View - Kowalewski, M. - Virginia Tech

LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMSglaciations would have altered the surface currentsof the world’s oceans, as the temperature gradientschanged and ice disappeared. Increased nutrientsupply would likely result from oceanographicreorganization, and this might have profoundlyaffected the pelagic primary-producing microorganismsand the nearshore benthic associations,just as eutrophication of oceanic waters does today.Increased productivity could account for theabundant and widespread mat development at thistime, an inferred increased phytoplanktonabundance, and changing diversity of the acritarchsas well. Energy flow, and hence trophic structures,is the only direct link between the many elementsof the Neoproterozoic biota.PHANEROZOIC: 0.545 TO 0.0 GACompared to most of the previous history oflife, Phanerozoic trophic structures were relativelyconstant throughout the entire eon (Table 3, 4),chiefly because all the phyla of animals, most ofthe other eukaryotes, and many photosynthesizingorganisms were present soon after the start of theeon (Valentine et al., 1991). Although trophicstructures varied, especially as complexityincreased through the addition of new species,skeletonization, and body plans in the Cambrian;and especially following mass extinctions, thefundamental energy flows did not change much.With regard to microfossils, six major evolutionarydevelopments of trophic importance occurred:1)evolution of benthic foraminifera and siliceousplankton in the Cambrian; 2) appearance of larger,symbiont-bearing foraminifera in late Paleozoicshallow carbonate shelf environments; 3) inferredsymbiosis between reef organisms and algalsymbionts; 4) emergence and diversification ofmajor groups of organic-walled, siliceous orcalcareous phyto- and zooplankton in the mid-Mesozoic; 5) Mesozoic radiation of deep-water, reef,and nearshore foraminifera; and 6) mass extinctionand rediversification of many species several times.In prokaryotes, mats and stromatolites weresubjected to increased grazing when bioturbatingand grazing metazoans evolved, first in the laterProterozoic and then in the early Phanerozoic(Awramik, 1971). Mats and even stromatolites haveremained abundant but are more restricted inmodern environments.Paleozoic: 545 to 248 Ma.—Cambrianecosystems differed from previous ones largelybecause new functional groups of animalsevolved. Microorganisms also developed some ofthe same features as the animals. These involvedfive important evolutionary events that relate totrophic structures: 1) radiation of skeletonizedorganisms at the start of the period; 2) appearanceof new body plans; 3) further diversification ofmicroplankton and suspension-feeding benthicanimals; 4) radiation of infaunal bioturbatinganimals; and 5) development of archaeocyathidreefs possibly facilitated by symbioses withphototrophs. While these developments were new,the fundamental trophic structures established inthe late Neoproterozoic remained intact.At the base of the Cambrian, skeletonization andornamentation evolved among protists (Culver,1994; Knoll, 1994; Lipps and Rozanov, 1996), largeralgae (Knoll and Lipps, 1993), and animals(Valentine et al., 1991). Animals radiated by way ofskeletonization, body plan proliferation throughdevelopmental gene duplications and rearrangements,and behavior (Crimes, 1992; Valentine et al.,1999). These radiations may have been in responseto changes in the marine environment—especiallyincreased primary productivity; acritarchs radiatedin concert with animals (Vidal and Moczydlowska-Vidal, 1997; Butterfield, 2001). Since phytoplanktonrespond chiefly to oceanic conditions, especiallywater mixing and nutrient supply, a change inprimary production may have initiated a cascade ofselection among other organisms in all environments,resulting in a rapid diversification. Today,benthic animals and protists respond quickly tochanges in water column primary production withchanges in metabolism, life histories, reproduction,and behavior (Erskian and Lipps, 1987; Graf, 1989;Anderson, 1993; Linke and Lutze, 1993; Altenbachet al., 1999; Schmiedl et al., 2000). The producerstoo were adapting to the selective pressures exertedon them by microzooplankton (protists, small81