The global oceanic dissolved organic matter (DOM) pool holds approximately as much carbon as can be found in atmospheric CO2. The compounds making up this huge carbon reservoir are still poorly characterized on a molecular level. Consisting mainly of C, H, N, O, P and S, they provide a major nutrient source for microbial life in the oceans. Most molecules are small enough to be directly taken up by bacteria who can obtain basic building blocks for growth as well as energy by oxidation of the DOM.
So far, little is known about the molecular composition of DOM and its relation to the prevailing microbial community of the Arctic Ocean. In particular, Svalbard fjord systems with their freshwater input from glacial meltwater and rivers have not been studied in this respect. We hypothesize that a coupling exists between molecular composition of DOM and microbial community composition.
During a Svalbard cruise in August 2010, we sampled the water column of four different fjords (Smeerenburgfjorden, Kongsfjorden, Van Meijenfjorden, Van Keulenfjorden) for a detailed analysis of molecular DOM composition as well as for the characterization of associated microbial communities. The sampling was conducted in different water depths from the freshwater inflow at the head towards the more marine mouth of the fjords. Dissolved organic carbon (DOC) was measured as a bulk parameter, while solid phase extracts of DOM were obtained and analyzed with FT ICR MS for organic chemical characterization of the water masses. Denaturing gradient gel electrophoresis, followed by sequencing of bands was used to examine the microbial community composition.
Applying these methods in combination with statistical tools, we address the following questions:
- Does freshwater input change the molecular fingerprint of the DOM in the fjords?
- Can we link molecular DOM composition to microbial community composition?

Sources and fate of dissolved black carbon in Svalbard fjords

Black carbon, defined as thermogenic organic matter, was first observed in the dissolved organic matter pool by Dittmar & Koch (2006). The condensed polyaromatic molecules are generally considered resistant to microbial degradation, but mass balances suggest that they are degraded in the environment.
On Svalbard, the input of black carbon from anthropogenic sources is limited and we hypothesize that erosion of coal by glaciers is a major source of black carbon to the water column and sediments of Svalbard fjords.
Samples of the 2010 cruise (see above) were analyzed for concentration and composition of dissolved black carbon. Complementary biotic and abiotic incubation studies on dissolution and degradation of coal were carried out under controlled conditions in the lab.
The main research questions of this project are:
- Can we trace the influx of coal derived DOM (black carbon) to Svalbard fjords?
- Can we identify biotic and abiotic sinks for dissolved black carbon in Svalbard fjords?

Dittmar & Koch (2006) Thermogenic organic matter dissolved in the abyssal ocean. Marine Chemistry 102: 208-217

Microbial production, degradation and transformation of DOM

Phytoplankton are the major producers of DOM in the ocean. While the bulk of freshly produced DOM is microbially degraded, a small part is resistant and can be exported to the deep ocean, where it persists for thousands of years and comprises one of the Earth’s major carbon reservoirs. Several processes such as photodegradation, microbial alteration and hydrothermal cycling are possible contributors to the formation of the refractory DOM. It is still largely unknown why this huge nutrient pool remains unused by the microbial community.
In a long-term mesocosm study, a phytoplankton bloom was induced by addition of a North Sea inoculum containing natural assemblages of phytoplankton and bacteria to DOC-free but nutrient-rich seawater medium. The degradation of the freshly produced organic matter and thus the production of refractory DOM by the microbial community is followed over the course of at least 12 months. All parameters including phytoplankton composition, DOC content, solid phase extracted DOM, nutrients, microbial cell counts and community composition are analyzed at regular intervals.
Additionally, water from bloom and non-bloom situations was sampled for the same parameters during a cruise to the North Sea in July 2011. Results from this environmental study will be interpreted with regard to the outcome of the mesocosm incubations.
The main questions underlying this project are:
- Can the reworking of fresh organic matter by microbes alone produce DOM with molecular fingerprint similar to deep sea DOM?
- Can we observe this process under controlled laboratory conditions within a timeframe of approximately one year?
- Can we find similarities between mesocosm DOM and the in situ situation in the North Sea?