View - Kowalewski, M. - Virginia Tech

LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMSSymbiosis with algal protists apparently wasimportant in the carbonate, tropical zones. Notonly were larger foraminifera present, butscleractinian corals and rudist bivalves developedextensive reef systems (Kauffman and Johnson,1988; Stanley, 1988). All of these organisms mayhave harbored photoendosymbionts. This type ofsymbiosis increases the ability of the host tosecrete large amounts of carbonate, and hence tobuild a massive skeleton (Cowen, 1983).The Mesozoic ended with a mass extinction thateliminated the symbiont-bearing but not otherbenthic foraminifera (Culver, in press), and reducedeach of the planktic microfossil groups to less than10% of their diversity. Likewise the reef builders ofthe Mesozoic perished completely in the extinctionevent, which seems to have affected shallower-,warmer-water organisms more than others.Cenozoic: 66 to 0 Ma.—The end-Mesozoicextinction left only a few members of eachmicrofossil group in pelagic and carbonateenvironments. From these few species offoraminifera, radiolaria, diatoms, coccolithophorids,and others, the Cenozoic biota (Pickering, 2000) wasestablished in the Paleocene. The species were quitedifferent in morphology, but their trophic roles wererestored. The trophic structures of the Cenozoicwere, as far as the fossil record indicates, more orless identical to those of the Mesozoic. After aboutthree million years, symbiont-bearing taxaappeared again in both benthic and plankticforaminifera and probably radiolaria; diatoms werethe dominant larger primary producer in eutrophicregions, while coccolithophorids dominated in theoligotrophic central gyres. Foraminifera againbecame important contributors to reefs. Deep-seabiotas were trophically and taxonomically similarto those of the Cretaceous but experienced a laterextinction as deep-water circulation changed at theend of the Paleocene. Although the biota changed,the foraminifera probably continued to cycledetrital material and prokaryotes to largerinvertebrates that preyed on them. At the end ofthe Eocene, shallow-water and pelagic biotassuffered another extinction that resulted in a sharpdecrease in diversity (Lipps, 1985). The microfossildiversity slowly recovered through the Oligocene,and by the Miocene the trophic structures seem ascomplex in terms of species involved as before.From the mid-Miocene on, glaciations in highlatitudes became increasingly intense until highlatitudeparts of continents were covered with largeice sheets. This refrigeration of Earth had littleeffect on the trophic structures in either benthic orpelagic microbiotas other than to shift the entirebiotas biogeographically toward lower latitudes andback again during warmer periods. Upwellingintensified along the equator, along the eastern sidesof continents, and around Antarctica. This allowedincreased primary production in these areas and aconcentration of silica deposition by the diatomsand radiolarians. Pleistocene glacial-interglacialshifts in upwelling intensity and bottom waterscaused restructuring of deep-water assemblages.EFFECTS OF EXTINCTIONSON FOOD WEBSExtinctions during the Phanerozoic changedthe microbial trophic structures enormously. Eachextinction, in general, appears to have had asimilar impact—the diversity of most elementsin shallow and pelagic ecosystems was reduceddrastically (Lipps, 1985; Culver, in press), and itmost commonly occurred rapidly (Table 5).Whether or not this happened because a particularpart of the food web was destroyed with ancillaryeffects cannot be determined. Species diversityof primary producers declined as much as that ofthe consumers and predators, and at the same time.Overall primary production may have declinedor the ecosystem changed so as to cause a decreasein carbon isotopic values (d’Hondt, 1998).Evidence from oxygen isotopic values of plankticforaminifera in the Mesozoic and Cenozoicindicates that the thermocline was commonlydegraded at mass extinction events, and this wouldcertainly have had a major effect on theavailability of nutrients for the photosyntheticorganisms. If they declined in productivity, thenthe entire trophic structure of much of the oceansmay well have collapsed too. Further studies of85