Sulphide stimulates nitrate reduction in benthic diatoms from a microbial mat

Diatoms are among the few eukaryotes known to store nitrate (NO₃⁻) and to use it for dissimilatory nitrate/nitrite reduction to ammonium (DNRA) to generate enegry in the absence of light and O₂. We used stable isotope incubations and in situ microsensor measurements over complete light cycles to study the diel activity transitions of the NO₃⁻-storing benthic diatom Craticula cuspidata in the submerged Middle Island Sinkhole, Lake Huron (USA). We found that this diatom links NO₃⁻ respiration to diel migration into deep (~4 cm) sulfidic sediments below the microbial mat. This pattern was accompanied by pronounced diel changes in the depth of sulphide consumption. During the day sulphide was consumed by anoxygenic photosynthesis and aerobic sulphide oxidation in the uppermost few mm. Surprisingly, the consumption zone moved downward in the evening and was deepest in the sediment at night. Thus, the sulphide consumption zone strikingly overlapped with the depth of DNRA-performing diatom residence. Using an enrichment of Craticula cuspidata, we found that the nitrate respiration via DNRA was ~10-fold higher in the presence of sulphide. Overall, our data therefore indicate that C. cuspidata and/or their microbiome link NO₃⁻ reduction to sulphide oxidation.