Nitrate uptake by foraminifera and use in conjunction with endobionts under anoxic conditions

Among eukaryotes, some benthic foraminiferal species have been reported to be capable of nitrate respiration, although little is known about their denitrification processes. In this study, we incubated the shallow-water benthic foraminifer Ammonia beccarii with isotopically labeled sodium nitrate (-73 parts per thousand, 0 parts per thousand, or +73 parts per thousand) under oxic or anoxic conditions to investigate how nitrate is used in foraminiferal cells and whether those signatures remain in amino acids. The delta N-15 values of amino acids from bulk cells incubated under anoxic conditions were correlated with those of nitrate in seawater and were enriched in N-15 by up to, similar to 50% compared to the isotopic compositions in seawater. There was no such relationship or enrichment in the cases of organic matter in the calcite tests under oxic or anoxic conditions or bulk cells under oxic conditions, suggesting that benthic foraminifera take up ambient nitrate under anoxic conditions and use it for denitrification; the remaining N-15-enriched intracellular nitrate pool is used for amino acid synthesis, probably by endosymbiotic microbes, as suggested by observation of the cellular ultrastructure. The degree of N-15 enrichment may depend on denitrification rates of the intracellular nitrate pool. Because the amino acids in the calcite test are synthesized by foraminifera, they were not enriched in N-15, even under anoxic conditions. Thus, differences between amino acid delta N-15 of bulk foraminiferal cells and organic matter in tests may serve as a proxy for denitrification in foraminiferal cells and microbial amino acid synthesis under oxygen-depleted conditions.