title: Metabolism in anoxic permeable sediments is dominated by eukaryotic dark fermentation.
abstract: Permeable sediments are common across continental shelves and are critical contributors to marine biogeochemical cycling. Organic matter in permeable sediments is dominated by microalgae, which as eukaryotes have different anaerobic metabolic pathways to prokaryotes such as bacteria and archaea. Here we present analyses of flow-through reactor experiments showing that dissolved inorganic carbon is produced predominantly as a result of anaerobic eukaryotic metabolic activity. In our experiments, anaerobic production of dissolved inorganic carbon was consistently accompanied by large dissolved H2production rates, suggesting the presence of fermentation. The production of both dissolved inorganic carbon and H2persisted following administration of broad spectrum bactericidal antibiotics, but ceased following treatment with metronidazole. Metronidazole inhibits the ferredoxin/hydrogenase pathway of fermentative eukaryotic H2production, suggesting that pathway as the source of H2and dissolved inorganic carbon production. Metabolomic analysis showed large increases in lipid production at the onset of anoxia, consistent with documented pathways of anoxic dark fermentation in microalgae. Cell counts revealed a predominance of microalgae in the sediments. H2production was observed in dark anoxic cultures of diatoms (Fragilariopsissp.) and a chlorophyte (Pyramimonas) isolated from the study site, substantiating the hypothesis that microalgae undertake fermentation. We conclude that microalgal dark fermentation could be an important energy-conserving pathway in permeable sediments.
authors.affiliation: Water Studies Centre, School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Bourke
authors.affiliation: Australian Centre for Research on Separation Science, School of Chemistry, Monash University, Wellington Road, VIC 3800, Australia.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Marriott
authors.affiliation: University of Southern Denmark, Nordic Centre for Earth Evolution, Odense M-5230, Denmark; Scottish Association for Marine Science, Oban PA37 1QA, UK; University of Aarhus, Arctic Research Centre, Aarhus, Denmark.
authors.forename: Ronnie N
authors.initials: RN
authors.lastname: Glud
authors.affiliation: University of Southern Denmark, Nordic Centre for Earth Evolution, Odense M-5230, Denmark; University of Southern Denmark, Villum Center for Bioanalytical Sciences, Odense M-5230, Denmark.
authors.forename: Harald
authors.initials: H
authors.lastname: Hasler-Sheetal
authors.affiliation: Department of Marine Biology, Texas A&M University. Galveston, TX, 77554 USA.
authors.forename: Manoj
authors.initials: M
authors.lastname: Kamalanathan
authors.affiliation: School of Biological Sciences, Monash University, Clayton, VIC 3800, Australia.
authors.forename: John
authors.initials: J
authors.lastname: Beardall
authors.affiliation: School of Biological Sciences, Monash University, Clayton, VIC 3800, Australia.
authors.forename: Chris
authors.initials: C
authors.lastname: Greening
authors.affiliation: Water Studies Centre, School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia.
authors.forename: Perran L M
authors.initials: PL
authors.lastname: Cook
keywords: Dark fermentation; Diatom; H2 production; Sand