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Keywords: Riverine bacteria; Planktonic bacteria; Sedimentary bacteria; Hydrodynamic; Dam; Multiscale morphology; Distance-decay relationship

Abstract: Riverine bacteria, including planktonic and sedimentary microorganisms, play important roles in geochemical cycling, climate change, and water environments. Nonetheless, over 95% of bacteria cannot be cultivated in laboratory settings. Our understanding and research on riverine bacteria are still in the early stages. The research on riverine bacteria represents an intersection of microbial ecology, river dynamics, and riverbed evolution. A comprehensive review on how hydrodynamics, river morphology changes, and dam construction impact riverine bacteria is urgently needed from the perspective of water-sediment movement and riverbed evolution. This review could guide future directions of this interdisciplinary research. Riverine bacteria can be categorized into planktonic bacteria, suspended in the water or attached to water particles, and sedimentary bacteria, adhering to the surface sediment or gravel surface in the riverbed. The review results indicate that, in terms of hydrodynamics, changes in flow velocity and discharge alter the chemical and nutrient concentrations in the water; external bacteria, relative to instream bacteria, are introduced into the main stream during processes such as channelization of flood land flow into the main channel and tributary confluence, thereby influencing the riverine microbial community. Dam construction not only alters water temperature and hydrodynamic conditions in the upstream and downstream channels but also intercepts sediment, reducing sediment influx and intensifying bed scouring and armoring in the downstream. It is suggested that microbial species abundance, especially sedimentary microorganisms downstream of dams, is significantly lower than the upstream, although controversy exists regarding this result. The multiscale morphology of rivers, including large spatial scales like mountains, hills, basins, plains, and river geomorphic units like rapids, deep pools, and shallow shoals, influences the composition and function of riverine microorganisms. In addition, at smaller microhabitat scales, the type and heterogeneity of riverbed substrates affect sedimentary microbial communities. Furthermore, the similarity in the composition of riverine microorganisms decreases with spatial distance, and planktonic and sedimentary microorganisms have different influencing factors and spatiotemporal characteristics, contributing to the complex spatiotemporal distribution patterns of riverine microorganisms. In summary, current research on riverine bacteria is in the early exploratory stage, and future studies should focus on the mechanistic exploration of diversity, gene functions, and community assembly mechanisms of planktonic and sedimentary microorganisms in connection with water-sediment movement. A more in-depth investigation into the multiscale impacts of river morphology and various types of hydraulic projects on riverine microbial communities is warranted.

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Year: 2024