Prokaryotic community composition in Makran cold seep sediments and the response to environment

Abstract:

[Objective] Cold seeps are mainly found along continental margins, which are characterized by the seepage of cold fluids rich in hydrocarbons dominated by methane and hydrogen sulfide. With unique geochemical environment, cold seep boasts unique prokaryotic community structure. However, the relationship between prokaryotic composition and cold seep environment is not clear. [Methods] In this paper, the key geochemical parameters, such as CH₄ in sediments and SO₄^2- and H₂S concentration in pore water, were measured along the depth profile based on the sediment samples from the active cold seep area of Makran continental margin. Through high-throughput sequencing of 16S rRNA gene, the community structure and spatial variation of prokaryotic microorganisms in cold seep sediments were systematically analyzed. [Results] Based on the methane- sulfate concentration, the sediments in the active seepage area of the Makran continental margin were divided into sulfate- reduction zone (SZ), sulfate- methane transition zone (SMTZ), and methanogenesis zone (MZ). It was found that the diversity and abundance of prokaryotic microorganisms decreased with the increase of depth through α diversity analysis and genetic quantification. According to the result of 16S rRNA gene sequencing, Gammaproteobacteria, Alphaproteobacteria, and Epsilonbacteraeota were the dominant prokaryotes in SZ, and the relative abundance of JS1, Chroloflexi, Lokiarchaeia, Bathyarchaeia, and Thermoprofundales which were related to organic matter degradation and used sulfate as electron donor was also high. ANME-1a, ANME-1b, and SEEP-SRB1 accounted for a large proportion in SMTZ, indicating that they might jointly mediate the anaerobic oxidation of methane coupled to sulphate reduction (SR-AOM). In addition, the prevalence of Bathyarchaeia and Chroloflexi indicated the potential of other hydrocarbons besides methane in the sediments. [Conclusion] Through the exploration of the prokaryotic communities in Makran cold seep, it is found that the bacterial and archaeal community compositions are closely related to the unique geochemical environment of the cold seep.