The ocean is one of the largest carbon reservoirs on Earth and therefore plays a key role in the global carbon cycle. At present, the ocean absorbs 30% of anthropogenic CO2, and it contains an amount of carbon in the form of oceanic dissolved organic matter (DOM; 660 Gt) similar to that in atmospheric CO2 (829 Gt). A net oxidation of only 1% of the seawater DOM pool in one year would be enough to produce a CO2 flux larger than that coming annually from fossil fuel combustion. Even though the importance of oceanic DOM for the global carbon cycle is well recognized, major gaps in our understanding of the chemical composition and the cycling of individual compounds persist.
Phytoplankton are the main source of DOM in the open ocean. During growth and decay, phytoplankton release up to 40% of primary production as DOM. This DOM release, composed mostly of labile DOM compounds, drives the linkage between phytoplankton and heterotrophic prokaryotes (HP), main DOM remineralizers in the sea. Phytoplankton-derived DOM is mostly composed of dissolved carbohydrates, but also of lipids and proteinaceous compounds. However, the amount of DOM released and its exact composition varies greatly among phytoplankton species and also on environmental factors such as nutrient limitation, solar radiation or temperature. On the other hand, roughly 50% of carbon that is fixed by photosynthesis in the ocean is channeled by heterotrophic prokaryotes (HP), either to produce biomass (HP production) or CO2 (HP respiration). Less known, however, is the role of HP as organic matter sources. The so-called microbial carbon pump states that HP consume labile DOM and produce refractory DOM, this is, compounds that resist further remineralization and are thus stored in the ocean for thousands of years, thus representing an important carbon sequestration process. Despite these recent insights, the underlying mechanisms that lead to the production of DOM by bacteria and its persistence in the ocean are still unclear.
The IMBERSea student will join our research group and help better understand the main drivers of DOM production by microbes in the ocean. This work would include:
- Single bacterial strains culturing and quantification/characterization of released DOM. The challenge in the project will be to recover enough DOM for all chemical analyses. For this reason, the IMBERSea student will be in charge of optimizing the culturing conditions and DOM extraction protocols in the experiments. She/he will grow different bacterial strains using a range of initial substrate concentrations (200 to 2000 µm C as glucose) to test the linearity of the response (is the quantity and quality of produced DOM linearly related to the initial concentration?), as well as to test the best method to separate cells from DOM (centrifugation vs. different filtration protocols).
- Helping in sample collection and processing of DOM and microbial activity/diversity in a timeseries study (microbial observatory “MOLA”, sampled once a month)
- Assessing the role of nutrient limitation (nitrogen vs. phosphorus) on DOM production/consumption by Mediterranean Sea microbes via experimental incubations using in situ communities
The research group also works in other research projects (e.g. DOM release by diazotrophic cyanobacteria, impact of DOM on photoheterotrophy) where the student could eventually collaborate
Therefore, the IMBERSea student will be introduced to laboratory work with tasks such as strain and natural communities culturing, microbial abundance (flow cytometry), DOM processing and characterization. DOM characterization includes its optical properties (DOM absorbance and fluorescence), quantification of amino acids and carbohydrates (colorimetry, HPLC) and C:N:P ratios. The student will also have the opportunity to participate in the lab seminars, meetings and journal club. We look for a motivated student, interested in microbial oceanography and willing to perform laboratory analyses in a rigorous, clean and organized manner.
