Salinity, microbe and carbonate mineral relationships in brackish and hypersaline lake sediments: A case study from the tropical Pacific coral atoll of Kiritimati

Abstract Microbiological activity can exert a substantial influence on carbonate mineral precipitation, but linking specific microbiological processes to carbonate minerals in an environmental setting is complex, as both abiotic and biotic factors ultimately influence carbonate mineral precipitation. The coral atoll of Kiritimati, Republic of Kiribati (1.9°N, 157.4°W), in the central tropical Pacific Ocean, contains hundreds of shallow water brackish to hypersaline lakes that contain a range of carbonate and evaporite minerals. Previous studies of Kiritimati lakes have investigated the microbial communities of finely laminated microbial mats and associated microbialites found in several of the more hypersaline lakes on the island. However, the microbial communities of the more brackish lakes are unknown. These brackish lakes precipitate metres of fine‐grained carbonate muds, which are useful for palaeoenvironmental reconstruction. Here, the relationships between carbonate abundance, mineralogy, water chemistry, and bacterial and archaeal communities are investigated in a suite of brackish to hypersaline lakes (8.7‐190 ppt) on Kiritimati. Next generation 16S rRNA gene sequencing of bacteria and archaea indicate that brackish lake sediments contain distinct microbial communities. In relation to carbonate precipitation, the relative abundance of Cyanobacteria, Choloroflexi and Deltaproteobacteria is greater in the brackish lake sediments, suggesting photosynthesis and sulphate reduction associated with these taxa may strongly influence alkalinity and carbonate precipitation in brackish lakes. The presence of dolomite in certain hypersaline lakes also coincided with the presence of a methanogenic family, indicating that methogenesis may contribute to dolomite precipitation in these lakes.