Putting the BIO in hydroBIOgeochemistry: current knowledge and future directions of river microbiome science

River corridors are a key element of the Earth system, acting as an important nexus of carbon and nutrient transport across terrestrial and aquatic interfaces, as well as hotspots for biogeochemical processes. From local to global scales, the emergent functioning of river corridors arises through the integration of hydrology and biogeochemistry. Associated microbiomes are primary catalysts connecting physical processes with material transformations in these systems. However, while the hydrology and biogeochemistry of river corridors have long been studied, community function at the genome level remains largely unexplored. In this review and synthesis of current river corridor microbiome data with an emphasis on DNA-based techniques, we cover recent advances and future directions linking microbial function to ecosystem processes. Surveying over 200 published investigations of river microbiomes, we identify current gaps in river corridor sampling including underrepresented geographic locations and hydrologic features. Focusing on studies utilizing DNA-based approaches, we parse gene-specific studies (e.g. 16S rRNA gene) from multi-omics studies (e.g. metagenomics, metatranscriptomics), underlining the need for genome-resolved functional understanding. Genome-resolved datasets from river microbiome studies are rare and often pursued in a narrow range of environmental conditions and geographic locations, with genome-resolved data coupled to expression data confined to a handful of studies. Data of this nature could complement previous studies by providing a window into functional aspects of microbial food webs that may be indicative of upstream hydrology and biogeochemistry. Review of current studies also revealed new frontiers of river corridor science that aim to incorporate microbial DNA-based data into hydro-biogeochemical models and leverage community science to increase our sampling and understanding across diverse river corridors.