Forms of RuBisCO in Cariaco's interface

A persistent chemoautotrophy maximum is present in many anoxic basins at the sulfidic layer’s upper boundary, but the microbiology and biogeochemistry that control this feature are poorly understood. In 13 of 31 cruises to the Cariaco Basin the particulate organic carbon pool was enriched in 13C (δ13CPOC as high as -16‰) within the oxic/sulfidic transitional layer compared to photic zone values (-23 to -26‰) and 13C enrichments persisted to depth. During these “heavy” cruises, dissolved O2 and [NO3- + NO2-] concentrations and their computed fluxes to the oxic/sulfidic interface were significantly lower than during “light” cruises. Cruises with isotopically heavy POC were more common between 2013 and 2015 when suspended particles below the photic zone tended to be nitrogen-rich compared to later cruises. Within the chemoautotrophic layer, nitrogen-rich particles (POC/PN< 10) were more likely to be 13C-enriched than nitrogen-poor particles among all cruises, implying these samples were dominated by living cells and fresh detritus rather than laterally transported or extensively decomposed biogenic debris. Dissolved inorganic carbon (DIC) assimilation during “heavy” cruises (n = 3) was faster and occurred deeper than during “light” cruises (n = 2). Metagenomics data from the chemoautotrophic layer during two cruises support prevalence of microorganisms carrying RuBisCO form II genes, which encode a carbon fixation enzyme that discriminates less against heavy isotopes than most other carbon fixation enzymes. Metatranscriptomics data confirm that form II RuBisCO genes were more highly expressed during both cruises and at depths where essential reactants coexist.