Both prokaryotes and microbial eukaryotes are key drivers of biogeochemical processes in oxygen deficient zones (ODZs). We combined genome-resolved metagenomics and metatranscriptomics to study the diversity, functions, activities, and potential interactions of prokaryotes and fungi in the eastern tropical North Pacific. Among the 426 high-quality metagenome-assembled genomes (MAGs) reconstructed, we identified novel bacterial lineages interfacing carbon (C), nitrogen (N), and sulfur (S) cycling. The highest level of nitric oxide (NO) reductase expression was found in a Rhodospirillales MAG, which expressed a complete dissimilatory pathway of sulfate reduction to sulfide at the secondary nitrite maximum. The highest level of nitrous oxide (N₂O) reductase expression was found in a Poribacteria MAG, of which the abundance and activity peaked at the oxic-anoxic interface and showed a positive correlation with N₂O reduction rates. This Poribacteria MAG lacks genetic capacity to reduce NO but it expressed the complete pathway for dissimilatory nitrate reduction to ammonia (DNRA), potentially fueling anaerobic ammonia oxidation (anammox) at the top of the anoxic layer. The eukaryotic metatranscriptomes revealed that 1/3 of the fungal activities were from early diverging fungi. Expressed even at anoxic depths, Dikarya GH7 was the dominant fungal hydrolytic enzyme and potentially fueled the growth of bacteria involved in nitrogen and sulfur cycling. These findings advance our understanding of the role of bacteria and fungi in linking C, N, and S cycling in ODZs.

Primary Presenter: Xuefeng Peng, University of South Carolina (xpeng@seoe.sc.edu)

Authors:

David Valentine, University of California, Santa Barbara  (valentine@ucsb.edu)