Characteristics of carbon-fixing microbial communities and pathways across different aquatic systems in the Tianjin...

Microbial carbon fixation is central to carbon cycling and carbon sink functioning in coastal aquatic ecosystems. Although carbon fixation pathways have been increasingly investigated across diverse aquatic environments, comparative evidence remains limited for hydrologically connected yet hydrochemically contrasting coastal groundwater and surface water systems. This study aimed to compare carbon-fixation-associated microbial communities and major carbon fixation pathways across groundwater, river water, and reservoir water in the Tianjin coastal region.

We integrated metagenomic sequencing with hydrochemical analyses to characterize carbon-fixation-associated microbial communities and six representative carbon fixation pathways. Surface waters were dominated by bacteria and showed relatively stable community composition, whereas groundwater communities comprised both bacteria and archaea and displayed pronounced spatial heterogeneity. The Calvin-Benson-Bassham cycle was prevalent across all water types, and the reductive tricarboxylic acid (rTCA) cycle was also widely distributed. Groundwater showed higher contributions of the Wood-Ljungdahl pathway, the archaeal 3-hydroxypropionate/4-hydroxybutyrate and dicarboxylate/4-hydroxybutyrate cycles, together with the rTCA cycle, indicating coexisting carbon fixation strategies. Pathway abundance and module completeness further suggested differences in pathway integrity among water types. Total dissolved solids, HCO3⁻, CO32⁻, and dissolved organic carbon were key correlates of carbon fixation gene distribution.

Carbon-fixation-associated microbial communities, pathway distributions, and pathway integrity differed markedly between coastal groundwater and surface waters. Groundwater exhibited enhanced non-CBB cycle potentials and more diversified carbon fixation strategies, highlighting the importance of groundwater processes in evaluating carbon sequestration potential and carbon cycling in hydrochemically heterogeneous coastal aquatic systems.