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This study analyzes the molecular composition of dissolved organic matter (DOM) in Prydz Bay by Fourier Transform Ion Cyclotron Resonance mass spectrometry to probe the carbon sequestration capacity in the continental shelf system. Concentrations of particulate organic carbon (POC), particulate nitrogen and dissolved organic carbon (DOC) with water depth show that POC could be mainly decomposed into DOC and/or microbially degraded. Highly labile DOC is further degraded and remineralized by microorganisms within the upper 200 m, as evidenced by a downward enrichment of 13CPOC and increases in the average molecular weight, oxygen atom number (O) and double bond equivalents of DOM molecules, indicating that biodegradation is the main driver for particulate organic matter and DOM evolution with water depth. Semi‐quantitative calculation demonstrates that ∼83% of POC was transformed to DOC as well as dissolved inorganic carbon (DIC), and ∼30% of DOC further to DIC via microbial degradation within the upper 200 m in summer, resulting in a relatively low total organic carbon content in sediments of Prydz Bay. The newly transformed DIC and residue DOC can be preserved in the deep layer due to the formation of well stratified and stable water body in summer of Prydz Bay, ultimately entering the regional circulation system instead of being released back into the atmosphere. This could be one of the most important processes determining the atmosphere CO2 uptake in the continental shelf system of Southern Ocean. Plain Language Summary: The continental shelf system in the Southern Ocean, especially with the development of polynyas, plays a critical role in the global carbon cycle and disproportionally accounts for almost half of the anthropogenic CO2 uptake by the biological pump, with ensuing deep‐sea sequestration by the form of particulate and dissolved organic carbon, and even CO2 due to microbial degradation of primary productivity. However, complex interactions between physical, chemical and biological processes significantly affect the fate of organic carbon through the water column, resulting in an uncertainty of vertical carbon export. In this study, we conduct an investigation on the dissolved organic matter sources and evolution with water depth in summer of Prydz Bay, a typical polynya‐developed continental shelf system in the East Antarctica. The results clearly show that biodegradation is the main factor controlling organic carbon mineralization and mainly occurs within the upper 200 m of water column, with a transformation into dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Integrated with regional circulation system and hydrological characteristics, our study further indicates that newly formed DIC and DOC can be preserved in the deep layer of Prydz Bay, then into the regional circulation system rather back to the atmosphere. Key Points: Novel evidence at molecular level shows the vital role of biodegradation during carbon sink in Prydz BayThe conversion of POC into DOC/DIC mainly occurs within the upper 200 m of water columnHighlighting regional circulation's contribution to carbon transport and sequestration in the Southern Ocean [ABSTRACT FROM AUTHOR]

Methane-oxidizing bacteria (MOB) have long been recognized as an important bioindicator for oil and gas exploration. However, due to their physiological and ecological diversity, the distribution of MOB in different habitats varies widely, making it challenging to authentically reflect the abundance of active MOB in the soil above oil and gas reservoirs using conventional methods. Here, we selected the Puguang gas field of the Sichuan Basin in Southwest China as a model system to study the ecological characteristics of methanotrophs using culture-independent molecular techniques. Initially, by comparing the abundance of the pmoA genes determined by quantitative PCR (qPCR), no significant difference was found between gas well and non-gas well soils, indicating that the abundance of total MOB may not necessarily reflect the distribution of the underlying gas reservoirs. 13C-DNA stable isotope probing (DNA-SIP) in combination with high-throughput sequencing (HTS) furthermore revealed that type II methanotrophic Methylocystis was the absolutely predominant active MOB in the non-gas-field soils, whereas the niche vacated by Methylocystis was gradually filled with type I RPC-2 (rice paddy cluster-2) and Methylosarcina in the surface soils of gas reservoirs after geoscale acclimation to trace- and continuous-methane supply. The sum of the relative abundance of RPC-2 and Methylosarcina was then used as specific biotic index (BI) in the Puguang gas field. A microbial anomaly distribution map based on the BI values showed that the anomalous zones were highly consistent with geological and geophysical data, and known drilling results. Therefore, the active but not total methanotrophs successfully reflected the microseepage intensity of the underlying active hydrocarbon system, and can be used as an essential quantitative index to determine the existence and distribution of reservoirs. Our results suggest that molecular microbial techniques are powerful tools for oil and gas prospecting. [ABSTRACT FROM AUTHOR]

Studying the response and recovery of marine microbial communities during mass extinction events provides an evolutionary window through which to understand the adaptation and resilience of the marine ecosystem in the face of significant environmental disturbances. The goal of this study is to reconstruct changes in the marine microbial community structure through the Late Devonian Frasnian‐Famennian (F‐F) transition. We performed a multiproxy investigation on a drill core of the Upper Devonian New Albany Shale from the Illinois Basin (western Kentucky, USA). Aryl isoprenoids show green sulfur bacteria expansion and associated photic zone euxinia (PZE) enhancement during the F‐F interval. These changes can be attributed to augmented terrigenous influxes, as recorded collectively by the long‐chain/short‐chain normal alkane ratio, carbon preference index, C30 moretane/C30 hopane, and diahopane index. Hopane/sterane ratios reveal a more pronounced dominance of eukaryotic over prokaryotic production during the mass extinction interval. Sterane distributions indicate that the microalgal community was primarily composed of green algae clades, and their dominance became more pronounced during the F‐F interval and continued to rise in the subsequent periods. The 2α‐methylhopane index values do not show an evident shift during the mass extinction interval, whereas the 3β‐methylhopane index values record a greater abundance of methanotrophic bacteria during the extinction interval, suggesting enhanced methane cycling due to intensified oxygen depletion. Overall, the Illinois Basin during the F‐F extinction experienced heightened algal productivity due to intensified terrigenous influxes, exhibiting similarities to contemporary coastal oceans that are currently undergoing globalized cultural eutrophication. The observed microbial community shifts associated with the F‐F environmental disturbances were largely restricted to the extinction interval, which suggests a relatively stable, resilient marine microbial ecosystem during the Late Devonian. [ABSTRACT FROM AUTHOR]