Doctoral thesis defense - Valentina Valdes

Valentina VALDES will defend her doctoral thesis on Tuesday October 31 at 14:00 - Amphithéâtre  (videoconferencing with the University of Conception, Chile)

“The biogeochemical role of zooplankton for nitrogen and phosphorus recycling in the ocean”

The interaction between metazooplankton metabolism and the microbial loop is recognized as one of the principal mechanisms controlling the quantity, composition and cycling of DOM in the sea. Zooplankton and bacteria are usually considered as separate groups despite their temporal and spatial co-existence and close interaction. Zooplankton may release dissolved organic and inorganic matter through sloppy feeding, leaching from fecal pellets and excretion. However, there is a lack of knowledge on the impact of these dissolved compounds released by zooplankton on biogeochemical cycles and on the microbial community structure. In this work, we have used an experimental approach with natural food sources and simulated in situ conditions, to determine the role that trophic interactions involving primary and secondary producers in the recycling of nitrogen and phosphorus in the pelagic system of central-southern Chile. First, we quantified the nitrogen and phosphorus compounds excreted by copepods fed with natural sized-fractioned diets. Then, we assessed the biogeochemical response of the microbial community to released nitrogen, including the nitrifying community, in terms of total and active cells, to the products excreted by copepods. Secondly, we explored the simultaneous interaction between copepod excretion and the biogeochemical response of active microbial community composition, over an oligotrophic gradient in the western tropical south Pacific (WTSP).

Our results show that DON was the main excretion product, with rates of up to 3.7 μmol L^-1 h^-1 under autumn/winter conditions. Meanwhile, in spring/summer copepods actively excreted nitrogen and phosphorus compounds, mainly in the form of ammonium and dissolved organic phosphorus (DOP), reaching excretion rates of up to 2.6 μmol L^-1 h^-1 and 0.05 μmol L^-1 h^-1, respectively. In both cases the main products excreted were closely related to the main item food available for copepods (nanoplankton and microplankton, respectively). In the autumn/winter experiments, ammonium was rapidly consumed by the microbial community. Ammonium consumption coincided with increased archaea (AOA) and bacteria ammoniaoxidizing (AOB) amoA transcript copies in copepods fed with the larger-sized fraction, whereas a different microbial community, probably heterotrophic, reacted to nitrogen input via excretion by copepods fed with the smaller-sized fraction in autumn. Moreover, during spring/summer a shift in the composition of active bacterial community was observed between periods and treatments, associated with the response of common-opportunistic seawater surface phyla Proteobacteria and Bacteroidetes.

In the WTSP, copepods contributed elevated levels of ammonium, DON and DOP highly labile. Copepod excretion can enhance the remineralization (ammonification nitrification) process and reshape the diversity and composition of the active bacterial community. These changes were characterized by shifts in Alteromonadales and SAR11 in both study regions (Melanesian archipelago and subtropical gyre) and differed substantially from the in-situ community. Enhanced bacterial activity in presence of copepods would further increase the available substrates for microbial and phytoplankton growth in a highly stratified system and provide an alternative source of phosphorus for diazotrophs blooms in the WTSP.

We concluded that nitrogen and phosphorus excreted by copepods can be used directly by microbial communities, including nitrifying microbes, providing significant remineralized nitrogen for sustaining new and regenerated production in the upper ocean of different marine ecosystems.

Thesis committee:

Pr. Stéphane BLAIN

Dr. Eric FOUILLAND

Dr. Humberto GONZALEZ 

Dr. Veronica MOLINA 

Dr. François CARLOTTI 

Dr. Camila FERNANDEZ

Dr. Fabien JOUX 

Dr. Ruben ESCRIBANO