Links Between Geological Processes, Microbial Activities&Evolution of Life

Abstract

Both the anaerobic and the aerobic oxidation of methane are fundamental microbial processes with far reaching influences on global element cycles and, consequently, on the physico-chemical nature of the hydro- and atmosphere. These processes lead to substantial removal of the radiatively active gas methane and are powerful factors in controlling the composition of ecosystems and the distribution of authigenic deposits at methane-rich sites. For instance, the sulfate-dependent anaerobic oxidation of methane (AOM) mediated by methanotrophic Archaea and sulfate-reducing Bacteria yields hydrogen sulfide and bicarbonate ions, which subsequently react with ions derived from pore waters and the water column to form sulfidic and carbonaceous minerals. The aerobic oxidation of methane, performed by obligate aerobic Bacteria particularly effective at oxic-anoxic boundaries, leads to the generation of carbon dioxide, which is a less radiative gas than methane but has higher residence times in the atmosphere. The geochemical characteristics of the contemporary Black Sea promote processes associated with the conversion of methane and are considered to resemble those of the Paleo/Mesoproterozoic global ocean. Geochemical and molecular microbiological considerations support the idea that the microbes involved in the AOM emerged before the Archaean - Proterozoic transition, and became more important in the Paleoproterozoic, when oceanic deep waters are thought to contain high amounts of methane and adequate amounts of sulfate. With the rise of atmospheric oxygen in the Paleo/Mesoproterozoic, substantial parts of the global ocean became oxygenated, which promoted the spreading of aerobic methanotrophs. Since then, biogeochemical remnants, like fossil seep deposits or lipid biomarkers strongly depleted in C-13, demonstrate the relevance of methanotrophy in Earth history. Methane in the contemporary Black Sea is mainly sourced from sedimentary gas reservoirs at emission sites like cold seeps and mud volcanoes. Detailed seismic and hydroacoustic investigations at deep-water cold seep areas on the Ukrainian shelf (northwestern Black Sea), the continental slope off Georgia (southeastern Black Sea) and at mud volcanoes located in the Sorokin Trough (northern Black Sea) provided insights into their subsurface structures, fluid migration pathways, and extents of gas plumes in the water column. This review considers recent studies on sources and migration pathways of methane, and its fate in the sediments in the water column of the Black Sea with special emphasis on the anaerobic and aerobic microbial methane consumption.