OceanographyVol. 20, No. 2 28 Answering Charles Dar win’ s prescient question has taken us nearly two cen- turies. Only in recent decades have methods and concepts been developed to explore the signicance omicrobes in the ocean’s web olie. Bacteria in aquatic ecosystems were rst recognized or their role in the decomposition oorganic material and the remineraliza- tion oinorganic nutrients, a role that only became ully accepted in the 1980s. Their importance as photosynthetic pro- ducers oorganic matter became evident when so-called blue-green algae were acknowledged as being bacteria, and the microscopic cyanobacterium othe genus Synechococcus was discovered to be abundant in the oceans—particularlyin the vast oligotrophic blue water where they are the dominant autotrophs. W e now kn ow that every liter o“clear blue water” is teeming with a billion microbes—bacteria, viruses, and pro- tists—ar exceeding all multi-cellular metazoa in abundance, biomass, meta- bolic activity, and genetic and biochemi- cal diversity. Their struggle or surv ival is maniest in a web omicrobial lie, the microbial loop, which is unction- ally intertwined with the more amiliar ood web oplants, herbivores, and car- nivores. It channels energy and carbon via bacteria to protozoa (Darwin’s inu- soria), to larger zooplankton such as copepods and krill, and on to shes and cetaceans. Indeed, when we eat mahi- mahi, we are the top predator in a ood web that has some oits beginnings in the microbial loop. About one-halothe oxygen in everybreath we take derives rom photosyn- thetic bacteria within the marine micro- bial loop. Bacteria manage to populate all parts othe ocean by capturing nutri- ents and energy rom diverse sources. Photosynthetic bacteria carry out much othe primary production oorganic matter in the central ocean basins. Heterotrophi c bacteria capture dis- solved organic molecules rom seawater as well as organic particles that they“digest” with enzymes. Some bacteria and archaea oxidize inorganic chemicals or energy, and the carbon they x into organic matter serves as basis or ood webs in diverse e cosystems, including some in seemingly uninhabitable envi- ronments. The “smoke” coming rom hot vents on the seafoor may even con- tain some bacteria and archaea (Box 1). Earth’s ocean is most certainly a sea omicrobes; without them it would be a very dierent place, less hospitable to all lie. Indeed, without the activity othese organisms, the cycles oNature would very quickly come to a halt. This is not the case or higher organisms: w hereas the near extinction othe great whales byshing undoubtedly modied the ecol- ogy othe Antarctic, it certainly did not bring it to a stop. MICROBIAL DOMINANCE Earth’s ocean is estimated to contain 10 29 bacteria (Whitman et al., 1998), a number larger than the estimated 10 21 stars in the universe. Their great numerical abundance makes up or their size, typically 0.2–0.6 µm in diameter. The total mass obacteria in the ocean LAwRENCE R. POMEROy (lpomeroy@ uga.edu) is Alumni Foundation Proessor Emeritu s, Institute oEcology, University oGeorgia, Athens , GA, USA . PEtER J. leB. wILLIAMs is Proessor Emeritus, School oO cean Sciences, University oWales, Bangor, UK. FAROOq AzAMis Distinguished Proessor, Scripps Institution oOceanography , La Jolla, C A, USA.JOhN E. hOBBIE is Senior Scholar, Te Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA. Te Microbial Loop By LAwRENCE R. POMEROy, PEtER J. leB. wILLIAMs, FAROOq AzAM, AND JOh N E. hOBBIE > sECtION II. IDEAs, CONCEPts, AND PARADIgMs > ChAPtER 2. thE MICROBIAL LOOP I presume that the numerous lower pelagic animals persist on the infusoria, which are known to abound in the open ocean: but on what, in the clear blue water, do these infusoria subsist?– Charles Darwin (1845) A sEA OF MICROBEs T iarticlehabeen p u blihedin Ocea n o gra p hy, Vo lu m e20, N u m ber2, aq u arterlyjo u ralo fT eO cean o grap hyso ciety. Co p yright2007byT eO cean o grap hyso ciety. A llrightreerved. Perm iio n igran tedto co p ythiarticlefo ru ein teachin gan dreearch. Rep u blicatio n , ytem m aticrep ro du ctio n , o rco llectivereditirbu tio n o fan yp o rtio n o fthiarticlebyp ho to co p ym achin e, rep o tin g, o ro therm ean ip erm ittedo n lyw iththeap p ro valo fT eO cean o grap hyso ciety. sen dallco rrep o n den ceto :in fo @ to . o rgo rT eO cean o grap hyso ciety, PO Bo x1931, Ro ckville, M D 20849- 1931, U sA .
