After viruses, bacteria are the next most abundant life form in the oceans today, occurring in densities of approximately 10 billion per liter of seawater in surface waters. The bacterio-plankton includes diverse taxa such as cyanobacteria and prochlorophytes, pro-teobacteria, and other groups.

Cyanobacteria, formerly called blue-green algae, are oxygen-producing photosynthetic bacteria that possess either chlorophyll a and phycobiliproteins, or chlorophylls a and b, as their primary light-harvesting pigments. Cyanobacteria are widely distributed in both marine and freshwater ecosystems and dominate the picoplankton-size class in marine environments. Cyanobacteria include unicellular and multicellular forms, the degree of morphological complexity ranging from simple, coccoid, and rod-shaped forms to more complex, differentiated forms that possess specialized compartments, such as reproductive cells, heterocysts, and akinetes.

Marine cyanobacteria possessing phyco-biliproteins include colonial, filamentous forms, such as Trichodesmium, and small coc-coid forms, such as Synechococcus and Syne-chocystis. Freshwater cyanobacteria include such forms as the filamentous Anabaena and Oscillatoria, and the colony-forming Microcystis. Microcystis produces neurotoxins and hepatotoxins, and ingesting water contaminated by these toxins can cause mortality in vertebrates and gastrointestinal illness in humans.

There are a few derived cyanobacterial groups that possess chlorophyll b, but lack phycobiliproteins. These taxa were initially called prochlorophytes, reflecting previous speculations that they might be living representatives of the ancestral group that gave rise to the chloroplasts of chlorophyte algae and green plants. More recently, prochlorophytes have been shown not to be a distinct group, but to have independently arisen from different cyanobacterial lineages. These groups include marine taxa such as the coccoid Prochlorococcus and Prochloron, and freshwater forms such as the filamentous Prochloro-


A diversity of heterotrophic bacteria of variable morphologies is found in the plankton of marine and freshwater ecosystems. Alpha-proteobacteria occur in both marine and freshwaters and include rods, vibrios, and filaments of various sizes. Beta-proteobacteria are dominant in freshwaters but rare in marine waters, and they are represented by straight to curved rods (1.5 ^m long by 1 ^m wide). Other proteobacteria are also present in lower abundances in both marine and freshwaters. The Cytophaga-Flavobacterium group is composed mostly of filaments 20 to 300 ^m long; it is the dominant group in marine waters, although members of this group also occur in freshwaters. Planctomycetales are large cocci, more than 1 ^m in diameter, that are found in both marine and freshwaters, often associated with macroaggregates.

Most of the earliest microfossils preserved in the fossil record are cyanobacterial, and most of them are represented by benthic mat-forming and stromatolite-building taxa. A few potential fossil planktonic forms are known; for example, Oscilltoriopsis obtusa is a 2-billion-year-old fossil from the Duck Creek Formation of Australia that morphologically resembles modern filamentous species of Oscillatoria.

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