Colonization Habitat Controls Biomass, Composition, and Metabolic Activity of Attached Microbial Communities in the Columbia River Hyporheic Corridor

Abstract

Hydrologic exchange plays a critical role in biogeochemical cycling within the hyporheic zone (the interface between river water and groundwater) of riverine ecosystems. Such exchange may set limits on the rates of microbial metabolism and impose deterministic selection on microbial communities that adapt to dynamically changing dissolved organic carbon (DOC) sources. This study examined the response of attached microbial communities (in situ colonized sand packs) from groundwater, hyporheic, and riverbed habitats within the Columbia River hyporheic corridor to "cross-feeding" with either groundwater, river water, or DOC-free artificial fluids. Our working hypothesis was that deterministic selection during in situ colonization would dictate the response to cross-feeding, with communities displaying maximal biomass and respiration when supplied with their native fluid source. In contrast to expectations, the major observation was that the riverbed colonized sand had much higher biomass and respiratory activity, as well as a distinct community structure, compared with those of the hyporheic and groundwater colonized sands. 16S rRNA gene amplicon sequencing revealed a much higher proportion of certain heterotrophic taxa as well as significant numbers of eukaryotic algal chloroplasts in the riverbed colonized sand. Significant quantities of DOC were released from riverbed sediment and colonized sand, and separate experiments showed that the released DOC stimulated respiration in the groundwater and piezometer colonized sand. These results suggest that the accumulation and degradation of labile particulate organic carbon (POC) within the riverbed are likely to release DOC, which may enter the hyporheic corridor during hydrologic exchange, thereby stimulating microbial activity and imposing deterministic selective pressure on the microbial community composition.IMPORTANCE The influence of river water-groundwater mixing on hyporheic zone microbial community structure and function is an important but poorly understood component of riverine biogeochemistry. This study employed an experimental approach to gain insight into how such mixing might be expected to influence the biomass, respiration, and composition of hyporheic zone microbial communities. Colonized sands from three different habitats (groundwater, river water, and hyporheic) were "cross-fed" with either groundwater, river water, or DOC-free artificial fluids. We expected that the colonization history would dictate the response to cross-feeding, with communities displaying maximal biomass and respiration when supplied with their native fluid source. By contrast, the major observation was that the riverbed communities had much higher biomass and respiration, as well as a distinct community structure compared with those of the hyporheic and groundwater colonized sands. These results highlight the importance of riverbed microbial metabolism in organic carbon processing in hyporheic corridors.

Keywords: 16S rRNA gene; biogeochemistry; biomass; composition; deterministic selection; dissolved organic carbon; hyporheic corridor; microbial communities; respiration; riverine.

FIG 1

ATP contents and rates of resazurin transformation to resorufin (Rru) for the groundwater (A, D), riverbed (B, E), and piezometer (C, F) sand colonized reactors for cross-feed experiment 1. Each data point shows the mean ± SD or range from triplicate or duplicate reactors.

FIG 2

ATP contents (A to C) and rates of resazurin transformation to resorufin (Rru) (D to F) for the desorption/cross-feed experiment.

FIG 3

DOC concentrations in the fluid phase of groundwater (A, D), riverbed (B, E), and piezometer (C, F) sand reactors from the two replicate cross-feed experiments. The green and red sections show the release or consumption of DOC based on the difference between the reactor DOC and the expected DOC content of the feed stock solution (blue sections). Values are the means ± SDs from 3 time points from triplicate reactors for experiment 1, and the means ± SDs from 6 time points from triplicate reactors for experiment 2.

FIG 4

DOC concentrations in the fluid phase of riverbed (A), groundwater (B), and piezometer (C) sand reactors for the desorption/cross-feed experiment. The green and red sections show the release or consumption of DOC based on the difference between the reactor DOC and the DOC content of the feed stock solution (indicated on x axis). Values show means ± SDs from 3 time points from triplicate reactors.

FIG 5

NMDS analysis of microbial community composition for the cross-feeding and desorption/cross-feed experiments. R and P values in each panel are results of ANOSIM of 16S rRNA gene amplicon dissimilarity. Results for reactors with different fluid sources are shown by the same symbol color because ANOSIM showed that fluid type did not significantly influence community composition (see Table 2). Different time points (days) are shown with the following symbols. For cross-feed 1: ○, 0; ♢, 14; △, 28; ▽, 35. For cross-feed 2: ○, 0; ♢, 7; △, 21; ▽, 35; □, 41. For desorption/cross-feed: ○, 0; ♢, 5; △, 19; ▽, 26.

FIG 6

Phylum-level microbial community compositions in the colonized sand reactors. Each bar shows the average result from 2 to 4 reactors that received different fluid sources during the incubation experiments. Results for reactors with different fluid sources were combined for each time point during each experiment because ANOSIM showed that fluid type did not significantly influence community composition (see Table 2). Chloroplast refers to the chloroplast from eukaryotic algae; non-PS Cyanobacteria refers to nonphotosynthetic cyanobacteria from the class ML635J-21 in the Greengenes and Silva 16S rRNA gene databases.

FIG 7

Proteobacterial class composition in the colonized sand reactors. Each bar shows the average result from 2 to 4 reactors that received different fluid sources during the incubation experiments. Results for reactors with different fluid sources were combined for each time point during each experiment because ANOSIM showed that fluid type did not significantly influence community composition (see Table 2).