Microcystis aeruginosa
Classification
Bacteria(Domain); Cyanobacteria (Phylum); Cyanophyceae (Class); Chroococcales (Order); Microcystaceae (Family); Mycrocystis (Genus); aeruginosa (species)
Species
Mycrocystis aeruginosa
Description and significance
Mycrocystis aeruginosa is a common unicellular colonial cyanobacteria found in fresh water environments. This bacterium produces harmful toxins that create health risks for populations living and harvesting in contaminated areas where M. aeruginosa blooms. Blooms occur when nutrient levels spike in aquatic environments or nutrient levels are selective toward Microcystis aeruginosa.
Genome structure
M. aeruginosa has a single circular genome consisting of 5 842 795 base pairs and has been entirely sequenced by researchers. In the genome are 6312 protein-encoding genes, two sets of rRNA genes, and 42 tRNA genes representing 41 tRNA species. Forty-five percent of the protein-encoding sequences showed sequence similarity to genes of known function, 32% were similar to hypothetical genes with a hypothetical function, and the remaining 23% had no apparent similarity and code for a unique function [2].
Cell structure and metabolism
Microcystis aeruginosa is a unicellular, planktonic freshwater cyanobacterium. The existence of intracellular structures, the gas vesicles, provides cells with buoyancy. These hollow, gas-filled structures can keep Microcystis cells close to the surface of water body, where there is optimal light and oxygen for growth. Thus, when the water column is stable, the colonies can accumulate at the water surface and form surface water blooms.[4] Cells range from 2.61 to 5.40μm in diameter, and can be either ovoid or spherical in shape. The extracellular covering of M. aeruginosa was divided into several layers: the cytoplasmic membrane or plasmalemma, the peptidoglycan layer, and the multilayered structure of the cell wall.[3] Microcystis aeruginosa is light dependent and oxygenic, but cells may live under the dark anaerobic conditions for periods of time in eutrophic lakes. Microcystis aeruginosa often form blooms in eutrophic lakes and reservoirs. Surface water blooms can cause anaerobic conditions below the surface in the water and thus make other phytoplankton including M. aeruginosa themselves live in an unfavorable environment. However, M. aeruginosa appeared to be more tolerant to dark anaerobic conditions, which may be important to the dominance of M. aeruginosa in eutrophic lakes. Shi et al (2007) reported that M. aeruginosa showed a slight increase in cell metabolic activity, no conspicuous death of cells, and absence of decay of chlorophyll-a fluorescence in individual and competition cases under dark anaerobic conditions.[6]
Ecology
Large occurrences of Microcystis aeruginosa are regularly found at the surface of water bodies in spring and summer. It is one of the most ecologically damaging species due to its toxicity to aquatic and terrestrial organisms. Cyanobacteria may adopt different strategies to reduce the possibility of being consumed by their upper trophic level of food web, such as morphology and intracellular toxins. Microcystins, a group of toxins are produced by Microcystis aeruginosa. Microcystins are cyclic peptides which are potent hepatotoxins for rodents and humans and are not allowed by many for grazing compounds. Further, we note recent reports showing that the ongoing invasion of freshwaters in North America by the filter-feeding zebra mussel, Dreissena polymorpha, is causing an increase in M. aeruginosa in low-nutrient lakes.[8] Moreover, anaerobic conditions below the surface water caused by bloom probably cannot sufficiently support fish or shellfish in the vicinity. Additionally, it pollutes the aquatic system, causes taste and odor problems in drinking water, and decreases the overall water quality. Some scientists reported that rice straw extract can inhibit the growth of M. aeruginosa due to the synergistic effects of various phenolic compounds in the rice straw, which indicated that environment-friendly bio-materials could be used for controlling the algal bloom of M. aeruginosa in eutrophic water.[5]
Current Research
Dreissena polymorpha, also known as the zebra mussel, is an ecologically disastrous invasive species found in the Great Lakes. Research has observed a correlation between M. aeruginosa blooms and populations of D. polymorpha in places such as the Saginaw Bay in Lake Huron. It was found that through selective filter feeding by the zebra mussels, the M. aeruginosa are rejected in the pseudofeces creating a selective pressure against other more desirable algae. Another research team also discovered that zebra mussels alter the N/P ratio in the water that creates another selective factor to help M. aeruginosa reproduce and thrive.[1][7] Both of these studies and more are showing a strong relationship between the two destructive species and management of this issue is continuing.
References
(1) Bykova, O., Laursen, A., Bostan, V., Bautista, J., McCarthy, L. 2006. Do zebra mussels (Dreissena polymorpha) alter lake water chemistry in a way that favours Microcystis growth? Science of the Total Environment. 371:362–372
(2) Kaneko, T., Nakajima, N., Okamoto, S., Suzuki, I., Tanabe, Y., Tamaoki, M., Nakamura, Y., Kasai, F., Watanabe, A., Kawashima, K., Akiko Ono, Y., Shimizu, Y., Takahashi, C., Minami, C., Fujishiro, T., Kohara, M., Katoh, M., Nakazaki, N., Nakayama, S., Yamada, M., Tabata, S., Watanabe, M.M. 2007. Complete Genomic Structure of the Bloom-forming Toxic Cyanobacterium Microcystis aeruginosa NIES-843. DNA Research. DNA Res 14: 247-256.
(3) Kim BH, Choi MK, Chung YT, Lee JB, Wui IS, 1997. Blue-green alga Microcystis aeruginosa Kütz. in natural medium. Bulletin of Environment Contamination and Toxicology, 59:35-43
(4) Mlouka, A., K. Comte, A.-M. Castets, C. Bouchier, and N. Tandeau de Marsac. 2004. The gas vesicle gene cluster from Microcystis aeruginosa and DNA rearrangements that lead to loss of cell buoyancy. J. Bacteriol. 186:2355-2365
(5) Park, M.-H., Han M.-S., Ahn C.-Y. Ahn, Kim H.-S., Yoon B.-D., Oh H.-M., 2006. Growth inhibition of bloom-forming cyanobacterium Microcystis aeruginosa by rice straw extract. Letters in Applied Microbiology 43:307–312
(6) Shi, X. L., Kong, F. X., Yu, Y., Yang, Z. 2007. Survival of Microcystis aeruginosa and Scenedesmus obliquus under dark anaerobic conditions. Marine and Freshwater Research, 58, 634–639
(7) Vanderploeg, H.A., Liebig, J.R., Wayne, W.C., Agy, M.A., Johengen, T.H., Fahnenstiel, G.L., Nalepa, T.F. 2001. Zebra mussel (Dreissena polymorpha) selective filtration promoted toxic Microcystis blooms in Saginaw Bay (Lake Huron) and Lake Erie. Can. J. Fish. Aquat. Sci. 58: 1208-1221.
(8) Wilson, Alan E., Sarnelle, Orlando, Neilan, Brett A., Salmon, Tim P., Gehringer, Michelle M., Hay, Mark E, 2005. Genetic Variation of the Bloom-Forming Cyanobacterium Microcystis aeruginosa within and among Lakes: Implications for Harmful Algal Blooms. Appl. Environ. Microbiol, 71: 6126-6133
