large subunit gene (rbcL) for ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), the enzyme responsible for primary carbon fixation during photosynthesis. Similar diel patterns of carbon fixation and rbcL gene expression were observed in three of four time series, with maxima for photosynthetic capacity (Pmax) and rbcL mRNA occurring between 10 a.m. and 1 p.m.. The lowest Pmax and rbcL levels were detected between 6 p.m. and 10:30 p.m.. A significant correlation was found between Pmax and form ID rbcL mRNA (R2 = 0.56) and forms IA and IB (R2 = 0.41 and 0.47, respectively). The correlation between the abundance of “diatom” rbcL and Pmax mRNA was modest (R2 = 0.49; n = 12) but improved dramatically (R2 = 0.97; n = 10) upon removal of two outliers which represented afternoon samples with high Pmax but lower mRNA levels. Clone libraries from reverse transcription-PCR-amplified rbcL mRNA indicated the presence of several chromophytic algae (diatoms, prymnesiophytes, and chrysophytes) and some eukaryotic green flagellates. Analogous results were obtained from amplified small rRNA sequences and secondary pigment analysis. These results suggest that diatoms were a major contributor to carbon fixation at LEO 15 at the time of sampling and that photosynthetic carbon fixation was partially controlled by transcriptional regulation of the RubisCO gene.
Nucleic acid sequence-based amplification (NASBA) is an isothermal method of RNA amplification that has been previously used in clinical diagnostic testing. A real-time NASBA assay has been developed for the detection of rbcL mRNA from the red tide dinoflagellate Karenia brevis. This assay is sensitive to one K. brevis cell and 1.0 fg of in vitro transcript, with occasional detection of lower concentrations of transcript. The assay did not detect rbcL mRNA from a wide range of nontarget organisms and environmental clones, while 10 strains (all tested) of K. brevis were detected. By the use of standard curves based on time to positivity, concentrations of K. brevis in environmental samples were predicted by NASBA and classified into different levels of blooms per the Florida Fish and Wildlife Conservation Commission (FWC) system. NASBA classification matched FWC classification (based on cell counts) 72% of the time. Those samples that did not match were off by only one class. NASBA is sensitive, rapid, and effective and may be used as an additional or alternative method to detect and quantify K. brevis in the marine environment.
This review addresses both historical and recent investigations into viral contamination of marine waters. With the relatively recent emergence of molecular biology-based assays, a number of investigations have shown that pathogenic viruses are prevalent in marine waters being impacted by sewage. Research has shown that this group of fecal-oral viral pathogens (enteroviruses, hepatitis A viruses, Norwalk viruses, reoviruses, adenoviruses, rotaviruses, etc.) can cause a broad range of asymptomatic to severe gastrointestinal, respiratory, and eye, nose, ear, and skin infections in people exposed through recreational use of the water. The viruses and the nucleic acid signature survive for an extended period in the marine environment. One of the primary concerns of public health officials is the relationship between the presence of pathogens and the recreational risk to human health in polluted marine environments. While a number of studies have attempted to address this issue, the relationship is still poorly understood. A contributing factor to our lack of progress in the field has been the lack of sensitive methods to detect the broad range of both bacterial and viral pathogens. The application of new and advanced molecular methods will continue to contribute to our current state of knowledge in this emerging and important field.
In order to assess the microbial water quality in canal waters throughout the Florida Keys, a survey was conducted to determine the concentration of microbial fecal indicators and the presence of human pathogenic microorganisms. A total of 19 sites, including 17 canal sites and 2 nearshore water sites, were assayed for total coliforms, fecal coliforms, Escherichia coli, Clostridium perfringens, enterococci, coliphages, F-specific (F+) RNA coliphages, Giardia lamblia, Cryptosporidium parvum, and human enteric viruses (polioviruses, coxsackie A and B viruses, echoviruses, hepatitis A viruses, Norwalk viruses, and small round-structured viruses). Numbers of coliforms ranged from <1 to 1,410, E. coli organisms from <1 to 130, Clostridium spp. from <1 to 520, and enterococci from <1 to 800 CFU/100 ml of sample. Two sites were positive for coliphages, but no F+ phages were identified. The sites were ranked according to microbial water quality and compared to various water quality standards and guidelines. Seventy-nine percent of the sites were positive for the presence of enteroviruses by reverse transcriptase PCR (polioviruses, coxsackie A and B viruses, and echoviruses). Sixty-three percent of the sites were positive for the presence of hepatitis A viruses. Ten percent of the sites were positive for the presence of Norwalk viruses. Ninety-five percent of the sites were positive for at least one of the virus groups. These results indicate that the canals and nearshore waters throughout the Florida Keys are being impacted by human fecal material carrying human enteric viruses through current wastewater treatment strategies such as septic tanks. Exposure to canal waters through recreation and work may be contributing to human health risks.
