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Dolomite nucleation and subsequent crystallization are kinetically-controlled processes. Modern dolomite-forming environments provide clues to the trigger factors that facilitate dolomite formation under Earth surface conditions. It has been documented that certain types of benthic microorganisms promoted the precipitation of protodolomite from sediment pore waters. As protodolomite is thought to be a possible precursor of sedimentary ordered dolomite, microbial mediation has thus been suggested as one interpretation of the occurrence of dolomite in modern sediments. To date, however, it is still unclear whether planktonic microorganisms could directly initiate protodolomite crystallization in the upper water column of present dolomite depositing environments. In this study, we report on the occurrence of authigenic protodolomite in the upmost sediments of a high-sulfate, Chinese inland saline lake (Lake Jibuhulangtu Nuur). This lake was therefore considered to be a natural laboratory to test the catalytic effect of planktonic aerobic heterotrophic bacteria on protodolomite formation. Laboratory mineralization experiments were conducted in a liquid medium that mimicked the ion concentrations and pH condition of lake surface water. The incubation experiments showed that aragonite formed in the abiotic systems, while protodolomite predominantly occurred in the bioreactors using either an enrichment culture or pure isolates of aerobic heterotrophic and halophilic bacteria from lake water. The resulting microbially-induced protodolomite crystals displayed spherical morphology and had MgCO 3 composition ranging from 42.7 mol% to 47.1 mol%. These protodolomite spherulites were formed by aggregation of randomly-distributed nano-crystals. Compared to synthetic abiotic protodolomite, microbially-induced protodolomite contained considerable amounts of organic matter, which might occur as intracrystalline inclusion or was located between nano-crystals of protodolomite spherulite. Our results support the emerging view that dissolved sulfate is not an inhibitor for the formation of low-temperature (proto-)dolomite. The presence of organic matter intimately associated with dolomite crystals may serve as a hallmark indicative of a biotically induced origin for some types of dolomite. [ABSTRACT FROM AUTHOR]

The distribution of archaeal ether lipids is thought to be governed by salinity in hypersaline and marine environments due to differences in their biological sources. However, little is known about their source and relationship with salinity in lacustrine environments. To explore this, we analyzed the distribution of specific archaeal lipids (isoprenoid GDGTs and diethers) of 11 ponds along a salinity gradient (0.8‰ to 279‰) in Inner Mongolia, NE China. Functional gene mcrA and compound-specific stable carbon isotope data were used to explore the biological source(s) of archaeal diether and tetraether lipids. Both lipid- and DNA-based methods showed methanogens to be abundant in ponds, which is consistent with the dominance of GDGT-0 relative to other isoprenoid GDGTs as well as the occurrence of archaeol (C 20 /C 20 DGD), hydroxyarchaeol and the mcrA gene. However, the occurrence of the extended archaeol (C 20 /C 25 DGD) also suggests the presence of Halobacteriales, non-halophilic methanogens and halophilic methanogens. This means that the biological sources of archaeal diethers and tetraethers are complicated. We subsequently evaluated the applicability of TEX 86 temperature and ACE salinity proxies in these settings; however, due to the mixed sources of archaeal diethers and tetraethers, these proxies are not applicable in these ponds and it appears that salinity is not the dominant control on archaeal ether lipid distributions in these saline ponds. [ABSTRACT FROM AUTHOR]