Prokaryotic activity in darc ocean mixing zones

Frontal systems in the surface layer of the ocean are well known for their elevated biological activity. Such boundary systems, however, are not only detectable as vertical boundary layers in the surface ocean, but may also exist in the deeper layers of the ocean as mixing zones of water masses. These deep ocean fronts or ecotones might be particularly important at ocean ridges, canyons and fracture zones. Based on preliminary measurements, we hypothesize that mixing of deep-water masses create `hot-spots` in prokaryotic diversity and activity with a significant influence on the overall biogeochemical cycles of the dark ocean, similarly to the well studied ecotones. Prokaryotic activity in the mixing zone and the parent water masses of the Charlie-Gibbs Fracture Zone in the North Atlantic will be assessed by determining heterotrophic biomass production, ectoenzyme activity and respiration as well as gene expression. Collectively, these data should allow for an in-depth view on the heterogeneity of prokaryotic organic matter cycling in the deep ocean by shedding light on the microbial ecology of deep-water boundary layers. As these mixing zones of major water masses are abundant, they might significantly stimulate overall matter cycling in the dark ocean.

The dark oceans water column (depth deeper than 200 m) is not homogeneous but composed of several waterbodies that lay on top of each other. These waterbodies, also called water masses, are formed in the surface of particular regions of the ocean and initially have very distinct features with a typical salinity, temperature, gas concentration and nutrients. Due to turbulences in the dark ocean by waves and other physical mechanisms, these water masses are able to mix with each other and thus the environmental conditions for the dark ocean microbes might drastically change. Likewise, the surface ocean has been divided into different oceanic provinces that share a set of similar physical and biological features. The idea of such a classification is to aid sampling in the vast space of the ocean, because it could make it possible to sample at few distinct places in the oceans and allow the extrapolation of the results from these stations to a larger area. While this approach seems to work for phytoplankton it is not clear as of yet whether such a classification into provinces is also relevant for bacteria and archaea, that make up the majority of microbes in the dark ocean. Based on model experiments we hypothesized that mixing of water masses leads to measurable variability in the activity of bacteria and archaea in the dark ocean by either enhancing or retarding the biomass productivity of the cells. For the North Atlantic we found that water mass mixing has a large influence on the distribution of nutrients and other organic matter but a relatively weak influence on the extent of the microbial activity. We also highlight water masses as strong predictors of the microbial community composition, i.e. each water mass harbors a relatively typical microbial assemblage. Still, we were able to decipher an impact of the provinces, defined for the surface ocean, on the environment and the dark ocean diversity of microbes in different deep water masses. Thus, our results suggest a far more complex dark ocean microbial biogeography than hitherto assumed.