Oxygen minimum zones.
Anthropogenic forces including agricultural runoff and the effects of climate change on ocean circulation patterns are contributing to the global expansion of 'dead zones' in the marine water column. These zones can span hundreds of meters water depth, with oxygen levels far below that needed to support animals including fish and marine mammals. Patricia has explored the methane cycle through these zones, using water samples collected at high spatial frequency. The goals of this work include pinpointing the locations and geochemistry that promote methane oxidizing bacteria. The primary study sites are the extended dead zone offshore Costa Rica, and in oxygen-depleted waters above Hydrate Ridge, Oregon.
Below: Victoria, Shana, Jake, Anne, David, and associated scientists on the 2010 Costa Rica cruise.
The 2010 cruise brought back approximately 160 filtered water column samples, collected at high spatial frequency through 12 different CTD casts. Samples were collected above the convergent margin, along the Costa Rica coast. Most of these samples were immediately processed for DNA extraction and molecular work, and some were strategically reserved in the knowledge that future technologies would allow new approaches. A primary question we asked was the role of methane oxidizers as a function of depth and the associated geochemistry of OMZs.
At left: General schematic of the concentrations of oxygen (solid line) and methane (dotted line) through water depth off the coast of Costa Rica. The heart of the oxygen minimum zone occurs from approximately 250 to 600 mbsl. In addition to lowered oxygen concentrations, this zone demonstrates slightly elevated methane concentrations. We asked if this geochemistry drives the establishment of unique marine methanotroph communities. As dead zones expand, we hope to track these communities to determine the source- or sink- strength of the ocean during global change.
The two primary planktonic marine methanotrophs, OPU1 and OPU3, identified in earlier work, show strikingly different residence patterns from one another, through all depth series analyzed. In particular OPU1 (orange, below left) is most abundant near the seafloor, where methane and oxygen levels are elevated. OPU3 (green, below left) spikes in abundance within the low oxygen core of the OMZ. ANOVA (right) through all 128 samples analyzed confirms that (low) oxygen and (elevated) methane are predictive for these clades.
We've expanded the above work, to include all 128 OMZ water column samples from Costa Rica as well as 60 water column samples from Hydrate Ridge, Oregon, to specifically probe the role of oxygen in the establishment of these lineages. When oxygen levels are below 0.16 mls -L, lineage OPU3 is more abundant than OPU1, in both environments. This suggests that oxygen is a global driver of these groups, and of their relative abundances. As dead zones expand, lineage OPU3 may represent a greater proportion of the planktonic methanotroph community.
Because we're curious...
What do these planktonic methanotrophs 'look like?' This is a natural question to ask - and surprisingly we (and other labs) have not been able to reliably cultivate either OPU1 or OPU3. Several labs, including our own, continue to try with a happy heart. The recent availability of the genome of OPU3 (Li et al, Padilla et al) is a boon to this goal. Shown at left is a FISH image of an enrichment culture in which OPU3 comprised approximately 10% of the total community. Although it ain't much, we were very happy to get this small glimpse of such a globally important organism. The organisms that are OPU3 are the red 'diplo' rods that have blue dots near the division plate. The other organisms (blue, green) are common marine species including Pseudoalteromonas.
Current and future directions.
* OPU3 appears to play a role in partial denitrification in methane - enriched, low oxic waters (see Padilla et al, 2017). We'll target sampling and geochemical analyses in line with this new knowledge.
* We'd still love to cultivate these globally important methane consumers. Samples from Costa Rica will be enriched using the moderately successful strategies learned from past attempts, in conjunction with an eye towards more appropriate cultivation vessels.
* We'd still love to cultivate these globally important methane consumers. Samples from Costa Rica will be enriched using the moderately successful strategies learned from past attempts, in conjunction with an eye towards more appropriate cultivation vessels.