title: Intracellular Nitrate of Marine Diatoms as a Driver of Anaerobic Nitrogen Cycling in Sinking Aggregates.
abstract: Diatom-bacteria aggregates are key for the vertical transport of organic carbon in the ocean. Sinking aggregates also represent pelagic microniches with intensified microbial activity, oxygen depletion in the center, and anaerobic nitrogen cycling. Since some of the aggregate-forming diatom species store nitrate intracellularly, we explored the fate of intracellular nitrate and its availability for microbial metabolism within anoxic diatom-bacteria aggregates. The ubiquitous nitrate-storing diatomSkeletonema marinoiwas studied as both axenic cultures and laboratory-produced diatom-bacteria aggregates. Stable15N isotope incubations under dark and anoxic conditions revealed that axenicS. marinoiis able to reduce intracellular nitrate to ammonium that is immediately excreted by the cells. When exposed to a light:dark cycle and oxic conditions,S. marinoistored nitrate intracellularly in concentrations >60 mmol L-1both as free-living cells and associated to aggregates. Intracellular nitrate concentrations exceeded extracellular concentrations by three orders of magnitude. Intracellular nitrate was used up within 2-3 days after shifting diatom-bacteria aggregates to dark and anoxic conditions. Thirty-one percent of the diatom-derived nitrate was converted to nitrogen gas, indicating that a substantial fraction of the intracellular nitrate pool ofS. marinoibecomes available to the aggregate-associated bacterial community. Only 5% of the intracellular nitrate was reduced to ammonium, while 59% was recovered as nitrite. Hence, aggregate-associated diatoms accumulate nitrate from the surrounding water and sustain complex nitrogen transformations, including loss of fixed nitrogen, in anoxic, pelagic microniches. Additionally, it may be expected that intracellular nitrate not converted before the aggregates have settled onto the seafloor could fuel benthic nitrogen transformations.
authors.affiliation: AIAS, Aarhus Institute of Advanced Studies, Aarhus University Aarhus, Denmark.
authors.forename: Anja
authors.initials: A
authors.lastname: Kamp
authors.affiliation: Department of Biology and Nordic Center for Earth Evolution, University of Southern Denmark Odense, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Stief
authors.affiliation: Department of Biology and Nordic Center for Earth Evolution, University of Southern DenmarkOdense, Denmark; Department of Biogeochemistry, Max Planck Institute for Marine MicrobiologyBremen, Germany.
authors.forename: Laura A
authors.initials: LA
authors.lastname: Bristow
authors.affiliation: Department of Biology and Nordic Center for Earth Evolution, University of Southern Denmark Odense, Denmark.
authors.forename: Bo
authors.initials: B
authors.lastname: Thamdrup
authors.affiliation: Department of Biology and Nordic Center for Earth Evolution, University of Southern DenmarkOdense, Denmark; Department of Biogeochemistry and Earth Science, Scottish Association for Marine ScienceOban, UK; Department of Bioscience, Arctic Research Centre, Aarhus UniversityAarhus, Denmark.
authors.forename: Ronnie N
authors.initials: RN
authors.lastname: Glud
keywords: DNRA; denitrification; low-oxygen environments; marine snow; nitrate respiration; nitrogen loss; stable isotopes