To determine the potential for bacteriophage-mediated gene transfer in the marine environment, we established transduction systems by using marine phage host isolates. Plasmid pQSR50, which contains transposon Tn5 and encodes kanamycin and streptomycin resistance, was used in plasmid transduction assays. Both marine bacterial isolates and concentrated natural bacterial communities were used as recipients in transduction studies. Transductants were detected by a gene probe complementary to the neomycin phosphotransferase (nptII) gene in Tn5. The transduction frequencies ranged from 1.33 × 10−7 to 5.13 × 10−9 transductants/PFU in studies performed with the bacterial isolates. With the mixed bacterial communities, putative transductants were detected in two of the six experiments performed. These putative transductants were confirmed and separated from indigenous antibiotic-resistant bacteria by colony hybridization probed with the nptII probe and by PCR amplification performed with two sets of primers specific for pQSR50. The frequencies of plasmid transduction in the mixed bacterial communities ranged from 1.58 × 10−8 to 3.7 × 10−8 transductants/PFU. Estimates of the transduction rate obtained by using a numerical model suggested that up to 1.3 × 1014 transduction events per year could occur in the Tampa Bay Estuary. The results of this study suggest that transduction could be an important mechanism for horizontal gene transfer in the marine environment.
Seasonal changes in the abundance of inducible lysogenic bacteria in a eutrophic estuarine environment were investigated over a 13-month period. Biweekly water samples were collected from Tampa Bay, Fla., and examined for prophage induction by mitomycin C treatment. At the conclusion of the study, we determined that 52.2% of the samples displayed prophage induction, as indicated by significant increases in viral direct counts compared with uninduced controls. Samples that displayed prophage induction occurred during the warmer months (February through October), when surface water temperatures were above 19°C, and no induction was observed in November, December, or January. This study presents clear evidence that there is seasonal variation in the number of inducible lysogenic bacteria in an estuarine environment.
To understand the ecological and genetic role of viruses in the marine environment, it is critical to know the infectivity of viruses and the types of interactions that occur between marine viruses and their hosts. We isolated four marine phages from turbid plaques by using four indigenous bacterial hosts obtained from concentrated water samples from Mamala Bay, Oahu, Hawaii. Two of the rod-shaped bacterial hosts were identified as Sphingomonas paucimobilis and Flavobacterium sp. All of the phage isolates were tailed phages and contained double-stranded DNA. Two of the phage isolates had morphologies typical of the family Siphoviridae, while the other two belonged to the families Myoviridae and Podoviridae. The head diameters of these viruses ranged from 47 to 70.7 nm, and the tail lengths ranged from 12 to 146 nm. The burst sizes ranged from 7.8 to 240 phage/bacterial cell, and the genome sizes, as determined by restriction digestion, ranged from 36 to 112 kb. The members of the Siphoviridae, T-φHSIC, and T-φD0, and the member of the Myoviridae, T-φD1B, were found to form lysogenic associations with their bacterial hosts, which were isolated from the same water samples. Hybridization of phage T-φHSIC probe with lysogenic host genomic DNA was observed in dot blot hybridization experiments, indicating that prophage T-φHSIC was integrated within the host genome. These phage-host systems are available for use in studies of marine lysogeny and transduction.
