Pressure effects, microbes

Heterotrophic bacteria and archaea (here: microbes) are critical drivers of the ocean’s biogeochemical cycles, active throughout the depth of the ocean. Their capabilities and limitations help determine the rates and locations at which carbon and nutrients are regenerated, as well as the extent to which organic matter is preserved (Hedges 1992). In the deep ocean, at bathy- and abyssopelagic depths (ca. 1000-6000 m), these communities are dependent upon the sinking flux of particulate organic matter (POM) from the surface ocean (Bergauer et al. 2018). This dependence means that heterotrophic microbial communities must produce the extracellular enzymes required to solubilize and hydrolyze high molecular weight (HMW) POM to sizes substrates suitable for cellular uptake. A recent global-scale investigation of deep-sea microbes in fact found that the genetic potential for exported (extracellular) enzymes among bacteria in deep waters was far greater than for communities in surface or mesopelagic waters (Zihao et al. 2020). We have new evidence that a substantial fraction of bacteria in bottom water from the North Atlantic Ocean use a specialized set of extracellular enzymes to rapidly take up HMW polysaccharides (Giljan et al. 2021), a substrate processing mechanism that would not be detected with the low molecular weight substrates used in most prior studies of microbial activity in the deep ocean (Nagata et al. 2010). However, our study – as well as the majority of prior investigations of deep sea microbial communities (e.g., Yokokawa et al. 2013) – was carried out with bottom water incubated at atmospheric pressures. Although pressure profoundly affects the activities of marine microbes (Tamburini et al. 2013), most investigations of microbial communities of the deep sea are carried out at atmospheric pressure (Nagata et al. 2010), due to limited availability of specialized equipment. Through our collaboration with the Danish Center for Hadal Research, we are positioned to characterize the effects on microbial communities and their extracellular enzymes of pressures characteristic of bathy- and abyssopelagic depths. At sea and in the lab, we will compare the effects of depressurization on the activities of enzymes produced by microbial communities of the deep ocean, as well as the effects of high pressure on surface-water derived enzymes and communities. Fieldwork will take place in Danish coastal waters, well as in the open North Atlantic and Pacific Oceans. Using pressurization systems and in situ incubations, we will measure hydrolysis rates of peptides and polysaccharides, two of the major classes of marine organic matter (Hedges et al. 2001). In collaboration with colleagues from the Max Planck Institute for Marine Microbiology, we will investigate possible pressure effects on selfish uptake of polysaccharides. These studies will greatly advance our understanding of pressure effects on the initial step of organic matter degradation by heterotrophic microbial communities of the ocean.