Aerobic and anaerobic ammonium oxidizers in the Cariaco Basin: Distributions of major taxa and nitrogen species across the redoxcline

Depth distributions of cells and functional gene copies from anaerobic ammonium-oxidizing (anammox) bacteria, aerobic ammonium-oxidizing bacteria (AOB) and archaea (AOA) in the Cariaco Basin, Venezuela were obtained using FISH and q-PCR assays. These distributions were compared to concentrations of dissolved ammonium (NH₄ ⁺), nitrite (NO₂ ^-), nitrate (NO₃ ^-), hydrogen sulfide (H₂S) and oxygen (O₂) along the redoxcline during 3 cruises. Cell counts of anammox bacteria and copies of their nitrite reductase gene (Scalindua-nirS) were consistently observed in 2 distinct layers: the suboxic zone (≤1.1 × 10⁶ cells l^-1) and the upper euxinic zone (≤4.7 × 10⁶ cells l^-1). We hypothesize that anammox bacteria vertically organize in response to NO₂ ^- sources, with NO₂ ^- for the shallower assemblage supplied by nitrification and for the deeper assemblage by denitrification. Peaks in AOA cell abundances (up to 14.8 × 10⁶ cells l^-1) consistently coincided with copy numbers of archaeal ammonia monooxygenase subunit A gene (archaeal amoA) in the lower oxic zone. Peak abundances of beta- and gammaproteobacterial AOB cells (up to 24.0 × 10⁶ cells l^-1) and one of their ammonia monooxygenase genes (β-amoA) overlapped above the shallow anammox peak. Our results suggest that anammox bacteria AOB, AOA and denitrifiers are metabolically interdependent and ultimately controlled by vertical fluxes of O₂, NO₃ ^-, NO₂ ^-, and NH₄ ⁺ in the Cariaco Basin.