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1. Ocean deoxygenation and copepods: coping with oxygen minimum zone variability

   https://doi.org/10.5194/bg-17-2315-2020  

   Wishner, K.F. ; Seibel, B. ; Outram, D. ( January 2020 , Biogeosciences)

   null (Ed.)

   Increasing deoxygenation (loss of oxygen) of the ocean, including expansion of oxygen minimum zones (OMZs), is a potentially important consequence of global warming. We examined present-day variability of vertical distributions of 23 calanoid copepod species in the Eastern Tropical North Pacific (ETNP) living in locations with different water column oxygen profiles and OMZ intensity (lowest oxygen concentration and its vertical extent in a profile). Copepods and hydrographic data were collected in vertically stratified day and night MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) tows (0–1000 m) during four cruises over a decade (2007– 2017) that sampled four ETNP locations: Costa Rica Dome, Tehuantepec Bowl, and two oceanic sites further north (21– 22 N) off Mexico. The sites had different vertical oxygen profiles: some with a shallow mixed layer, abrupt thermocline, and extensive very low oxygen OMZ core; and others with a more gradual vertical development of the OMZ (broad mixed layer and upper oxycline zone) and a less extensive OMZ core where oxygen was not as low. Calanoid copepod species (including examples from the genera Eucalanus, Pleuromamma, and Lucicutia) demonstrated different distributional strategies (implying different physiological characteristics) associated with this variability. We identified sets of species that (1) changed their vertical distributions and depth of maximum abundance associated with the depth and intensity of the OMZ and its oxycline inflection points; (2) shifted their depth of diapause; (3) adjusted their diel vertical migration, especially the nighttime upper depth; or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept). These distribution depths changed by tens to hundreds of meters depending on the species, oxygen profile, and phenomenon. For example, at the lower oxycline, the depth of maximum abundance for Lucicutia hulsemannae shifted from  600 to  800 m, and the depth of diapause for Eucalanus inermis shifted from  500 to  775 m, in an expanded OMZ compared to a thinner OMZ, but remained at similar low oxygen levels in both situations. These species or life stages are examples of “hypoxiphilic” taxa. For the migrating copepod Pleuromamma abdominalis, its nighttime depth was shallow ( 20 m) when the aerobic mixed layer was thin and the low-oxygen OMZ broad, but it was much deeper ( 100 m) when the mixed layer and higher oxygen extended deeper; daytime depth in both situations was  300 m. Because temperature decreased with depth, these distributional depth shifts had metabolic implications. The upper ocean to mesopelagic depth range encompasses a complex interwoven ecosystem characterized by intricate relationships among its inhabitants and their environment. It is a critically important zone for oceanic biogeochemical and export processes and hosts key food web components for commercial fisheries. Among the zooplankton, there will likely be winners and losers with increasing ocean deoxygenation as species cope with environmental change. Changes in individual copepod species abundances, vertical distributions, and life history strategies may create potential perturbations to these intricate food webs and processes. Present-day variability provides a window into future scenarios and potential effects of deoxygenation. 

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2. Anaerobic methane oxidation is an important sink for methane in the ocean's largest oxygen minimum zone

   https://doi.org/10.1002/lno.11235  

   Thamdrup, Bo ; Steinsdóttir, Herdís G. R. ; Bertagnolli, Anthony D. ; Padilla, Cory C. ; Patin, Nastassia V. ; Garcia‐Robledo, Emilio ; Bristow, Laura A. ; Stewart, Frank J. ( June 2019 , Limnology and Oceanography)

   We investigated methane oxidation in the oxygen minimum zone (OMZ) of the eastern tropical North Pacific (ETNP) off central Mexico. Methane concentrations in the anoxic core of the OMZ reached ~ 20 nmol L⁻¹at off shelf sites and 34 nmol L⁻¹at a shelf site. Rates of methane oxidation were determined in ship‐board incubations with³H‐labeled methane at O₂concentrations 0–75 nmol L⁻¹. In vertical profiles at off‐shelf stations, highest rates were found between the secondary nitrite maximum at ~ 130 m and the methane maximum at 300–400 m in the anoxic core. Methane oxidation was inhibited by addition of 1μmol L⁻¹oxygen, which, together with the depth distribution, indicated an anaerobic pathway. A coupling to nitrite reduction was further indicated by the inhibitory effect of the nitric oxide scavenger 2‐phenyl‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (PTIO). Metatranscriptomes from the anoxic OMZ core supported the likely involvement of nitrite‐reducing bacteria of the NC10 clade in anaerobic methane oxidation, but also indicated a potential role for nitrate‐reducing euryarchaeotal methane oxidizers (ANME‐2d). Gammaproteobacteria of the Methanococcales were further detected in both 16S rRNA gene amplicons and metatranscriptomes, but the role of these presumed obligately aerobic methane oxidizers in the anoxic OMZ core is unclear. Given available estimates of water residence time, the measured rates and rate constants (up to ~ 1 yr⁻¹) imply that anaerobic methane oxidation is a substantial methane sink in the ETNP OMZ and hence attenuates the emission of methane from this and possibly other OMZs.

