Riverine microbial communities in an upper watershed: Integrating volcanic and anthropogenic influences

Abstract

ABSTRACT Riverine microbial communities are shaped by complex natural and anthropogenic factors. This study investigates these dynamics in the Upper Cimanuk River, Indonesia, a system originating from the active volcanoes Papandayan and Guntur and flowing through agricultural and settlement areas. We analyzed water quality, land use, and microbial community structure using 16S ribosome-ribonucleic acid (rRNA) gene sequencing across eight sites. Multivariate analyses revealed a clear spatial separation into the volcanic hazard zones ( kawasan rawan bencana , KRB) and a non-volcanic zone (non-KRB). The KRB exhibited extreme conditions, including low potential of Hydrogen (pH), high total dissolved solids (TDS), and elevated heavy metals, acting as a stringent environmental filter that selected for specialized extremophiles such as Hydrogenophaga and Acidiphilium , key taxa involved in sulfur oxidation and acid tolerance. In contrast, non-volcanic zones were dominated by heterotrophic bacteria (e.g., Bacteroidota, Actinobacteriota ) associated with terrestrial organic matter inputs. Crucially, the highest microbial diversity occurred in transitional zones, observed Shannon diversity indices ranged from 1.63 to 2.08, substantially higher than the 0.50–2.62 range recorded in the most extreme volcanic and homogeneous non‑volcanic reaches, respectively. At the same time, TDS in the high‑KRB segments often exceeded 40,000 mg/L, reinforcing the role of intense geogenic input as a key environmental filter along the river. These areas function as microbial ecotones integrating chemosynthetic and heterotrophic processes. Two key gradients structured communities: a geogenic gradient (TDS, phosphate) and an anthropogenic gradient (chemical and biological oxygen demand). Consequently, KRB zonation functions not only as a risk map but also as an effective proxy for ecological conditions. This enables targeted management to protect critical ecosystem services, from water provisioning to biogeochemical cycling.

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