A simple method that combines guanidinium isothiocyanate RNA extraction and probing with antisense and sense RNA probes is described for analysis of microbial gene expression in planktonic populations. Probing of RNA sample extracts with sense-strand RNA probes was used as a control for nonspecific hybridization or contamination of mRNA with target DNA. This method enabled detection of expression of a plasmid-encoded neomycin phosphotransferase gene (nptII) in as few as 104Vibrio cells per ml in 100 ml of seawater. We have used this method to detect expression of the ribulose-1,5-bisphosphate carboxylase large-subunit gene (rbcL) in Synechococcus cultures and natural phytoplankton populations in the Dry Tortugas, Florida. During a 36-h diel study, rbcL expression of the indigenous phytoplankton was greatest in the day, least at night (1100, 0300, and 0100 h), and variable at dawn or dusk (0700 and 1900 h). These results are the first report of gene expression in natural populations by mRNA isolation and probing. This methodology should be useful for the study of gene expression in microorganisms released into the environment for agricultural or bioremediation purposes and indigenous populations containing highly conserved target gene sequences.
We investigated the possibility for natural transformation in the marine environment by using broad-host-range plasmid multimers and a high-frequency-of-transformation (HFT) Vibrio strain as the recipient. Water and sediment samples were taken from Tampa Bay, the eastern Gulf of Mexico, the Florida Shelf near Miami, and the Bahamas Bank. In water column microcosms, transformation frequencies ranged from 1.7 × 10-6 to 2.7 × 10-10 transformants per recipient, with highest frequencies occurring when low levels of nutrients (peptone and yeast extract) were added. The presence of the ambient community either reduced transformation frequency by an order of magnitude or had no effect. In sterile sediments, nutrient additions had no consistent effect on transformation, with transfer frequencies similar to those observed in the water column. Transformation was not observed in any sediment experiment when the ambient microbial community was present. These findings are the first report of natural plasmid transformation in seawater and in the presence of the ambient microbial community. This process may be a mechanism for the acquisition of small, nonconjugative plasmids, which are commonly found in aquatic bacteria. Our data also suggest that natural transformation may be more likely to occur in the water column than in native marine sediments, contradicting prior conclusions based on studies with sterile sediments.
The substrate specificity of the DNA-binding mechanism(s) of bacteria in a Florida reservoir was investigated in short- and long-term uptake studies with radiolabeled DNA and unlabeled competitors. Thymine oligonucleotides ranging in size from 2 base pairs to 19 to 24 base pairs inhibited DNA binding in 20-min incubations by 43 to 77%. Deoxynucleoside monophosphates, thymidine, and thymine had little effect on short-term DNA binding, although several of these compounds inhibited the uptake of the radiolabel from DNA in 4-h incubations. Inorganic phosphate and glucose-1-phosphate inhibited neither short- nor long-term binding of [3H]- or [32P]DNA, indicating that DNA was not utilized as a phosphorous source in this reservoir. RNA inhibited both short- and long-term radiolabeled DNA uptake as effectively as unlabeled DNA. Collectively these results indicate that aquatic bacteria possess a generalized nucleic acid uptake/binding mechanism specific for compounds containing phosphodiester bonds and capable of recognizing oligonucleotides as short as dinucleotides. This binding site is distinct from nucleoside-, nucleotide-, phosphomonoester-, and inorganic phosphate-binding sites. Such a nucleic acid-binding mechanism may have evolved for the utilization of extracellular DNA (and perhaps RNA), which is abundant in many marine and freshwater environments.
The turnover of extracellular DNA was investigated in oligotrophic springs of the Crystal River and the eutrophic Medard Reservoir of southwest Florida. The Medard Reservoir possessed large populations of bacterioplankton and phytoplankton (6.8 × 109 cells per liter and 28.6 μg of chlorophyll a per liter, respectively), while the Crystal River springs only contained a fraction of the microbial biomass found in the Medard Reservoir. Although dissolved DNA values were greater in the Medard Reservoir, higher rates of DNA removal resulted in similar extracellular DNA turnover times in both environments (9.62 ± 3.6 h in the Crystal River and 10.5 ± 2.1 h in the Medard Reservoir). These results indicate that regardless of trophic status or microbial standing stock, extracellular DNA turns over rapidly in subtropical planktonic freshwater environments. Therefore, recombinant DNA sequences from released genetically engineered microorganisms might not be expected to survive for long periods of time in freshwater planktonic environments.
