IS FORAMINIFERAL DENITRIFICATION PERFORMED BY THE EUKARYOTE OR BY PROKARYOTIC ASSOCIATES?

The process of denitrification (conversion of nitrate to gaseous compounds, including N₂) removes bioavailable nitrogen, particularly in marine sediments, making it a key process in the marine nitrogen budget. Benthic foraminifera reportedly perform complete denitrification, a process previously considered nearly exclusively performed by bacteria and archaea. If the ability to denitrify is widespread among these diverse and abundant protists, a paradigm shift is required for marine biogeochemistry and microbial ecology with implications reverberating to paleoecology and paleontology. However, the mechanisms of foraminiferal denitrification are unclear: it is possible that the ability to perform complete denitrification is due to symbiont metabolism in some foraminiferal species. Using a variety of experimental, ecological, microscopic, molecular, and chemical methods, we analyzed 5 foraminiferal species (Bolivina argentea, Buliminella tenuata, Nonionella stella, Fursenkoina cornuta, undescribed allogromiid) to glean details on foraminiferal denitrification. During experiments including oxic, anoxic, and suboxic treatments (either with or without antibiotics), intracellular nitrate concentrations decreased while a general increase in δ¹⁵N-NO₃^- occurred, indicating nitrate respiration and/or denitrification. Where measurable, nitrogen isotopic data from freshly collected specimens suggest nitrate consumption occurs within foraminifera in situ. The genes nirS and nirK, which are involved with dissimilatory nitrite reduction, were also detected. Overall, results suggest that denitrification occurs in a range of foraminiferal species, including some lacking endobionts and some possessing endobionts, implying that microbial associates may be involved with but are not solely responsible for denitrification in foraminifera. The apparent widespread ability of foraminifera to denitrify suggests a biogeochemistry/microbial ecology paradigm shift must be considered, as suggested by earlier authors. If the ability of foraminifera to denitrify is an ancient trait, this early evolving protist taxon likely competed well in Neoproterozoic ecosystems potentially allowing diversification into all marine habitats. Supported by NSF grant MCB-0702491.