Microbial Ecological Niche Partitioning Affects N2 gas Production in the Largest Marine Oxygen Minimum Zone

Clara A Fuchsman1, Justin Leonard Penn2, Allan Devol3, Hilary I Palevsky4, Curtis A Deutsch2, Richard Keil1, Bess B Ward5 and Gabrielle Rocap1, (1)University of Washington, School of Oceanography, Seattle, WA, United States, (2)University of Washington Seattle Campus, School of Oceanography, Seattle, WA, United States, (3)University of Washington, Seattle, WA, United States, (4)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (5)Princeton University, Department of Geosciences, Princeton, NJ, United States
Abstract:
Up to half of oceanic N2 production occurs in oxygen minimum zones (OMZs). In the Eastern Tropical North Pacific OMZ in April 2012, we measured a nine station coast to open ocean transect of N2 gas in the heart of the ETNP OMZ. Depth profiles of excess N2 gas had dual maxima located at the top of the OMZ and at 300m. An ecosystem biogeochemical model of the ETNP was also found to produce dual maxima at stations with a shallow OMZ. The model indicated that high N2 production rates caused the upper N2 maxima while long water residence time caused the deeper maxima. At a low productivity open ocean station where dual N2 maxima were observed, we obtained a depth profile of metagenomic sequences from both free living and >30 μm fractions to determine which N2 producing microbes were living in these three ecological niches. We use a phylogenetically-aware approach to identify metagenomic sequences by placing them on reference trees, which allows us to utilize them in a semi-quantitative manner. Overall, genes for denitrification (napA, nirS, nirK, qnor, nosZ) were enriched on particles while anammox was free-living. However, separation of genes into phylotypes indicated that the system is more complicated. For example, 4 out of 5 N2O reductase denitrifier phylotypes were actually free-living, while the fifth, most abundant phylotype was particle-attached. In the water column, denitrifier and anammox genes were spatially separated with depth with denitrifiers focused on the top section of the OMZ and with anammox becoming abundant slightly deeper and being more dominant at the deep N2 maxima. Interestingly, different phylotypes of denitrifiers have different depth profiles, implying individual adaptations and niches. The presence of measurable ammonia (>200 nM) at the top 20m of the OMZ along with the very low numbers of anammox bacteria is consistent with recent shoaling of the OMZ at the time of sampling. Thus the spatial separation of denitrifiers and anammox at the top of the OMZ may be a transient event due to differences in growth rate and response time. Additionally, assembly of metagenomic reads and clustering of the resulting contigs by tetranucleotide frequency was used to identify denitrifying bacteria and link together gene phylotypes.
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