Salinity-triggered compositional and metabolic responses of autotrophic prokaryotes in Tibetan lacustrine sediments

Abstract

Background Autotrophic prokaryotes are crucial participants in the global carbon cycle, and autotrophic carbon fixation contributes approximately 50% of the global net primary production in aquatic ecosystems per year. Salinity is a prominent regulator structuring microbial communities in diverse aquatic ecosystems. However, little information is available regarding the compositional and physiological response of autotrophic microbial communities to salinity change. Here, we used genome-resolved metagenomics to study autotrophic microbial communities in 25 Tibetan lacustrine sediments with a salinity gradient (from 0.54‰ to 82.6‰). Results117 metagenome-assembled genomes (MAGs) with carbon fixation potential belonging to 12 phyla were retrieved, of which approximately 21% were not affiliated with the known orders, suggesting taxonomically diverse autotrophic assemblages in sediments. The total abundance of these putative autotrophs decreased significantly with increasing salinity, and the variation of sediment autotrophic communities was mainly driven by salinity, pH and TOC. Notably, a change in the predominant lineage from Betaproteobacteria to Deltaproteobacteria was observed along the salinity gradient, and the dominant pathway for carbon fixation shifted from the Calvin-Benson-Bassham (CBB) cycle to more energy efficient Wood-Lungdahl (WL) pathway with glycolysis from Entner-Doudoroff to more exergonic Embden-Meyerhof-Parnas, demonstrating that the physiological efficiency increases from freshwater to hypersaline autotrophic communities. Metabolic inference revealed major links for carbon fixation to the oxidation of reduced sulfur compounds, ferrous iron and carbon monoxide, denitrification and nitrogen fixation in these MAGs, as well as the occurrence of dissimilatory sulfate reduction and the WL pathway dominating hypersaline sediments, greatly extending the understanding of metabolic versatility and diverse ecological niches of autotrophic microorganisms. Conclusions This study provided a systematic attempt to characterize the response of carbon fixation pathways to salinity and the knowledge essential for revealing ecological roles of autotrophic prokaryotes in aquatic habitats. These findings suggest with increased salinity, physiological efficiency of the autotrophic community increases, which has important implications for understanding the carbon budget in aquatic ecosystems.

Closed genomes uncover a saltwater species ofCandidatusElectronema and shed new light on the boundary between marine and freshwater cable bacteria

“…Electronema halotolerans, and has been previously suggested as a potential discriminant between marine and freshwater cable bacteria 12 (Figure 2-3, Dataset S1). The medium quality MAGs KV 43 and SY1 44 , which are closely related to Ca. Electronema halotolerans, encoded also the NhaA gene (Figure 2), confirming its importance for survival of bacteria in saltwater environment 45 .…”