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LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMS(Fig. 1). In the grazing component, larger primaryproducers (microphytoplankton) are taken byherbivorous zooplankton, which are in turn eatenby larger carnivores ranging from small crustaceansand other invertebrates to fish and whales. Thisstandard view of trophic resources in the sea ismisleading and does not apply to most of Earth’shistory. Another part of the food web sinks to thesea floor where it becomes available for benthicorganisms. Fecal pellets, aggregations of otherorganisms into marine snow, and the dead and evenlive bodies of organisms comprise the sinking flux.This flux removes a variable amount of carbonfrom the water column.The bacterial loop dominates over the grazingfood web in the oligotrophic parts of the oceans.The grazing web prevails in eutrophic situations,geographically and seasonally (Kiorboe, 1993;Shinada et al., 2001), but even here the bacterialloop is significant. In eutrophic areas, phytoplanktonare larger, resulting in a web with fewer levels orlinks. Because smaller phytoplankton are stillpresent, more complex webs coexist.Benthic Food Webs.—The benthic environmentsof the world’s oceans are far more diverseand heterogeneous than pelagic ones. This isreflected in the species diversity of groups withboth benthic and pelagic members. For example,among Recent foraminifera, well over 4000 benthicspecies are known but only 42 or so species areplanktic. In prokaryotes, where species have noteven been described, pristine marine sedimentcontains far more genomic diversity than the watercolumn (Torsvik et al., 2002). The benthic habitatsfor microorganisms include all known marineenvironments (see Nybakken, 2001, for summary).Perhaps the most significant habitats through timeand even today are microbial mats (Figs. 2–4, 6).While these mats have changed over the past 3.5billion years, they still appear very similar acrossthe ages. The other major environments of theworld’s oceans have undergone significant andconstant oceanographic modifications through timeas continental positions and climates have changed.At any period in the history of the oceans,thousands of distinctive habitats and individualtrophic structures existed. Today, because of theprominent thermal gradients from north to south(Buzas et al., 2002; Crame, 2002) and shallow todeep, the latitudinal array of the continents, andthe evolutionary complexity and diversity oforganisms, habitat heterogeneity is perhaps at leastas great as at any other time in Earth history.MICROORGANISMS AND THEEVOLUTION OF MARINETROPHIC STRUCTURESNothing, of course, is known of the earliesttimes on Earth from its formation approximately4.55 Ga to about 3.8 Ga, other than what can beinferred from extraterrestrial bodies that fell toTABLE 2—Marine pelagic organisms. Primary producers indicated with an asterisk.Groups Size (microns) Main Taxa Age RangeNekton 2×10 3 –3.3×10 7 Invertebrates, fish, reptiles, mammals Cambrian–RecentZooplankton 2×10 3 –2×10 3 Invertebrates, vertebrate larvae Cambrian–RecentMicroplankton 20–2×10 2 Acritarchs*, dinoflagellates*, diatoms*,silicoflagellates*, radiolarians,foraminifera, tintinnidsProterozoic; chieflyMesozoic–RecentNanoplankton 2–20 Coccolithophorids* Mesozoic–RecentPicoplankton 0.2–2 Prokaryotes*, cyanobacteria* Archean–Recent73