Abstract

The nitrogen cycle is involved in key aspects of the biogeochemistry of marine organic matter and therefore intervenes in the chemical composition of any planktonic microbial community. At the center of this cycle, ammonium (NH4+) is produced by microbial activity during the degradation of dissolved organic matter in a process called ammonification. Tightly linked to this process, a diverse nitrifying community composed by archaea and bacteria ensures the production of nitrate by oxidizing ammonium in the euphotic and deep layers of the ocean. Although a seemingly lineal relationship, this link between organic matter degradation and inorganic nitrogen regeneration holds several unanswered questions for oceanography. How does dissolved organic matter originated by specific functional groups influence the potential for ammonium release via ammonification? What is the potential for ammonification of anthropogenic organic matter from antropogenic origin and how does it compare with “natural” DOM? Can differential ammonification rates alter the community structure of ammonium oxidizers in the water column (archaea vs bacteria)? One of the main concerns in the current research of N cycle is the large fraction of antropogenic N that is annually introduced in marine environments. One of the most conspicuous sources is aquaculture through fish excretion, uneaten food degradation and fluxes of associated fauna. The DOM annually generated in salmon farming for instance can exceed the contribution of carbon of an entire phytoplankton bloom. However, there is almost no information of the fate of aquaculture-derived DOM and its potential for nitrogen generation. Because the lability of any kind of DOM is also governed by photochemical processes in the euphotic zone, the cycling of nitrogen (N) and ammonification rates can show specific responses to solar radiation. The Chiloe Caucahue area in northern Patagonia (42.5ºS) is a natural laboratory for exploring the effect of photochemical degradation of DOM on marine microbial communities and to assess the problem of antropogenic DOM inputs. This area is heavily impacted by solar radiation (UV Radiation, 280-400 nm, can reach the first 2 to 10m of the water column) and by aquaculture activity, which probably affects the quality of DOM being released to the water column. Through substrate availability and adaptability to light intensity, the photochemical transformations of DOM and the subsequent changes in its composition and lability are closely linked to the capacity of microbial groups to use nitrogen in the euphotic zone. is therefore important to determined the rates of ammonium regeneration as a function of irradiation stress and compared it to net biological ammonification. Here we propose to assess the link between the quality of DOM (as defined by its origin) and net ammonium regeneration in the water column. By examining DOM-case studies of diatom, diazotrophic and anthropogenic footprint dominance, we will evaluate the rates of ammonium production and its oxidation into nitrate, hence the capacity of ecosystems to channel DIN inputs. Simultaneously we will deepen our understanding of the ecology and interactions between archaea and bacteria ammonium oxidizers. We will apply a multidisciplinary strategy combining laboratory and field experiments, hence linking culture-based experimentation to natural marine communities. Biogeochemical (e.g. discrete determination of NH4 and NO3, isotopic enrichments and natural isotope studies) will be complemented with state of the art molecular approaches (next-generation sequencing of barcode 16S rRNA genes , Real-Time PCR on functional and phylogenetic markers in DNA and cDNA). We expect our results to contribute to the understanding of the implications of DOM reactivity for specific and crucial microbial functional groups and help assessing the current debate on the relevance of bacterial versus archaeal nitrification from a multiscale perspective (from the ecosystem to the cell). We also believe that this research can represent a turning point in our conception of photodegradation and nutrient cycling in the upper water column. Because this research will also touch aquaculture related problems, its results are likely to be of interest to policy makers and general public.