Pyrite Oxidation
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PYRITE OXIDATION
Radiolaria (see entry Radiolaria) build mostly tests from opal (hydrated silica), some from silica and organic material, and some from strontium sulphate. They are important index fossils and a source of marine silica cements, chert (see entry Cherts), etc. Foraminiferans (see entry Foraminifera) have typically multichambered tests, although some are unilocular. Foraminiferans are important as biostratigraphic index fossils and as a reliable source of untransported carbonate shell material for stable-isotope analyses. Further, the Paleozoic Acritarchs (nonacid soluble organic structures, potentially cysts) and Chitinozoa (chitinous organic-walled flask-like chambers) potentially are also protozoan microfossils. Besides the fossils of heterotrophic taxa, specifically several Chromist taxa (diatoms, golden algae) build important microfossils such as the diatoms (see entry Diatoms), the Coccolithophorids (see entry Chroococcidiopsis), and the golden algae (Figure 2).
Diversity and biogeography: “everything is everywhere”? Two main hypotheses about the causes of nonrandom distribution of microorganisms continue to obtain conflicting experimental support: the idea that “everything is everywhere” is based on the assumption that the enormous dispersal capabilities of microorganisms allow them to spread into virtually any habitat. Absence of taxa is caused by unfavorable local conditions that prevent them from getting established everywhere. In a contrasting model, the rate of dispersal of microorganisms is not sufficiently high to overcome historical dispersal limitations and human influence. This allows for existence of endemic taxa many of which remain to be discovered. Adjusting the species concepts is a necessary precondition for drawing conclusions about derived theories such as flagellate biogeography and the “everything is everywhere” debate. The predominant view of a low to moderate number of nanoflagellate taxa, most of which seem to be globally distributed and ubiquitous, is increasingly replaced by the view of a tremendous number of taxa with a much more restricted distribution, both with respect to habitat type and geographic distance. Summary Protozoa are a loose grouping of organisms with usually unicellular organization and heterotrophic mode of nutrition. The systematic position of the Protozoa and related organisms remained and still remains unclear for many lineages. Functional groups based on rough morphological characters comprise amoeboid, flagellated, and ciliated organisms. Protozoa are a major component in any ecosystem and are responsible for the majority of eukaryotic respiration and production whereas their autotrophic counterparts are important primary producers. Protists are key mediators in the enhancement of nutrient flow by regulating decomposition rates and specific metabolic pathways to the benefit of plants and microorganisms in
soil and aquatic habitats. They build important microfossils both in the marine and in freshwaters.
Bibliography Adl, S. M., Simpson, A. G.
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