Oxygen Minimum Zone Team

Areas of lowest oxygen in the ocean are referred to as Oxygen Minimum Zones (OMZs).  They are vast and can exert a large influence on marine organisms because oxygen is essential for aerobic life.  The largest source of oxygen to the ocean is the atmosphere. Therefore, oxygen concentrations are typically highest at the water-atmosphere interface and decrease with depth. The largest sink for oxygen in the ocean is the biology. OMZs often occur along upwelling margins – these are areas where cold, oxygen-deplete water gets transported up from the deep ocean to the surface ocean. However, these waters also bring lots of nutrients to the phytoplankton and fuel production. Therefore, as the aerobic bacteria feed on this sinking production (i.e., organic matter) just below the ocean’s surface, they further deplete the oxygen as they respire. The combined effect of oxygen-poor source waters and high oxygen demand by the bacteria can lead to these massive midwater oxygen minima.  These minima typically occur between 200 to 1,000 meters below the sea surface.

These low oxygen concentrations not only influence the organisms in the water column (i.e., pelagic organisms), but also have the potential to influence the communities on the sea floor (i.e., benthic organisms). Where OMZs intercept the seafloor, benthic faunas are known to exhibit altered animal communities, including reductions in abundance, altered composition, decreases in species richness and increases in species dominance (e.g., very few species are able to live at such low oxygen). Microbial communities also change drastically with differences in oxygen concentration, and as oxygen decreases bacterial groups appear that can breath alternatives to oxygen. Finding new bacterial diversity in undersampled environments like the OMZ can lead to finding new bacterially-produced antibiotics. The core of the OMZ off southern California is concentrated at 650 m water depth, but there is recent evidence that the depth of the upper boundary is shoaling into shallower seafloor habitats. This shoaling will alter the structure and function of microbial and animal communities along the seafloor off San Diego.  The goals of the OMZ Team are to assess the sensitivity of the microbial and animal communities to present gradients in oxygen and carbon dioxide and project how future changes in oxygen and carbon dioxide will alter San Diego’s seafloor.

To sample the seafloor, the OMZ Team will use a sediment multicore. The multicore has a steal frame and carries up to eight sampling tubes. The multicore is equipped with a piston so when it hits the ground, the tubes are slowly pushed into the mud. The mud is then sealed in the tubes by stoppers on the top and the bottom. When the sediment returns to the ship, we can use the mud for different types of analyses. We will be collecting samples to examine different components of the benthic community including bacteria, protists and different size classes of multicellular animals (meiofauana and macrofauna). Bacteria that are isolated from these samples will be tested for the production of small molecules that are active in anti-cancer and antibiotic screens. DNA from the sediments will be used to determine where along environmental gradients the bacteria produce these small molecules. We will also be looking at the chemistry within the sediments to get a sense of the local chemical environment the infaunal organisms experience, as well as looking at the general characteristics of the sediment such as grain size and organic carbon content. These physical and chemical parameters will help us interpret the observed biological trends. 


Photo: A multi-core being deployed from the A-frame off the stern (i.e., back) of a ship.  
Photo credit: Noah Brookoff

Photo: Example of a multicore tube retrieved with sediments from the seafloor. Once on the ship, we extrude the sediments from the core. This involves using a wooden platform to push the sediments up and out the top of the core. Photo credit: Noah Brookoff

Photo: Example of crude bacterial isolation plate. Sediment samples are diluted and treated with heat and desiccation in order to selectively culture antibiotic-producing actinomycete bacteria. Photo credit: Kelley Gallagher