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LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMSTABLE 1—The chief fossilizable prokaryotes and protists and their trophic functions and geologic age.Modified from Lipps (1993).Bacteria or Protists Habitat Function Age RangeHeterotrophic bacteria All Consumers Archean–RecentAutotrophic bacteria,especially cyanobacteriaAll Primary producers Paleoproterozoic–RecentAcritarchsChiefly onshore/offshore pelagicPrimary ProducersMesoproterozoic–RecentDinoflagellatesOceanic, nearshore,lacustrinePrimary ProducersPaleoproterozoic?–RecentCoccolithophorids andrelated calcareous algaeOceanic, eutrophicto oligotrophicPrimary ProducersTriassic–RecentSilicoflagellates Oceanic Primary Producers;secondary consumers?Cretaceous–RecentDiatomsOceanic, chieflyeutrophic; lacustrinePrimary ProducersCretaceous–RecentForaminiferaBenthicPelagicHerbivorous,omnivorous,carnivorousCambrian–Recent.Jurassic–Recent forpelagic forms.Radiolarians Oceanic Herbivorous, carnivorous Cambrian–RecentTintinnids Neritic, oceanic Herbivorous, carnivorous Ordovician–Recentmust have been somewhat similar in function.Pelagic Food Webs.—By most definitions, thepelagic environment comprises the water columnof the open oceans seaward of the continental shelf.Organisms present there may be planktonic (passivedrifters that do not swim very much, at leasthorizontally) or nektonic (swimming forms). Theseorganisms range in size from the tiniest picoplanktonthrough nano- and microplankton to the relativelylarge zooplankton on which huge vertebrates graze(Table 2). At all size ranges, the herbivores,carnivores, and other consumers are supportedultimately by mostly tiny primary producers.Prokaryotes, commonly constituting the largestbiomass in the water column (Christaki et al.,2001), are supplied with food and nutrients by largeamounts of dissolved organic matter (DOM) andparticulate organic matter (POM) derived fromnumerous sources, as well as viruses, protists, andother living organisms. They are heterogeneouslydistributed even within a single water sample, withsome habitats within the water particularly rich inthem; for example, marine snow, detritus, andphytoplankton find abundant bacteria attackingthem. Bacteria constitute the major pathway foroceanic primary production (about 50%), whichtakes the form of the so-called bacterial loop(Fig.1) (Pomeroy , 1974; Azam, 1998). This loopis variable and dynamic in its functions (Falkowskiet al., 1998; Riemann et al., 2000; Shinada et al.,2001; and it is largely self-sustaining andindependent of larger organisms.Two other pathways exist in pelagic systems:the “grazing food web” and the “sinking flux”71