![]() ![]() Recently, however, 77 previously undocumented fossil seep sites, comprising more than 150,000 vents to the atmosphere, were mapped in Alaska ( 11). Thus far, mud volcanoes and microseepage in low-latitude countries, such as Italy, Greece, Romania, Azerbaijan, and Taiwan, have received the most research attention ( 7, 9, 10). The global seepage area and the number of seep sites on different continents remain uncertain. Major emissions are related to natural fossil methane formed in sedimentary basins (both microbial and thermogenic methane) and, subordinately, to geothermal areas ( 5). Macroseeps are localized flows and gas vents, both on land and on the seafloor, which emit up to 10 2 tons CH 4 year −1, while mud volcanoes represent the largest visible expression of geologic methane emission, producing flows of up to 10 3 tons CH 4 year −1 ( 6, 7, 8). Microseepage is the slow, diffuse loss of CH 4 over wide areas on the order of 10 0 to 10 2 mg m 2 day −1. The emission of geologic CH 4 occurs from seepage, including microseeps, terrestrial macroseeps, geothermal/volcanic emissions from the Earth's crust, and marine seeps. Recent estimates appraise the total methane emitted naturally from all geologic sources at more than 50 Tg year −1 and potentially approaching 80 Tg year −1 ( 5). The second-largest natural source is geologic methane, which is emitted from coal beds, natural gas deposits, and gas hydrates. Natural wetlands are the largest source of atmospheric methane, contributing 175 to 217 Tg CH 4 year −1 ( 1, 2). The global methane emission is currently estimated between 550 and 678 Tg year −1 ( 1, 2). Since 2007, CH 4 levels have been rising again ( 3), indicating a recent imbalance between CH 4 sources and sinks that is not yet fully understood ( 1, 4). A sustained increase in atmospheric CH 4 levels in the 1980s was followed by a slowdown in the increase in the 1990s and a general stabilization from 1999 to 2006. Since the beginning of the Industrial Era, the atmospheric CH 4 concentration has increased by a factor of 2.5 ( 2). Methane (CH 4) is an important greenhouse gas responsible for about 20% of the warming induced by long-lived greenhouse gases ( 1, 2). Apparently, microbial communities attenuating methane fluxes from these local but strong CH 4 sources in floodplains of high-latitude rivers have a large proportion of potentially novel, psychrotolerant methanotrophs, thereby providing a challenge for future isolation studies. Nearly all of these sequences affiliated with type I MB, including the Methylobacter-Methylovulum-Methylosoma group, lake cluster 2, and several as-yet-uncharacterized methanotroph clades. A total of 53,828 pmoA gene sequences of seep-inhabiting methanotrophs were retrieved and analyzed. The diversity of methanotrophs in this habitat was further assessed by pyrosequencing of pmoA genes, encoding particulate methane monooxygenase. Most (95.8 to 99.3%) methanotroph cells were type I (gammaproteobacterial) MB. Fluorescence in situ hybridization detected 10 7 methanotrophic bacteria (MB) per g of mud (dry weight), which accounted for up to 20.5% of total bacterial cell counts. Although the seeps were characterized by low in situ temperatures (3.5 to 5☌), relatively high rates of methane oxidation (15.5 to 15.9 nmol CH 4 ml −1 day −1) were measured in mud samples. The δ 13C value of methane released from these seeps varied between −71.1 and −71.3‰, suggesting its biogenic origin. CH 4 flux from most (90%) of these gas ebullition sites did not exceed 1.45 g CH 4 h −1, while some seeps emitted up to 5.54 g CH 4 h −1. A complex system of muddy fluid-discharging and methane (CH 4)-releasing seeps was discovered in a valley of the river Mukhrinskaya, one of the small rivers of the Irtysh Basin, West Siberia. ![]()
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