The origin, diversification, adaptation, and extinction of organisms can only be understood by considering the biotic interrelationships within successive ecosystems. In spite of their inconspicuous appearance, smaller protistans are of prime importance to the global ecosystems, for marine food chains and atmospheric oxygen are both direct functions of photosynthesis by the plant protists. Other plant and animal protistans also are important components of many food chains. Unicellular and undifferentiated in nature, the protistans vary widely in other respects, and some demonstrated opposing reactions in the Permian-Triassic.

During the Early Paleozoic, phytoplankton included acritarchs and Prasinophyceaen green algae; their numbers were sharply reduced at the end of the Devonian, perhaps as a response to the rise of land plants. At present the submerged benthic plants along the coasts locally inhibit phytoplankton growth by their more successful competition for nutrients and light; such an effect was intensified in the Devonian by the first retention of nutrients in an important and extensive terrestrial biomass. External metabolites produced by modern phytoplankton inhibit bacterial development, hence the Late Paleozoic reduction in phytoplankton and resultant absence of these bactericidal compounds allowed bacterial increase. The increase in bacteria and decrease in phytoplankton food supply probably combined in decimating zooplankton levels. Planktonic radiolaria had been important, and tintinnids were present in the Early Paleozoic but became scarce by the Permian when radiolarian diversity was only one-fifth that of the Devonian. Zooplankton remained scarce through the Triassic. With the expansion of phytoplankton in the Jurassic (especially the coccolithophorids and dinoflagellates), zooplankton also diversified. Radiolarians, calpionellid tintinnids, and planktonic foraminifera all became of rock-forming abundance locally.

During the Late Paleozoic the scarcity of phytoplankton allowed an increase and diversification of benthic algae. The coralline red algae and the blue-greens (represented by stromatolites) were important during the Carboniferous, whereas dasyclad green algae expanded greatly during the Permian and Triassic. These photosynthetic plants did not replace phytoplankton in the food chain, for filter-feeding invertebrates could not utilize calcareous benthic algae. Planktonic larval stages of other invertebrates were vulnerable to the changes in productivity. Although some specialized benthonic foraminifera such as the Fusulinacea disappeared, other groups rose and disappeared in the Triassic. Some smaller foraminifera and various other invertebrates that persisted probably utilized detritus, benthic algae, or bacteria as food. Many of the agglutinated or porcelaneous foraminifera and even the hyaline Nodosariacea showed little reduction from the Carboniferous into the Mesozoic. The rate of taxonomic turnover of other protists and invertebrates continued to be high in the Triassic, until by the Rhaetic the foraminiferal assemblage had attained an aspect mch like that of the Early Jurassic.

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Judging from modern representatives, neither relative land and sea positions nor water depth nor temperature could have been the single controlling factor in the protistan Permian-Triassic changes, as these highly adaptive organisms now are found at all latitudes in waters from intertidal to hadal depths and at wide variations in salinity. Seemingly, their abundance and diversity were most strongly influenced, then as now, by the availability of nutrients or food supply and thus controlled by the various biotic interrelationships.

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