Catch and release: Hyporheic retention and mineralization of N‐fixing Nostoc sustains downstream microbial mat biomass in two polar desert streams

Abstract

Much work has been performed to investigate controls on nitrogen (N) uptake in streams, yet the fate of assimilated N is comparatively poorly resolved. Here, we use in-stream fixed N as an isotopic tracer to study the fate of assimilated N in glacial meltwater streams. We characterized d 15 N signatures of Oscillatorean, Chlorophyte, and N-fixing Nostoc mats over the lengths of two streams, and transported particulate organic matter (POM) in one. POM was isotopically most similar to Nostoc, which always had values near the atmospheric standard (d 15 N ffi 0&), suggesting N-fixation. Other mat types were depleted upstream, and became progressively enriched downstream (plateauing at d 15 N ffi 0&), indicating a shift in N source. These results collectively show that Nostoc-derived N is mobilized, mineralized, and increasingly assimilated downstream as more depleted glacier-derived N is exhausted, demonstrating the importance of organic matter processing to balancing elemental budgets, and improving our understanding of nutrient cycling in lotic environments. Additional Supporting Information may be found in the online version of this article.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementNitrogen (N) pollution is a widespread environmental problem, and considerable effort over the last few decades has been invested in understanding how stream systems store, transform, and export N. However, most of this work has focused on microbial uptake and incorporation of N, rather than what happens to the N downstream. In this study, we traced N originating from N-fixing cyanobacteria through Antarctic glacial meltwater streams to demonstrate that N incorporated into microbial mats is repeatedly recycled and exported downstream, maintaining mat communities as other sources of N are exhausted. Our results demonstrate the potential importance of N-fixation and organic matter cycling in supporting primary production in N-limited streams.

Stable isotopic composition of top consumers in Arctic cryoconite holes: revealing different position in supraglacial trophic network

“…during collecting. The differences in δ 15 N between both consumers could suggest δ 15 N enrichment in food for rotifers caused by consumption of bacteria or DOM enriched in δ 15 N. This assumption is supported by Nagarkar et al (2004) and Kohler et al (2018), who described that cyanobacteria have higher content of proteins and a higher δ 15 N values typical for nitrogen fixing organisms. The other potential 265 explanation of the observed pattern is tardigrades' consumption of algae, which can vary in δ 15 N depending on their C:N ratio (Adams and Sterner, 2000).…”

Evaluating Alternative Metacommunity Hypotheses for Diatoms in the McMurdo Dry Valleys Using Simulations and Remote Sensing Data

“…These predictions could be tested by comparing field observations of diatom assemblages in the different mat types with observations from potential sources of immigrants. Two likely vectors for diatom dispersal in this system are aeolian transport (Šabacká et al, 2012;Diaz et al, 2018) and drift in the water column in streams (Cullis et al, 2014;Kohler et al, 2018). If NM dynamics govern the structure of the diatom metacommunity, and wind/saltation is the primary driver of diatom dispersal, then these modeling outcomes suggest the composition of the pool of dispersing diatoms in both aeolian and drift samples will more closely represent the extant community in nearby, downwind or downstream orange mats than black mats.…”

“…Higher concentrations of major ions and silica occur in the longer streams driven by the greater extent of weathering reactions occurring in the hyporheic zone as the water flows from the glacier to the lake (Gooseff et al, 2002). The primary sources of nutrients for the cyanobacterial mats are nitrate deposited on the glacier surfaces associated with auroral activity, weathering of apatite releasing phosphorus in hyporheic sediments, and recycling of nutrients from mat biomass that has become entrained in hyporheic sediments (McKnight et al, 2004;Kohler et al, 2018). In streams with abundant mats, nutrient concentrations are low (Table 1) compared with streams where geomorphic conditions, such as unstable sandy substrates, do not support mat growth (McKnight et al, 2004).…”

“…Many MDV diatom genera are considered to be aerophilic (e.g., Luticola, Hantzschia, Humidophila, and Muelleria), and thus adapted to survive the challenges associated with both aeolian transport (Diaz et al, 2018) and the desiccation-rehydration and freeze-thaw cycles associated with life in MDV cyanobacterial mat habitats (McKnight et al, 1999). Streams have also been shown to be an important vector for the movement of organic matter, including diatoms, downstream toward the terminal lakes (Cullis et al, 2014;Kohler et al, 2018). Black mats are probably more easily mobilized than orange mats (e.g., by high wind or high flow events) and appear to be more sensitive to changes in the hydrologic regime (Kohler et al, 2015(Kohler et al, , 2018Gooseff et al, 2017).…”

“…Streams have also been shown to be an important vector for the movement of organic matter, including diatoms, downstream toward the terminal lakes (Cullis et al, 2014;Kohler et al, 2018). Black mats are probably more easily mobilized than orange mats (e.g., by high wind or high flow events) and appear to be more sensitive to changes in the hydrologic regime (Kohler et al, 2015(Kohler et al, , 2018Gooseff et al, 2017). Thus, there is the potential for differences in the dispersal of diatoms among patches of cyanobacterial mats in the MDV landscape to mediate how local (i.e., "alpha-scale" or "patchscale") community assembly processes scale to maintain the biodiversity of the diatom metacommunity (i.e., "gamma-scale" or "regional-scale").…”