    

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3. Genome‐resolved viral ecology in a marine oxygen minimum zone

   https://doi.org/10.1111/1462-2920.15313  

   Vik, Dean ; Gazitúa, Maria Consuelo ; Sun, Christine L. ; Zayed, Ahmed A. ; Aldunate, Montserrat ; Mulholland, Margaret R. ; Ulloa, Osvaldo ; Sullivan, Matthew B. ( November 2020 , Environmental Microbiology)

   Oxygen minimum zones (OMZs) are critical to marine nitrogen cycling and global climate change. While OMZ microbial communities are relatively well‐studied, little is known about their viruses. Here, we assess the viral community ecology of 22 deeply sequenced viral metagenomes along a gradient of oxygenated to anoxic waters (<0.02 μmol/l O₂) in the Eastern Tropical South Pacific (ETSP) OMZ. We identified 46 127 viral populations (≥5 kb), which augments the known viruses from ETSP by 10‐fold. Viral communities clustered into six groups that correspond to oceanographic features. Oxygen concentration was the predominant environmental feature driving viral community structure. Alpha and beta diversity of viral communities in the anoxic zone were lower than in surface waters, which parallels the low microbial diversity seen in other studies. ETSP viruses were largely endemic, with the majority of shared viruses (87%) also present in other OMZ samples. We detected 543 putative viral‐encoded auxiliary metabolic genes (AMGs), of which some have a distribution that reflects physico‐chemical characteristics across depth. Together these findings provide an ecological baseline for viral community structure, drivers and population variability in OMZs that will help future studies assess the role of viruses in these climate‐critical environments.

    

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4. Imprint of trace dissolved oxygen on prokaryoplankton community structure in an oxygen minimum zone

   https://doi.org/doi:10.3389/fmars.2020.00360  

   Medina Faull, L. ; Mara, Paraskevi ; Taylor, G.T. ; Edgcomb, V.P. ( January 2020 , Frontiers in marine science)

   null (Ed.)

   The Eastern Tropical North Pacific (ETNP) is a large, persistent, and intensifying oxygen minimum zone (OMZ) that accounts for almost half of the total area of global OMZs. Within the OMZ core (350–700 m depth), dissolved oxygen is typically near or below the analytical detection limit of modern sensors (10 nM). Steep oxygen gradients above and below the OMZ core lead to vertical structuring of microbial communities that also vary between particle-associated (PA) and free-living (FL) size fractions. Here, we use 16S amplicon sequencing (iTags) to analyze the diversity and distribution of prokaryotic populations between FL and PA size fractions and among the range of ambient redox conditions. The hydrographic conditions at our study area were distinct from those previously reported in the ETNP and other OMZs, such as the ETSP. Trace oxygen concentrations (0.35 mM) were present throughout the OMZ core at our sampling location. Consequently, nitrite accumulations typically reported for OMZ cores were absent as were sequences for anammox bacteria (Brocadiales genus Candidatus Scalindua), which are commonly found across oxic-anoxic boundaries in other systems. However, ammonia-oxidizing bacteria (AOB) and archaea (AOA) distributions and maximal autotrophic carbon assimilation rates (1.4 mM C d􀀀1) coincided with a pronounced ammonium concentration maximum near the top of the OMZ core. In addition, members of the genus Nitrospina, a dominant nitrite-oxidizing bacterial (NOB) clade were present suggesting that both ammonia and nitrite oxidation occur at trace oxygen concentrations. Analysis of similarity test (ANOSIM) and Non-metric Dimensional Scaling (nMDS) revealed that bacterial and archaeal phylogenetic representations were significantly different between size fractions. Based on ANOSIM and iTag profiles, composition of PA assemblages was less influenced by the prevailing depth-dependent biogeochemical regime than the FL fraction. Based on the presence of AOA, NOB and trace oxygen in the OMZ core we suggest that nitrification is an active process in the nitrogen cycle of this region of the ETNP OMZ. 

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5. Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

   https://doi.org/10.1029/2022GB007330  

   Rahman, Shaily ; Shiller, Alan M. ; Anderson, Robert F. ; Charette, Matthew A. ; Hayes, Christopher T. ; Gilbert, Melissa ; Grissom, Karen R. ; Lam, Phoebe J. ; Ohnemus, Daniel C. ; Pavia, Frank J. ; et al ( June 2022 , Global Biogeochemical Cycles)

   Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg⁻¹) and increase to 70–80 and 140–150 nmol kg⁻¹in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa_xs) formation and dissolution rates were tracked by normalizing particulate excess²³⁰Th activities. Th‐normalized pBa_xsfluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m⁻² year⁻¹average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa_xsburial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year⁻¹. Estuarine mixing processes may add another 3–13 Gmol year⁻¹. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year⁻¹), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance.

    

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