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Archive for currents

Untangling microbial evolution in the oceans: How the interaction of biological and physical timescales determine marine microbial evolutionary strategies

Posted by mmaheigan 
· Wednesday, March 11th, 2020 

Marine microbes are the engines of global biogeochemical cycling in the oceans. They are responsible for approximately half of all photosynthesis on the planet and drive the ‘biological pump’, which transfers organic carbon from the surface to the deep ocean. As such, it is important to determine how marine microbes will adapt and evolve in response to a changing climate in order to understand and predict how the global carbon cycle may change. However, we still lack a mechanistic understanding of how and how fast microorganisms adapt to stressful and changing environments. This is particularly challenging due to the diversity of organisms that live in the ocean and the dynamic nature of the oceans themselves—microbes are at the whim of ocean currents and so get transported large distances fairly quickly. For the first time, a new study published in PNAS provides a prediction on the controls of microbial evolutionary timescales in the oceans.  The authors hypothesize that there is a trade-off for marine microbes between ability to evolve to long-term changes versus respond to shorter term variability. Their results suggest that marine microbes commonly experience conditions that favor a short-term strategy at the cost of long-term adaptation. This trade-off determines evolutionary timescales and provides a foundation for understanding distributions of microbial traits and biogeochemistry.

Illustration of trade-off in evolutionary strategy as a function of environmental variability. Trajectories where individuals perceived high environmental variability (a & b) exhibited low selective pressure for any one environment but allowed for high environmental tracking. Trajectories where individuals perceived a more stable environment (c&d) had high selective pressure for ’new environments’ (high probability of a selective sweep) but these individuals exhibited poor environmental tracking. Panels a and c show trajectories where selective sweeps were highly probable (red), likely (yellow), and had a low probability (grey). Panels b and d show the estimated persistence of non-genetic modifications necessary for environmental tracking, where grey indicates unrealistically long timescales.

 

Authors:
Nathan G. Walworth (University of Southern California)
Emily J. Zakem (University of Southern California)
John P. Dunne (Geophysical Fluid Dynamics Laboratory, NOAA)
Sinéad Collins (University of Edinburgh)
Naomi M. Levine (University of Southern California)

The Equatorial Undercurrent influences the fate of the Oxygen Minimum Zone in the Pacific

Posted by mmaheigan 
· Tuesday, November 12th, 2019 

While the ocean as a whole is losing oxygen due to warming, oxygen minimum zones (OMZs) are maintained by a delicate balance of biological and physical processes; it is unclear how each one of them is going to evolve in the future. Changes to OMZs could affect the global uptake of carbon, the generation of greenhouse gases, and interactions among marine life. Current generation coarse-resolution (~1°) climate models compromise the ability to simulate low-oxygen waters and their response to climate change in the future because they fail to reproduce a major ocean current, the Equatorial Undercurrent (EUC). These shortcomings lead to an overly tilted upper oxygen minimum zone (OMZ) (Figure 1), thus exaggerating sensitivity to circulation changes and overwhelming other key processes like diffusion and biology. The EUC also plays a vital role in feeding the eastern Pacific upwelling region, connecting it to global climate variability.

Figure: Top: The boundary of the Oxygen Minimum Zone (OMZ) along the Equator is unrealistically tilted for current generation (coarse resolution) climate models, and improves with increased horizontal resolution. The tilt is due to a bad representation of the Equatorial Undercurrent in the coarse model, also seen in other coarse models. The exaggerated tilt of the OMZ boundary at the Equator leads to increased inter-annual variability of the depth of the upper OMZ boundary, via changes in the zonal flow (left). This phenomenon is found in most CMIP5 models (right) and could be responsible for the current inability to predict the change in OMZ extent for the next century.

A recent high‐resolution climate model study in Geophysical Research Letters significantly improved the representation of both the EUC and OMZ, suggesting that the EUC is a key player in OMZ variability. This study emphasizes the importance of improving transport processes in global circulation models to better simulate oxygen distribution and predict future OMZ extent. The results of this study imply that the fundamental dynamics maintaining this key ocean current could be categorically misrepresented in the current generation of climate models, potentially influencing the ability to predict future climate variability and trends.

 

Authors:
Julius J.M. Busecke (Princeton University)
Laure Resplandy (Princeton University)
John P. Dunne (NOAA/GFDL)

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Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.