Nitrification plays a key role in the marine nitrogen (N) cycle, including in oceanic oxygen minimum zones (OMZs), which are hot spots for denitrification and anaerobic ammonia oxidation (anammox). Recent evidence suggests that nitrification links the source (remineralized organic matter) and sink (denitrification and anammox) of fixed N directly in the steep oxycline in the OMZs. We performed shipboard incubations with ¹⁵N tracers to characterize the depth distribution of nitrification in the Eastern Tropical North Pacific (ETNP). Additional experiments were conducted to investigate photoinhibition. Allylthiourea (ATU) was used to distinguish the contribution of archaeal and bacterial ammonia oxidation. The abundance of archaeal and β‐proteobacterial ammonia monooxygenase gene subunit A (amoA) was determined by quantitative polymerase chain reaction. The rates of ammonia and nitrite oxidation showed distinct subsurface maxima, with the latter slightly deeper than the former. The ammonia oxidation maximum coincided with the primary nitrite concentration maximum, archaeal amoA gene maximum, and the subsurface nitrous oxide maximum. Negligible rates of ammonia oxidation were found at anoxic depths, where high rates of nitrite oxidation were measured. Archaeal amoA gene abundance was generally 1 to 2 orders of magnitude higher than bacterial amoA gene abundance, and inhibition of ammonia‐oxidizing bacteria with 10 μM ATU did not affect ammonia oxidation rates, indicating the dominance of archaea in ammonia oxidation. These results depict highly dynamic activities of ammonia and nitrite oxidation in the oxycline of the ETNP OMZ.

Nitrification, the oxidation of ammonium ( ) to nitrite ( ) and to nitrate ( ), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with ¹⁵N‐labeled substrates to measure rates of and oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of and oxidation rates were primarily controlled by and availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, oxidation was negligible or below detection, but high rates of oxidation were observed. oxidation displayed extremely high affinity for both and oxygen. The positive linear correlations between oxidation rates and in situ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ concentrations or supply by adjusting their population size, which determines the oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N₂O concentration, along with independently reported simultaneous direct N₂O production rate measurements, suggests that AOA were predominantly responsible for oxidation, which was a major source of N₂O production at oxygen concentrations > 5 µM.

Growing interest in understanding the relevance of marine fungi to food webs, biogeochemical cycling, and biological patterns necessitates establishing a context for interpreting future findings. To help establish this context, we summarize the diversity of cultured and observed marine planktonic fungi from across the world. While exploring this diversity, we discovered that only half of the known marine fungal species have a publicly available DNA locus, which we hypothesize will likely hinder accurate high-throughput sequencing classification in the future, as it does currently. Still, we reprocessed >600 high-throughput datasets and analyzed 4.9 × 10⁹ sequences (4.8 × 10⁹ shotgun metagenomic reads and 1.0 × 10⁸ amplicon sequences) and found that every fungal phylum is represented in the global marine planktonic mycobiome; however, this mycobiome is generally predominated by three phyla: the Ascomycota, Basidiomycota, and Chytridiomycota. We hypothesize that these three clades are the most abundant due to a combination of evolutionary histories, as well as physical processes that aid in their dispersal. We found that environments with atypical salinity regimes (>5 standard deviations from the global mean: Red Sea, Baltic Sea, sea ice) hosted higher proportions of the Chytridiomycota, relative to open oceans that are dominated by Dikarya. The Baltic Sea and Mediterranean Sea had the highest fungal richness of all areas explored. An analysis of similarity identified significant differences between oceanographic regions. There were no latitudinal gradients of marine fungal richness and diversity observed. As more high-throughput sequencing data become available, expanding the collection of reference loci and genomes will be essential to understanding the ecology of marine fungi.

Ammonia-oxidizing archaea (AOA) have been reported at high abundance in much of the global ocean, even in environments, such as pelagic oxygen minimum zones (OMZs), where conditions seem unlikely to support aerobic ammonium oxidation. Due to the lack of information on any potential alternative metabolism of AOA, the AOA community composition might be expected to differ between oxic and anoxic environments. This hypothesis was tested by evaluating AOA community composition using a functional gene microarray that targets the ammonia monooxygenase gene subunit A (amoA). The relationship between environmental parameters and the biogeography of the Arabian Sea and the Eastern Tropical South Pacific (ETSP) AOA assemblages was investigated using principal component analysis (PCA) and redundancy analysis (RDA). In both the Arabian Sea and the ETSP, AOA communities within the core of the OMZ were not significantly different from those inhabiting the oxygenated surface waters above the OMZ. The AOA communities in the Arabian Sea were significantly different from those in the ETSP. In both oceans, the abundance of archaeal amoA gene in the core of the OMZ was higher than that in the surface waters. Our results indicate that AOA communities are distinguished by their geographic origin. RDA suggested that temperature (higher in the Arabian Sea than in the ETSP) was the main factor that correlated with the differences between the AOA communities. Physicochemical properties that characterized the different environments of the OMZ and surface waters played a less important role, than did geography, in shaping the AOA community composition.