A direct comparison of [3H]thymidine incorporation with DNA synthesis was made by using an exponentially growing estuarine bacterial isolate and the naturally occurring bacterial populations in a eutrophic subtropical estuary and in oligotrophic offshore waters. Simultaneous measurements of [3H]thymidine incorporation into DNA, fluorometrically determined DNA content, and direct counts were made over time. DNA synthesis estimated from thymidine incorporation values was compared with fluorometrically determined changes in DNA content. Even after isotope dilution, nonspecific macromolecular labeling, and efficiency of DNA recovery were accounted for, [3H]thymidine incorporation consistently underestimated DNA synthesized by six- to eightfold. These results indicate that although the relationship of [3H]thymidine incorporation to DNA synthesis appears consistent, there are significant sources of thymine bases incorporated into DNA which cannot be accounted for by standard [3H]thymidine incorporation and isotope dilution assays.
The mechanisms of utilization of DNA by estuarine microbial populations were investigated by competition experiments and DNA uptake studies. Deoxyribonucleoside monophosphates, thymidine, thymine, and RNA all competed with the uptake of radioactivity from [3H]DNA in 4-h incubations. In 15-min incubations, deoxyribonucleoside monophosphates had no effect or stimulated [3H]DNA binding, depending on the concentration. The uptake of radioactivity from [3H]DNA resulted in little accumulation of trichloroacetic acid-soluble intracellular radioactivity and was inhibited by the DNA synthesis inhibitor novobiocin. Molecular fractionation studies indicated that some radioactivity from [3H]DNA appeared in the RNA (10 and 30% at 4 and 24 h, respectively) and protein (approximately 3%) fractions. The ability of estuarine microbial assemblages to transport gene sequences was investigated by plasmid uptake studies, followed by molecular probing. Although plasmid DNA was detected on filters after filtration of plasmid-amended incubations, DNase treatment of filters removed this DNA, indicating that there was little transport of intact gene sequences. These observations led to the following model for DNA utilization by estuarine microbial populations. (i) DNA is rapidly bound to the cell surface and (ii) hydrolyzed by cell-associated and extracellular nonspecific nucleases. (iii) DNA hydrolysis products are transported, and (iv) the products are rapidly salvaged into nucleic acids, with little accumulation into intracellular nucleotide pools.
Dissolved DNA and microbial biomass and activity parameters were measured over a 15-month period at three stations along a salinity gradient in Tampa Bay, Fla. Dissolved DNA showed seasonal variation, with minimal values in December and January and maximal values in summer months (July and August). This pattern of seasonal variation followed that of particulate DNA and water temperature and did not correlate with bacterioplankton (direct counts and [3H]thymidine incorporation) or phytoplankton (chlorophyll a and 14CO2 fixation) biomass and activity. Microautotrophic populations showed maxima in the spring and fall, whereas microheterotrophic activity was greatest in late summer (September). Both autotrophic and heterotrophic microbial activity was greatest at the high estuarine (low salinity) station and lowest at the mouth of the bay (high salinity station), irrespective of season. Dissolved DNA carbon and phosphorus constituted 0.11 ± 0.05% of the dissolved organic carbon and 6.6 ± 6.5% of the dissolved organic phosphorus, respectively. Strong diel periodicity was noted in dissolved DNA and in microbial activity in Bayboro Harbor during the dry season. A noon maximum in primary productivity was followed by an 8 p.m. maximum in heterotrophic activity and a midnight maximum in dissolved DNA. This diel periodicity was less pronounced in the wet season, when microbial parameters were strongly influenced by episodic inputs of freshwater. These results suggest that seasonal and diel production of dissolved DNA is driven by primary production, either through direct DNA release by phytoplankton, or more likely, through growth of bacterioplankton on phytoplankton exudates, followed by excretion and lysis.
