Archive for ODZ

The arsenic respiratory cycle in pelagic waters of Oxygen Deficient Zones

Posted by mmaheigan 
· Wednesday, October 30th, 2019 

Oxygen Deficient Zones (ODZs) are naturally occurring functionally anoxic regions of the open ocean which can act as proxies for early Earth’s anoxic ocean. Without free oxygen, microorganisms in these regions use alternative electron acceptors to oxidize organic material. These functionally anoxic regions are also hotspots for chemoautotrophic pathways. Some microorganisms can use arsenic based compounds to oxidize organic material, and others can couple nitrate reduction with arsenic oxidation supporting autotrophic carbon fixation thus linking arsenic respiration with carbon and nitrogen cycling. While arsenic concentrations in modern oceans are relatively low, the Precambrian ocean likely had periods of high arsenic concentrations. Integrating over time and space of anoxic waters, arsenic-based metabolisms may have had significant implications for the biogeochemical cycling of not only arsenic, but also carbon and nitrogen.

Figure 1: Arsenotrophic genes identified in the Eastern Tropical North Pacific Oxygen Deficient Zone. (A) Genomic complement for dissimilatory arsenate reduction assembled from metagenomes which likely supports respiration of organic matter. (B) Genomic complement for putative chemoautotrophic arsenite oxidation pathway assembled from metagenomes which may couple with nitrate reduction to support organic matter production. (C) Relative abundance of genes associated with arsenite oxidase (aioA), dissimilatory arsenate reduction (arrA), and forward dissimilatory sulfite reductase (dsrA) associated with sulfur reduction; abundance shown as a relative contribution to the total microbial community as estimated by abundance of RNA polymerase genes (rpoB). The genes arrA and forward-dsrA are more abundant in the particulate fraction, whereas aioA is more abundant in the free-living fraction. (D) Relative abundance of genes in the microbial community for the more abundant genes aioA-like and reverse form of dsrA associated with sulfur oxidation. aioA-like genes are relatively more abundant within the particulate fraction, with no strong partitioning between fractions identified for the reverse-dsrA genes. Arsenical reduction and chemoautotrophic arsenical oxidation are likely performed by different microbial groups within the ODZ communities.

Recent work in PNAS identified gene sequences for a complete arsenic respiratory cycle from Eastern Tropical North Pacific (ETNP) ODZ metagenomes. The authors identified arsenotrophic genes for dissimilatory arsenate reduction from one group of microorganisms and genes for a putative chemoautotrophic arsenite oxidation pathway from another group within the ETNP ODZ microbial community. Analysis of genomic sequences from a free-living sample and from particulate-associated sample indicate niche differentiation of these pathways—arsenate reduction genes enriched within the particulate fraction and arenite oxidation enriched in the free-living water column. In addition to the presence of these genes in metagenomes, the authors identified the active expression of these arsenotrophic genes in publicly available metatranscriptomes from the ETNP and Eastern Tropical South Pacific ODZs. Theyalso found an abundance of sequences in the ETNP ODZ for the gene aioA-like, which is a closely related enzyme to arsenite oxidase (aioA), but with an unconfirmed function. The identification of these actively expressed genes in modern ODZs enables further investigation of these cycles that were likely important in early oceans. These findings also highlight that there are still yet to be discovered respiratory pathways in ODZs. Arsenotrophy, in conjunction with other niche respiratory pathways – both known and as yet undiscovered – likely sum to a considerable contribution of energy flow and elemental cycling through these anoxic systems.

Authors:
Jaclyn Saunders (University of Washington; present affiliation Woods Hole Oceanographic Institution)
Clara Fuchsman (University of Washington; present affiliation Horn Point Laboratory)
Cedar McKay & Gabrielle Rocap (University of Washington)

 

See related University of Washington press-release

Ocean microbes drive fluctuating nutrient loss

Posted by mmaheigan 
· Tuesday, May 28th, 2019 

The removal of bioavailable nitrogen (N) by anaerobic microbes in the ocean’s oxygen deficient zones (ODZs) is thought to vary over time primarily as a result of climate impacts on ocean circulation and primary production. However, a recent study in PNAS using a data-constrained model of the microbial ecosystem in the world’s largest ODZ revealed that internal species oscillations cause local- to basin-scale fluctuations in the rate of N loss, even in a completely stable physical environment. Such ecosystem oscillations have been hypothesized for nearly a century in idealized models, but are rarely shown to persist in a three-dimensional ocean circulation model.

Figure caption. Ecological variability in the basin-scale rate of nitrogen loss over time (left) and in the local-scale contribution of autotrophic anammox to total N loss (right) in a model with unchanging ocean circulation. In the left panel, colors represent model simulations with different biological parameters. In the right panel, colors represent distinct locations within the ODZ in the standard model simulation.

