Nitrogen fixation facilitates stream microbial mat biomass across the McMurdo Dry Valleys, Antarctica

Drifting along: using diatoms to track the contribution of microbial mats to particulate organic matter transport in a glacial meltwater stream in the McMurdo Dry Valleys, Antarctica

“…Evidence for this is in the work of Heindel et al (2021) , where diatoms were directly observed in the hyporheic sediments (the identity of which correspond well to that observed for the POM in this study) and were linked with greater concentrations of nitrogen ( Singley et al, 2021 ). Given this background, the present work provides a sort of “missing link” for patching together the observations of nitrogen fixation in the surface mats ( McKnight et al, 2007 ; Kohler et al, 2018 , 2023 ) and the diatoms and enhanced nitrogen concentrations in the hyporheic zone ( Heindel et al, 2021 ; Singley et al, 2021 , 2023 ).…”

“…The results of the SourceTracker analyses agree that at least a portion of the transported POM is directly derived from the black mats. Given the limited nutrients released from the glacier and lack of allochthonous inputs, this is likely an important mechanism for nutrient regeneration in Von Guerard Stream and across Taylor Valley (Kohler et al, 2018(Kohler et al, , 2023. It is thought that much of this nutrient regeneration from POM takes place in the hyporheic zone, where it can be degraded and provide nutrients via upwelling for downstream communities.…”

“…Two distinct, macroscopic vegetation types dominate many MDV streams; orange (or red) microbial mats formed of filamentous oscillatorian cyanobacteria and adhering to sediments within stream channels, and accumulations of black mucilagenous colonies of Nostoc weakly attached to sediments along stream margins ( Alger et al, 1997 ; Kohler et al, 2015c ). These latter ‘black mats’ are particularly important to local nutrient cycles given their ability to fix nitrogen, and the remineralization of both mat types in the hyporheic zone are critical to balancing downstream nutrient budgets ( Kohler et al, 2018 , 2023 ) given high rates of biological uptake ( McKnight et al, 2004 ).…”

“…Previously, diatoms present in hyporheic sediments were found to be most similar to black mat diatom communities (Heindel et al, 2021), leading to the hypothesis that mat-derived diatoms in the POM are retained in stream sediments following hyporheic exchange. This scenario would not only potentially account for the elevated content of biogenic silica (2-8%, Heindel et al, 2021) and nitrogen (~40 g N m −2 , Singley et al, 2023) found in the hyporheic sediment, but would also have implications for MDV diatom dispersal.…”

Nitrogen fixation across the aquascape: current perspectives, future priorities

“…This suggests that the epiphytes were likely cyanobacteria and the relative importance of epiphytic N 2 fixation to the lagoon N budget suggested that these cyanobacteria provide a key ecosystem service to help maintain the health of seagrass communities. Similarly, Kohler et al (2023) measured the importance of N 2 fixation by Nostoc in the N budget of Antarctic streams. They identified three common microbial mat types in pristine streams of the of the McMurdo Dry Valley: black mats typically dominated by Nostoc, orange mats dominated by other cyanobacteria such as Leptolyngbya, Oscillatoria, and Phormidium, and green mats dominated by chlorophytes.…”