Biogeographic distribution, ecotype partitioning and controlling factors of Chloroflexi in the sediments of six hadal trenches of the Pacific Ocean.

The hadal trenches are "hot spots" for mineralization of organic matter in the deep ocean. Chloroflexi are one of the most dominant and active taxa in trench sediments, serving as important drivers of carbon cycles in hadal trenches. However, current understanding on hadal Chloroflexi is largely restricted to individual trench. This study systematically analyzed the diversity, biogeographic distribution, ecotype partitioning as well as environmental drivers of Chloroflexi in the sediments of hadal trenches, by reanalyzing 16S rRNA gene libraries of 372 samples from 6 trenches around the Pacific Ocean. The results showed that Chloroflexi averagely account for 10.10 % and up to 59.95 % of total microbial communities in the trench sediments. Positive correlations between relative abundance of Chloroflexi and depths down the vertical sediment profiles were observed in all of the sediment cores analyzed, suggesting the increasing significance of Chloroflexi in deeper sediment layers. Overall, trench sediment Chloroflexi were mainly composed of the classes Dehalococcidia, Anaerolineae and JG30-KF-CM66, and four orders i.e. SAR202, Anaerolineales, norank JG30-KF-CM66 and S085, were identified as core taxa that were dominant and prevalent in the hadal trench sediments. A total of 22 subclusters were identified within these core orders, and distinct patterns of ecotype partitioning related with depths down the vertical sediment profiles were observed, suggesting the great diversification of metabolic potentials and environment preference of different Chloroflexi lineages. The spatial distribution of hadal Chloroflexi were found to be significantly related with multiple environmental factors, while depths down the vertical sediment profiles explained the highest proportion of variations. These results provide valuable information for further exploring the roles of Chloroflexi in biogeochemical cycle of the hadal zone, and lay the foundation for understanding the adaptive mechanisms and evolutionary characteristics of microorganisms in hadal trenches.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.