View - Kowalewski, M. - Virginia Tech

LIPPS AND CULVER—TROPHIC ROLE OF MARINE MICROORGANISMSalso probably widely distributed in Earth’s seas.Pelagic realms of the Archean must have ratherquickly evolved a prokaryotic biota (Table 4).Prokaryotic trophic interactions would have beenless complex than modern ones, chiefly becauselarger plankton and sources of organic debris wereabsent, and diversity was probably lower.Nevertheless, this pelagic system would have beenfueled by organic matter from the prokaryotes,from other ecosystems elsewhere on Earth, fromviruses, from co-occurring species, and from thecontinued infall of meteorites. The system wouldhave operated quite independently of the benthos,although the export of parts of it to the benthosmust have taken place and DOM and POM fromthe benthos may have been mixed into the openwaters (Fig. 1).After their initial evolution, prokaryotes likelyradiated into most benthic marine habitats. Therecord reveals only sparse prokaryotic fossils andfilms, mats, and stromatolites, known from severallocations, which represent shallow-watercommunities. These, however, cannot be fullyrepresentative of the range of microbial habitats inthe Archean oceans because they represent so fewof the possible Archean environments and the knownhabitats that microbes occupied later in their history.Evidence of predation in these communities is absenttoo, although surely the producers were harvestedby other prokaryotes and possibly protists, eitherdirectly or as decomposed matter.PROTEROZOIC: 2.5 TO 0.545 GADuring the two billion years of the Proterozoic,trophic structures evolved from simple typesdominated by prokaryotes to those that were theimmediate precursors of modern marine types(Table 3, 4), which included large metazoans andabundant larger primary producers in the benthosand a poorly preserved but likely complex groupof plankton. Trophic relationships changed throughtime as newly evolved kinds of organismsinterposed themselves into previously existingcommunities. The fossil record throughout thePaleoproterozoic and Mesoproterozoic is stillrelatively unknown, although there is enough fossilevidence to indicate that protists had becomeimportant parts of the benthic and pelagicecosystems (Vidal and Moczydlowska-Vidal, 1997;Javaux et al., 2001). In the Neoproterozoic, fossilspermit reasonable reconstructions of trophicinteractions.Global events in the Proterozoic may have hada significant impact on marine ecosystems. Theseevents include a poorly known but extensive glacialperiod about 2.4 Ga, the so-called Snowball Earth I(Kirschvink et al., 2000). Oxygen became plentifulin the atmosphere and oceans between 2.2 and1.8Ga, and this may have spurred protists todiversify and occupy more habitats. Fossil protistsare present in the record starting about 1.8 Ga (Vidaland Moczydlowska-Vidal, 1997) and indicaterelatively complicated ecologic relationships. Thefirst protistan groups known later in the record alsoappear during this eon (Javaux et al., 2001). Mostsignificantly, a second set of Snowball Earthglaciations took place about 750 to 600 Ma.Snowball Earth II ended just before the appearanceof metazoans in the most complicated ecosystem ofthe first seven-eighths of Earth history.Paleoproterozoic: 2.5 to 1.6 Ga.—During theearly Paleoproterozoic, cyanobacteria likelyappeared and produced free oxygen by way ofphotosynthesis. Thus, for the first time in Earthhistory, at about 2.2 Ga, the hydrosphere andatmosphere became significantly oxygenated.Acritarchs, the first protist fossils, occur in rocksas old as 1.8 Ga (Vidal and Moczydlowska-Vidal,1997). Most acritarchs were cysts of planktic algaealthough some may have been prokaryotes orbenthic protists (Butterfield, 1997). Clearly thepelagic ecosystem changed drastically from onemade up almost solely of prokaryotes to one thatincluded oxygenic-photosynthesizing nano- tomicrophytoplankton (Table 4). Although no fossilsare known, microzooplankton was likely presentas well, as unskeletonized protists that preyed onboth prokaryotes and phytoplankton (Christaki etal., 2001). Ciliates, flagellates, and sarcodines ortheir ancestors are possible candidates for this role.Benthic communities of prokaryotes werecommon in shallow waters and probably elsewhere75