Publications

4619

Abstract Anoxic and hypoxic environments serve as habitats for diverse microorganisms, including unicellular eukaryotes (protists) and prokaryotes. To thrive in low-oxygen environments, protists and prokaryotes often establish specialized metabolic cross-feeding associations, such as syntrophy, with other microorganisms. Previous studies show that the breviate protist Lenisia limosa engages in a mutualistic association with a denitrifying Arcobacter bacterium based on hydrogen exchange. Here, we investigate if the ability to form metabolic interactions is conserved in other breviates by studying five diverse breviate microcosms and their associated bacteria. We show that five laboratory microcosms of marine breviates live with multiple hydrogen-consuming prokaryotes that are predicted to have different preferences for terminal electron acceptors using genome-resolved metagenomics. Protist growth rates vary in response to electron acceptors depending on the make-up of the prokaryotic community. We find that the metabolic capabilities of the bacteria and not their taxonomic affiliations determine protist growth and survival and present new potential protist-interacting bacteria from the Arcobacteraceae, Desulfovibrionaceae, and Terasakiella lineages. This investigation uncovers potential nitrogen and sulfur cycling pathways within these bacterial populations, hinting at their roles in syntrophic interactions with the protists via hydrogen exchange.

Abstract Analysing the nitrogen (15ε) and oxygen (18ε) isotope effects of anaerobic ammonium oxidation (anammox) is essential for accurately assessing its potential contribution to fixed-N losses in the ocean, yet the 18ε of anammox remains unexplored. Here, we determined the previously unexplored 18ε of anammox using a highly enriched culture of the marine anammox species “Ca. Scalindua sp”. Because Scalindua significantly accelerated oxygen isotope exchange between NO2− and H2O, we introduced a new rate constant for anammox-mediated oxygen isotope exchange (keq, AMX = 8.44 ~ 13.56 × 10−2 h−1), which is substantially faster than abiotic oxygen isotope exchange (keq, abio = 1.13 × 10−2 h−1), into a numerical model to estimate the 18ε during anammox. Based on our experimental results, we successfully determined the 18ε associated with: (1) conversion of NO2− to N2 (18εNO2- → N2 = 10.6 ~ 16.1‰), (2) NO2− oxidation to NO3− (18εNO2- → NO3- = −2.9 ~ −11.0‰, inverse fractionation), (3) incorporation of oxygen from water during NO2− oxidation to NO3− (18εH2O = 16.4 ~ 19.2‰). Our study underscores the possibility that unique anammox oxygen isotope signals may be masked due to substantial anammox-mediated oxygen isotope exchange between NO2− and H2O. Therefore, careful consideration is required when utilizing δ18ONO3- and δ18ONO2- as geochemical markers to assess the potential contribution of anammox to fixed-N losses in the ocean.