MAIN RESEARCH INTERESTS

• Marine organic biogeochemistry

• Origin and stability of dissolved organic matter

• Production of refractory organic matter by microorganisms

• Microbial biomarkers

General research questions and approaches

Dissolved organic matter (DOM) in the ocean contains as much carbon as the earth atmosphere. This huge pool of energy- and nutrient-rich compounds provides an important base for microbial life in the water column. But only part of the DOM is rapidly turned over by microorganisms. A significant fraction of DOM resists rapid microbial degradation and accumulates in the oceans over thousands of years. The question is: Why?

One hypothesis is that the stability of DOM is intrinsic and due to specific molecular structures. We know that even freshly produced DOM contains molecules with resistant structures that are likely to persist in the ocean for thousands of years, e. g. bacterial cell wall components or thermogenic material. Then again, we also know that most of the dissolved molecules in the ocean are small enough to be directly taken up by bacteria. Apart from resistant molecular structures there must be additional factors that influence the bioavailability of DOM, e. g. the low concentration of individual molecules.

An important step to identify the reasons for DOM stability is its molecular characterization. So far, only a tiny fraction of bulk DOM is characterized at the molecular level. We use different analytical approaches which provide complementary information on the composition of DOM. Chemical approaches include ultrahigh resolution mass spectrometry (FT-ICR-MS, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry) to determine molecular formula of individual molecules in bulk DOM and liquid chromatography (UPLC®, Ultra Performance Liquid Chromatography; HPLC, High Performance Liquid Chromatography) to separate and quantify specific known compounds. As microorganisms play an important role in production and processing of DOM, we combine chemical and microbiological approaches to elucidate the driving forces of DOM dynamics in experimental and natural systems.