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Ecological importance of bacteria In most systems from small ponds to the oceans bacteria have the largest collective biomass, most numbers of organisms, and play key roles in all areas from production to consumption, including interchange of electrons during metabolism. They are in virtually all habitats, from free-living to attached, external and internal to all organisms, and their metabolites are important whether positive products or lethal toxins. Jacob Larsen summarized their importance in this statement: Image of Larsen’s summary by D.J. Patterson, Marine Biological Laboratory, Wood’s Hole MA USA
Planktonic bacteria Heterotrophic bacteria, both active and inactive, seem to be present in most freshwater lakes at concentrations in the range of 10^5 to 10^7 ml^-1 (Kuznetsov 1970). Samples of unfiltered (“whole”) lake water sampled at close intervals (1 – 10 cm depth intervals) often vary greatly in bacterial concentration, with ‘microstratification’ (Baker 1973) evident in water density gradients, usually within the thermocline or metalimnion. The heterotrophic communities are also associated with autotrophic plankton, either in precisely the same layers (‘plates’ or ‘blankets’), or in nearby strata. Iron oxidizing bacteria. A variety of chemoautolithotrophic bacteria capable of oxidizing iron- or sulphur-containing minerals can be isolated from acidic mine drainage or ore exposed to water and the atmosphere. The mineral-oxidizing bacteria found in these conditions at ambient temperatures are ubiquitous, the most commonly encountered characterized as Thiobacillus ferrooxidans, `Leptospirillum ferrooxidans', Thiobacillus thiooxidans and more recently Thiobacillus caldus. T. ferrooxidans (iron and sulphur oxidizer) and `L. ferrooxidans' (iron oxidizer) can oxidize ore such as pyrite when growing in pure culture. T. thiooxidans and T. caldus are both sulphur oxidizers that cannot oxidize pyrite alone but grow on the sulphur released after the iron has been oxidized. Also found in similar environments is a variety of acidophilic heterotrophs or facultative heterotrophs in the genera Acidiphilium, Acidocella and `Ferromicrobium'. Oxidation of ore by consortia of bacteria generally takes place at a higher rate than with pure cultures (Rawlings et al. 1999). Photosynthetic bacteria At present (2015) seven distinct groups of photosynthetic bacteria are recognized. All but one contain bacteriochlorophyll (Bchl), one or more of Bchl a through Bchl g, and are anoxygenic. The only oxygenic group has chlorophyll (Chl) instead of Bchl, and is oxygenic (Cyanobacteria). Green sulfur bacteria, e.g. Chlorobium and Pelodictyon, are strict anaerobes found only in anoxic habitats such as the hypolimnia of mesotrophic to eutrophic lakes, and subsurface layers of salt marsh mats. Purple sulfur bacteria such as Chromatium are restricted to low concentrations of dissolved oxygen (microaerobes) and anoxic habitats. They often lie above the green sulfurs in microlayers of lakes and salt marsh mats. Purple nonsulfur bacteria such as Rhodospirillum, Rhodobacter and several others are diverse in their nutrition and habitats They are mainly anaerobic photoheterotrophs that survive under at low light intensity with several organic substrates. Some ferment. Many are also photoautotrophs or chemoheterotrophs that grow in microaerobic or even aerobic conditions. Some species switch modes of metabolism das a result of habitat conditions. One of another species occupies various diverse habitats include freshwater and marine aquatic habitats, also soils, sewage lagoons and plants. Acidobacteria, first recognized in 1997, are soil dwellers that was described in 2007. Some are acidophlic such as Choroacidobacterium. Heliobacteria were discovered by a field class at Indiana University in 1981 given the name Heliobacterium chlorum. They live in soils and contain Bchl g, and lack intracytoplasmic memberanes so that all their pigments are in the cytoplasmic membrane. Green and red filamentous bacteria, formerly known as green non-sulfurs, are anaerobes that can be photoautotrophs, chemoorganotrophs or photoheterotrophs. Cyanobacteria are the first known oxygenic photosynthesizers and contain Chl a along with two water-soluble ‘phycobilin’ pigments’ = phycocyanin, a blue pigment, and phycoerithrin, a red pigment. the phycobilins absorb in the light spectrum where chlorophyll a is least efficient, thus increase the light harvesting ability of cyanobacteria at low light intensity. Many if not all cyanobacteria are capable of switching between anoxygenic and oxygenic photosynthesis.
References Baker, A.L. 1973. Microstratification of phytoplankton in selected Minnesota lakes. PhD Dissertation, University of Minnesota. Kuznetsov, S.I. 1970. The Microflora of Lakes and its Geochemical Activity. [English translation by the Israel Program for Scientific Translations, Jerusalem, Israel; C.H. Oppenheimer, Ed.] University of Texas Press (503 pp). Rawlings, D.E., H. Tributsch and G. S. Hansford 1999. Reasons why ‘Leptospirillum ’-like species rather than Thiobacillus ferrooxidansare the dominant iron-oxidizing bacteria in many commercial processes for the biooxidation of pyrite and related ores. Microbiology 145:5-13.
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