IU 625 (S.
IU 625) is a cyanobacterium also called Anacystis nidulans
. It is a non-motile, unicellular, rod-shaped microorganism which is similar to Gram-negative bacteria in cell wall structure, cell division and the ability to harbor plasmids (Lau and Doolittle, 1979
). Owing to the ubiquitous nature of cyanobacterial species and their adaptability to various ecological environments, they serve as excellent models for the investigation of various biological processes, including membrane transport and physiological changes in photosynthesis due to changes in environmental conditions (Sauer et al., 2001
; Zdrou and Tromballa, 1981
). They also serve as good indicators of environmental pollution, especially heavy metal contamination. Studies have found that heavy metals such as barium, aluminum, lead, cadmium, and mercury are toxic to S.
IU 625 (Lee et al., 1991
; Lee and Lustigman, 1996
). However, even essential metals such as Mn2+
inhibit the growth of S.
IU 625 when present at high concentrations (Lee et al., 1993
; Lee and Lustigman, 1996
). Furthermore, the toxicity of selenium and mercury combinations have been evaluated (Lee et al., 2002
Mercury (Hg) has no known biological function (Weiss-Magasic et al., 1997
) and is the most lethal heavy metal on the Environmental Protection Agency T
ist (TAL). In humans, mercury poisoning can cause diseases such as Minamata Disease/Hunter Russell syndrome (which results in reduced senses and speech impairment) or Pink Disease (painful limbs) (Davidson et al., 2004
). The severity of symptoms depends on the elemental form of mercury, dosage, and duration of exposure. Mercury is highly toxic because of its high affinity for thiols, which play a vital role in protein function (Bruins et al., 2000
; Kozak and Forsberg, 1979
). It also non-specifically binds to DNA, inducing single-strand breaks (Williams et al., 1987
), and affects oxidative phosphorylation and membrane permeability (Brown et al., 1983
). Mercury exhibits high toxicity to both light and dark photosynthetic reactions in phototrophic organisms (Lu et al., 2000
Water, soil, and rocks can contain mercury. Oxidized forms of mercury, such as Hg2+
, can be reduced to Hg0
, which is volatile at room temperature, or methylated to form methylmercury (Barkay et al., 2003
), which is subject to bioaccumulation in the food chain (Davidson et al., 2004
). Aquatic microorganisms often methylate mercury, ultimately accumulating in fish at the top of the food chain, providing the primary route for human organomercury exposure (Davidson et al., 2004
). Bioaccumulation of mercury in microalgae occurs in two stages: short-term physical absorption at the cell surface and long-term uptake involving intracellular accumulation (Inthorn et al., 2002
Because of the toxicity and ubiquity of metals in the environment, microorganisms have developed multiple ways of managing both essential and non-essential metals. The levels of these metals must be carefully regulated to ensure sufficient supply while avoiding toxicity – a process often referred to as metal homeostasis. Microorganisms can manage excess metals by simultaneously effluxing metal ions and restricting uptake (Brown et al., 1983
; Brocklehurst et al., 1999
). Some microorganisms can make metals less toxic by sequestering or enzymatically reducing the metal to a less toxic form. The latter is the case in most mercury resistant microorganisms (Brown et al., 1983
). Since bacteria are likely to encounter toxic Hg2+
concentrations in their natural environments, mercury resistance determinants encoded by mer
genes can be found among a number of species of bacteria (Nies, 1999
; Silver, 1998
), including close relatives of S.
IU 625, such as S. elongatus.
Our previous work showed that mercury inhibits the growth of S
. IU 625 (Lee et al., 1992a
) and suggested that S.
IU 625 removes mercury from culture supernatants (Lee et al., 2002
). In this study we expand on our previous findings, reporting that S.
IU 625 recovers from mercury-induced pigmentation and morphological defects and describe the temporal dynamics of the reduction of mercury concentration in cell supernatants and the association of mercury with S.
IU 625 cells.