Nitrate respiration and diel migration patterns of diatoms are linked in sediments underneath a microbial mat.

Elisa Merz,Dirk De Beer,Thomas Hübener,Gregory J Dick,Sten Littmann,Sharon Grim,Kirk Olsen,Judith M Klatt,Dack Stuart,Hannah K Marchant,Gaute Lavik

doi:10.1111/1462-2920.15345

Abstract

Diatoms are among the few eukaryotes known to store nitrate (NO3 - ) and to use it as an electron acceptor for respiration in the absence of light and O2 . Using microscopy and 15 N stable isotope incubations, we studied the relationship between dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and diel vertical migration of diatoms in phototrophic microbial mats and the underlying sediment of a sinkhole in Lake Huron (USA). We found that the diatoms rapidly accumulated NO3 - at the mat-water interface in the afternoon and 40% of the population migrated deep into the sediment, where they were exposed to dark and anoxic conditions for ~75% of the day. The vertical distribution of DNRA rates and diatom abundance maxima coincided, suggesting that DNRA was the main energy generating metabolism of the diatom population. We conclude that the illuminated redox-dynamic ecosystem selects for migratory diatoms that can store nitrate for respiration in the absence of light. A major implication of this study is that the dominance of DNRA over denitrification is not explained by kinetics or thermodynamics. Rather, the dynamic conditions select for migratory diatoms that perform DNRA and can outcompete sessile denitrifiers.

Highlights

In illuminated benthic ecosystems, microphytobenthos (MPB), a general grouping of benthic microalgae, drives primary production via photosynthesis
The migration rhythm is linked to the assimilation and respiration of inorganic nitrogen (Koho et al, 2011), which can have large effects on the nitrogen (N)-cycle, the balance between denitrification, dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and nitrification (Daims et al, 2015)
Most of the 15NO3− could be recovered in the 15Nammonium (15NH4+) pool and only a small amount was reduced to N2, indicating that DNRA was responsible for 98%–99% of the NO3− consumption in all of the incubations (Fig. 1)

Summary

Introduction

Microphytobenthos (MPB), a general grouping of benthic microalgae, drives primary production via photosynthesis. We used stable isotope incubations and light microscopy to determine how DNRA is linked to diel light dynamics and diatom migration behaviour.

Results

Conclusion