Dissolved organic matter (DOM) in the ocean is one of the largest carbon reservoirs on Earth (700 Gt), similar in size to atmospheric CO2. Despite its importance for global element cycles, our understanding of sources and sinks, composition and dynamics of the DOM pool is still limited. Some compounds of DOM (e.g. amino acids and carbohydrates) are highly dynamic and are turned over rapidly (minutes to days in the upper layers of the water column). In contrast, a large fraction of the DOM pool is exported to the deep sea, where it resides for thousands of years (refractory DOM). DOM consists of a complex mixture of thousands of different molecules most of them in very low concentrations (pico- to femtomolar), making DOM one of the biggest challenges in analytical chemistry.
DOM serves as a major source of nutrients and energy for microbial life in the ocean. We hypothesize that the microbial community and the composition of DOM are closely interrelated and that the microbial imprint in DOM can be traced through the major deep water masses back to its main origin in the microbially most active surface layers. The main research questions of this project are:
(1) Can we identify specific links between the microbial community structure and the molecular composition of DOM?
(2) Is there a universal imprint of microbial alteration in refractory DOM?
(3) Can we identify oceanic regimes based on DOM molecular fingerprints?
Samples will be taken during three RV Polarstern cruises (ANT XXVIII -2/ -4/ -5) to the Southern and the Atlantic Ocean during the austral summer 2011/2012. The Southern Ocean is characterized by close proximity of different water masses and is highly dynamic with respect to its microbial community structure and DOM composition. Main objective of the project will be a detailed molecular characterization of DOM in natural water samples from the Southern Ocean. Sampling will be carried out in close cooperation with microbiologists of the DFG-funded Collaborative Research Centre (SFB/TRR51) on the Roseobacter clade, who will work on the same water samples. That way, we will have access to complementary information on abundance and composition of prevailing microbial communities.
The project is funded by the German Research Foundation (DFG) in the frame of the Priority Programme “Antarctic Research”.
Microbial degradation of natural marine dissolved organic matter: A chemostat experiment
A significant fraction of marine dissolved organic matter (DOM) resists rapid microbial degradation and accumulates in the ocean for thousands of years. It is not clear why microorganisms do not use this huge pool of carbon, energy and nutrients. One hypothesis is that the extremely low concentration of individual molecules limits their degradation. We tested this hypothesis in chemostat experiments, where we provided different concentrations of natural DOM, and monitored quantitative and qualitative changes in DOM and microbial communities.