 

These emergent ecosystem dynamics arise at the oxic-anoxic interface from O2-dependent resource competition between aerobic and anaerobic microbes, and leave a unique geochemical fingerprint: infrequent spikes in ammonium that are observable in nutrient measurements from the ODZ. Non-equilibrium ecosystem behavior driven by competition among aerobic nitrifiers, anaerobic denitrifiers, and anammox bacteria also generates fluctuations in the balance of autotrophic versus heterotrophic N loss pathways that help reconcile conflicting field observations.

These internally driven fluctuations in microbial community structure partially obscure a direct correspondence between the chemical environment and microbial rates, a universal assumption in biogeochemical models. Because of the fundamental nature of the underlying mechanism, similar dynamics are hypothesized to occur across wide-ranging microbial communities in diverse habitats.

 

Authors:
Justin L. Penn (University of Washington)
Thomas Weber (University of Rochester),
Bonnie X. Chang (University of Washington, NOAA)
Curtis Deutsch (University of Washington)

 

See also the OCB2019 plenary session: Anthropogenic changes in ocean oxygen: Coastal and open ocean perspectives (Monday, June 24, 2019)

Autonomous measurement of N-loss in the Eastern Tropical North Pacific ODZ: An Invitation for Collaboration

Posted by mmaheigan 
· Thursday, January 10th, 2019 

By Mark A. Altabet (SMAST/U. Mass. Dartmouth), Craig McNeil, and Eric D’Asaro (both at APL / U. Washington)

Oxygen deficient zones (ODZs) constitute a small fraction of total oceanic volume yet play an important role in regulating global ocean carbon and nitrogen cycles. They are critical for regulating the ocean’s nitrogen budget, as loss of biologically available nitrogen to N2 gas (N-loss) within ODZs is estimated to be 30 to 50% of the global total. However, temporal and spatial variability in N-loss rates have been undersampled by ship-based process studies leaving substantial uncertainty in overall rates. While local and short-term regulation of N-loss by O2 and organic matter availability is well documented, little is known about the larger scale temporal/spatial variability in N-loss that may result from physical forcings such as remote ventilation, seasonal variability in vertical exchange with the near-surface layer, and mesoscale eddies. Understanding the impact of larger scale physical forcings on N-loss as mediated through O2 and organic flux is needed to fully understand the causes and consequences of any future ODZ expansion. To achieve this, we need sustained observations by a distributed array capable of detecting synoptic variability.

To address these issues, a new NSF-funded project will carry out a multiyear, autonomous float-based observational program to answer the following questions:

  • How does biogenic N2 production in ODZs vary over weekly to annual time scales and space scales of 10s to 1000s km?
  • What are the major scales of variability and their associated oceanographic phenomena and how do they relate to control by organic matter flux and O2 concentration?
  • How does this variability influence regionally integrated N-loss?

Figure 1. The ETNP ODZ roughly defined by O2 <1.5 μmol/kg (orange, World Ocean Atlas 2013). Our new NSF-funded project will sample across these patterns of spatial and temporal variability for 2 years with 10 subsurface ODZ floats (red/blue) each measuring profiles of T, S, O2 (50 nmol/kg LOD) and N2 (0.1 μmol/kg precision), and the in situ rate of N2 change. Four Argo floats with O2 sensors and BioOptical floats provided by collaborators will supplement this array. Bright bar symbols are the planned deployment positions; dimmed bar symbols suggest possible displacements after 2 years. Ship-based measurements (yellow stars) along the deployment cruise track (magenta) will be used for float sensor calibration and identification of ETNP source water properties. The 2-year track of our prototype GasFloat is also shown (black line).

This project will exploit our ability to make in situ, ultra-high precision measurement of N2 concentration (~0.1 umol/kg) and use commercially available O2 sensors to measure O2 in the 10s of nM range. Our study area is the Eastern Tropical North Pacific (ETNP), the largest ODZ and the region of our successful pilot deployments (Figure 1). Over a multi-year period, our study will determine in situ nM-level O2 and biogenic N2 on float profiles distributed throughout the ETNP and encompassing geographic gradients in O2 and surface productivity. For the first time, our study will also determine in situ N-loss rates from changes in N2 concentration during one- to two-week Lagrangian float deployments drifting along a constant density surface (Figure 2). A pilot two-year float (‘GasFloat, Figure 1) deployment in the ETNP has documented our ability to do so. Critically, our float-based approach more closely matches the frequency and distribution of observations to the expected variability in biogenic N2 production, as compared to prior work. This study will also dramatically increase the data density in this region by acquiring >500 profiles/drifts for N2 and >1000 profiles for nM O2.

Figure 2. (a) Schematic of float system to be deployed (b) Example of float mission including 2-week isopycnal drift.

We anticipate float deployment in summer 2020 via a UNOLS vessel. Investigators interested in collaborative participation through contribution of autonomous instrumentation and/or making shipboard measurements are encouraged to contact the lead PI Mark Altabet at maltabet@umassd.edu.  Similarly, students interested in graduate research opportunities through this project should contact the lead PI.

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