Biogeochemistry of marine dissolved organic matter: molecular composition, reactivity and new methods

Abstract

Dissolved organic matter (DOM) is an ultimate chemical product of all life on earth. It integrates energy, carbon dioxide and nutrients into a vast compositional and structural variety of molecules further modified by biological, chemical, and physical processes. In the ocean, organic matter production depends mainly on the photosynthetic activity of autotrophs and most of it is immediately consumed and respired by heterotrophs. Some of this fresh organic matter, however, escapes immediate turnover and accumulates in dissolved form in the entire water column. During isopycnal transport and seasonal convective overturn, microbial, photochemical, and physical processes remove most of the fresh DOM. The remaining organic matter is an old, chemically poorly characterized heterogeneous mixture of small, partially oxidized and unsaturated molecules: refractory DOM. The main topic of this thesis is the chemical characterization of DOM: elemental composition and reactivity with regard to environmental boundary conditions as well as causalities of persistence. All studies involved substantial chemical and physical gradients of temperature, pressure, salinity, irradiation, biological communities, and nutrients. These gradients allowed for testing the main research hypotheses with different end members to obtain functional relationships between the physico-chemical variables and the observed properties of DOM. Different methods were applied to achieve these aims. High resolution inorganic and organic mass spectrometry, chromatography, statistical analysis and modeling were performed on samples obtained from oceanic research cruises. Additional seasonal surveys in an estuarine system and experimental setups addressed the influence of the various physico-chemical boundary conditions on the chemical composition and phase distribution of DOM. The most comprehensive study of this work included more than 200 samples from the tropical to the polar open ocean and from the surface to the seafloor and represents the so far largest consolidated dataset for ultrahigh resolution organic mass spectrometry in the ocean. A method was established that enabled for the first time separation and quantification of organic phosphorus and sulfur in marine DOM in a coupled chromatography mass spectrometry system. It was shown that the compositional diversity of DOM, i.e., the contributions from the heteroatoms phosphorus and sulfur, was reflected in the chemical properties of the molecules as revealed by polarity separation. Further, the method was shown to be applicable for determining metal ions that are also part of the chemical entity of DOM. However, not all investigated metal ions showed a strong and selective affinity for organic matter, e.g., uranium. A rare isotope of uranium, 236U, determined for the first time in an oceanic depth profile, was demonstrated to be a suitable transient tracer in oceanographic studies, reflecting an anthropogenic marker for water mass circulation. Very different compounds, surface active sulfonic acids, were identified as part of the total DOM pool in a sea surface microlayer study. Although sulfonic acids are widely known as potential contaminants in surface waters, this study demonstrated the analytical capability of ultrahigh resolution organic mass spectrometry and fragmentation to study thousands of DOM molecules and their responses to changing physico-chemical conditions, e.g., the ionic strength of the aqueous phase. An even deeper insight into the composition and long-term transformation of DOM was achieved by comparing the molecular signatures of DOM samples from the East Atlantic and Southern Ocean. Using statistical tools, it was demonstrated that distinct patterns of mass peak magnitude changes could be related to the consecutive ageing of this mixture of molecules. A modeling of the degradation rates of individual DOM molecules demonstrated that the chemical composition of the bulk DOM changes with age towards a proposed island of stability . The broad distribution of these degradation rates is proposed as an extension of the contemporary perception of marine DOM cycling and reworking. Bringing together inorganic and organic biogeochemistry as well as (molecular) microbiology to study the complex biogeochemical interactions in the ocean will be an important future research direction in marine sciences. The combined efforts from multidisciplinary research groups are a prerequisite to resolve the unanswered questions on the response of the microbial communities, the fate of anthropogenic carbon dioxide, the chemical processes and equilibria in the ocean, and its crucial feedback mechanisms in a changing climate.