The effect of 5-fluoro-2′-deoxyuridine (FdUrd) on [methyl-3H] thymidine incorporation by bacterioplankton populations in subtropical freshwater, estuarine, and oceanic environments was examined. In estuarine waters, intracellular isotope dilution was inhibited by FdUrd, which enabled us to estimate both intracellular and extracellular isotope dilution. In 2 of 10 cases, extracellular isotope dilution was significant. At low concentrations of [methyl-3H]thymidine or [6-3H]thymidine, FdUrd completely inhibited incorporation of radioactivity into protein and RNA. At high concentrations of [3H]thymidine, however, FdUrd had little effect on labeling patterns. The dihydrofolate reductase inhibitors amethopterin and trimethoprim had no effect on macromolecular labeling patterns. These results suggest that thymidylate synthase is not involved in nonspecific labeling and that FdUrd inhibits nonspecific labeling by blocking some other enzyme involved in thymidine catabolism. In oligotrophic oceanic and freshwater samples, FdUrd did not inhibit intracellular isotope dilution or [3H]thymidine labeling of protein and RNA, but caused some inhibition of [3H]thymidine incorporation into DNA. The ability of FdUrd to inhibit nonspecific macromolecular labeling during [3H]thymidine incorporation was significantly correlated (r = 0.84) with total thymidine incorporation (in picomoles per liter per hour). The results are discussed in terms of applications of FdUrd to routine bacterial production measurements and the general assumptions of [3H]thymidine incorporation.
A method was developed for the determination of dissolved DNA in aquatic environments. The method is based upon the concentration of dissolved DNA by ethanol precipitation of 0.2-μm-pore-size filtered water. The DNA in concentrated extracts was quantified by the fluorescence of Hoechst 33258-DNA complexes. Fluorescence not attributable to DNA was corrected for by DNase I digestion of the extracts and averaged 25% of the total fluorescence for all samples. The effectiveness of the procedure for concentrating dissolved DNA was demonstrated by the efficient (>90%) recovery of internal standards. Concentrations of dissolved DNA from a variety of marine and freshwater environments ranged from 0.2 to 44 μg/liter, with the highest values being obtained for estuarine and river environments. The method is simple, specific for DNA, and more sensitive than previously described methods for the determination of extracellular DNA.
[3H]thymidine incorporation, the rate of reduction of iodonitrotetrazolium violet (INT) to INT formazan normalized to DNA, and the ratio of ATP to DNA were adapted to measure the activity of attached and unattached microbial assemblages of Bayboro Harbor, Fla. Activity measurements by [3H]thymidine incorporation were made of cells attached to polystyrene culture dishes, in unfiltered water samples, and in the <1-μm-filtered fraction. In most cases, the activity of attached cells was greater than that of unattached cells either in unfiltered water samples or in the <1-μm fraction. The calculated thymidine incorporation rates for cells in the >1-μm fraction were higher than those for cells either in unfiltered water or in the <1-μm-filtered fraction. By the rate of reduction of INT to INT formazan normalized to DNA and by ATP-to-DNA ratios, attached cells were also more active than cells in unfiltered water samples. These results indicate that the microenvironment afforded by attachment is a more beneficial habitat for microbial growth. Reasons for greater activity by natural populations of attached bacteria are discussed.
Three independent techniques, [3H]thymidine incorporation, the reduction rate of p-iodonitrotetrazolium violet (INT) to INT formazan normalized to DNA, and the ratio of ATP to DNA, were adapted to measure the activity of attached and unattached estuarine bacteria. In experiments employing the estuarine isolate Vibrio proteolytica, nutrient concentrations were manipulated by varying the concentration of peptone-yeast extract. In the presence of exogenous nutrients, the activity of free-living cells was greater than that of attached cells as measured by [3H]thymidine incorporation and ATP/DNA ratios. In the absence of peptone-yeast extract, however, the activity of attached cells surpassed that of free-living cells as determined by [3H]thymidine incorporation and INT formazan normalized to DNA. Of the three techniques, [3H]thymidine incorporation was deemed most sensitive for detecting changes in activity resulting from slight differences in nutrient concentration. By this technique, attached cells were much less sensitive to changing nutrient concentrations than were free-living cells. Below a threshold concentration, attached cell activity remained constant, while the activity of unattached cells decreased as a function of decreasing nutrient concentration. The results suggest that loss of cell surface area available for substrate uptake due to attachment may be an important factor in determining the relative activities of attached and free-living cells.