TY - JOUR T1 - Deterministic assembly processes govern seasonal and spatial variation in microbiomes across hydrologically-connected hyporheic zones JF - bioRxiv DO - 10.1101/088518 SP - 088518 AU - Emily B. Graham AU - Alex R. Crump AU - Charles T. Resch AU - Sarah Fansler AU - Evan Arntzen AU - David W. Kennedy AU - Jim K. Fredrickson AU - James C. Stegen Y1 - 2016/01/01 UR - http://biorxiv.org/content/early/2016/11/18/088518.abstract N2 - Subsurface zones of groundwater and surface water mixing (hyporheic zones) are regions of enhanced rates of biogeochemical cycling, but ecological processes influencing hyporheic microbiomes through space and time remain unknown. We sampled attached and planktonic microbiomes in the Columbia River hyporheic zone across seasonal hydrologic change within three hydrologically-connected, yet physicochemically-distinct geographic zones (inland, nearshore, river). Although microbiomes remained dissimilar through time across all zones and habitat types (attached vs. planktonic), consistent presence of certain heterotrophic taxa suggested dispersal and/or common selective pressures among all zones. We used statistical null models and co-occurrence network analysis, respectively, to demonstrate a pronounced impact of deterministic assembly on microbiomes in all data subsets and to elucidate taxa most affected by these processes. The composition of one network cluster of nearshore organisms exhibited a seasonal shift from heterotrophic to autotrophic microorganisms, and the abundance of taxa within this cluster also correlated positively with active microbial biomass and metabolism, possibly indicating that these taxa have strong influences over biogeochemical reactions within the hyporheic zone. Taken together, our research demonstrates a predominant role for deterministic assembly across highly-connected environments and provides insight into niche dynamics associated with seasonal changes in hyporheic microbiome composition and metabolism.Originality-Significance Statement Subsurface zones of groundwater and surface water mixing (hyporheic zones) are hotspots of biogeochemical activity and strongly influence carbon, nutrient and contaminant dynamics within riverine ecosystems. Hyporheic zone microbiomes are responsible for up to 95% of riverine ecosystem respiration, yet the ecology of these microbiomes remains poorly understood. While significant progress is being made in the development of microbially-explicit ecosystem models, poor understanding of hyporheic zone microbial ecology impedes development of such models in this critical zone. To fill the knowledge gap, we present a comprehensive analysis of hyporheic zone microbiomes through space and time. We quantify ecological drivers of microbiome change and identify taxa that may be particularly important to hyporheic zone biogeochemical function. Despite pronounced hydrologic connectivity throughout the hyporheic zone, we find that ecological selection deterministically governs microbiome composition within local environments and that comparatively high-organic C conditions during surface water intrusion into the hyporheic zone may support heterotrophic metabolisms, succumbing to autotrophy during time periods of groundwater discharge. These results provide new opportunities to develop microbially-explicit ecosystem models that incorporate the hyporheic zone and its influence over riverine ecosystem function. ER -