Boreskov

OP‐37THE DAWN OF THE AEROBIC BIOSPHERE: ECOLOGY OF NEOPROTEROZOICAND EARLY PALEOZOIC BIOTARozhnov S.V.Borissiak Paleontological Institute RAS, Moscow, RussiaThe history of the Proterozoic biota is characterized first of all by replacement ofanaerobic communities with aerobic ones in many habitats and by massive incorporation ofeukaryotes into prokaryote ecosystems. The evolution of the aerobic biosphere was spatiallyand temporally intermittent. The modern ocean contains 1% of all free oxygen on Earth,while the atmosphere contains the remaining 99%. During the early Proterozoic thedistribution of free oxygen was different. The oxygen content of the sea water varied widelybetween habitats, while small fresh water bodies could have been completely oxygenated.The oxygen content in the atmosphere increased in discrete steps. Each sharp increase wasassociated with an abrupt change in the homeostatic feedback loop between the rate ofoxygen release into the atmosphere and the rate of fixation of atmospheric oxygen. Duringthe Paleo‐ and Mesoproterozoic the oxygen content of the atmosphere fluctuated at thelevel not exceeding one percent. This resulted in a strong UV radiation at the surfaces ofboth sea and land, which allowed existence of the phototrophic zone only at depths greaterthan 1‐10 m, while the contact zone between the ocean waters and atmosphere, whereorganics was quickly mineralized by UV‐generated superoxide radicals, was oxygendepleted.This zone acted as a barrier considerably impeding diffusion of water oxygen intothe air. The Neoproterozoic biota was characterized by glaciations and sharp fluctuations inthe abundance of producers. During interglacials waters from melted ice transported largeamounts of nutrients from the land into the oceans, causing mass development ofproducers. The benthic fauna of the Vendian serves as an indicator of oxidizing conditions inthe corresponding habitats. Producers at that time period were represented by algalbacterialmats, multicellular algae, and bacterial‐algal plankton. During the day time thealgal‐bacterial mats saturated the bottom water with oxygen, enabling the existence ofmulticellular eukaryotic benthic fauna. The distribution of benthic communities ofmulticellular eukaryotes during the Vendian was determined by the mosaic distribution ofoxygen in water and by its diurnal fluctuations, resulting from the low oxygen content of theatmosphere. These conditions determined the biology, physiology, and morphology ofconsumers. Multicellular consumers at that time had low metabolic rate and their activitywas low, which was an adaptation to the extremely low oxygen levels during the night. Thisis reflected in the morphology of Vendian multicellular organisms: bodies with large surfacearea, often flattened, without appendages suitable for vigorous locomotion, feeding via94