[Differential Responses of Indigenous Microorganisms to Salinity Changes in Lake Sediments： A Case Study of the Baiyangdian Lake and Qinghai Lake].

Abstract

The impact of salinity stress on methane-oxidizing microorganisms in lake sediments has been recognized as influential in altering both the community structure and function, thereby affecting methane emissions in lake water. Denitrifying anaerobic methane oxidation （DAMO） is a crucial process within the lake sediment, involving a consortium of known DAMO characteristic microorganisms and associated carbon and nitrogen-transforming functional bacteria. However, the variations in DAMO flora in lake sediments under diverse salinity conditions and their implications on greenhouse gas production remain inadequately explored. To address this gap, a comparative study was conducted on the indigenous flora in sediments from two lakes with contrasting salinities, namely the saline Qinghai Lake and the freshwater Baiyangdian Lake. The α-diversity analysis revealed higher Shannon and Observed-Richness indices in the Baiyangdian Lake sediment compared to the Qinghai Lake sediment, while the Inv-Simpson index showed the opposite trend. While the dominant microbial populations were similar at the phylum level, significant differences were observed in the abundance and proportion of common microbial populations. Notably, the salinity of lake water exhibited a negative correlation with microbial diversity and methane-oxidizing microbial diversity in sediments. Further investigation on the indigenous DAMO microflora in the lake sediments under different salinity conditions revealed distinct patterns. Specifically, anaerobic microcultures were carried out at different salinity gradients （0, 11, and 110 g·L^-1）, with CH₄ and KNO₃ as substrates. The rates of nitrite formation and nitrate consumption were lower in the Qinghai Lake sediment compared to those in the Baiyangdian sediment with respect to individual salinity. CO₂ production in the Baiyangdian sediment decreased with increasing salinity. Additionally, for the Baiyangdian sediment, methane oxidation and nitrous oxide production were lower in the high salinity group than those in the low salinity group. In the Qinghai Lake sediment, the most active methane oxidation was observed at the original sample's salinity （11 g·L^-1）, with the highest methane oxidation reaching 481.67 μmol·L^-1 and the lowest nitrous oxide production around 1.69 μmol·L^-1. The methane oxidation capacity of the Qinghai Lake sediment cultured at its original salinity was better than that of other salinity groups, and the yield of nitrous oxide was also lower. These results indicated that DAMO flora in high salinity lakes could adapt to the salinity of their habitats after long-term screening and domestication. Therefore, the significant difference in indigenous microorganisms in the sediments of saltwater and freshwater lakes lead to the differences in their response to changes in the salinity environment and further affect the DAMO process in lakes. Quantitative analysis of DAMO characteristic bacterial genes revealed varying abundances of ANME-2d Methanoperedens nitroreducens and NC10 Methylomirabilis oxyfera in the sediments of the Qinghai Lake and Baiyangdian Lake, influenced by salinity treatments. The DAMO bacteria activity and process were inhibited with increasing salinity in the Baiyangdian Lake sediment, whereas in the Qinghai Lake sediment, the highest DAMO microbe abundance and rate were observed at the salinity of the original sample （11 g·L^-1）. Manipulating the salinity of the original sample resulted in the inhibition of the DAMO process in the lake sediment, leading to lower DAMO microbe abundance, reduced methane oxidation, and increased nitrous oxide production. These findings underscore the marked impact of lake salinity on indigenous microorganisms in lake sediments, with implications for their responses to changes in the salinity environment and subsequent effects on the DAMO process in lakes. This provides insights for predicting lake greenhouse gas emissions in the context of climate warming.