Molecular composition of Southern Ocean dissolved organic matter and its relation to structure and activity of prevailing microbial communities
Final Report Abstract
In this project we aimed at a better understanding of the role of microorganisms in shaping the molecular composition of dissolved organic matter (DOM). Marine DOM contains two orders of magnitude more carbon than all living biomass in the ocean combined. Despite its importance for global element cycles, our understanding of sources and sinks, composition and dynamics of the DOM pool is still limited. DOM is the main carbon and energy source for heterotrophic marine microorganisms, and microbial production and transformation processes also contribute to the molecular composition of DOM. The Southern Ocean is characterized by close proximity of different water masses, including major global deep water masses, and spatial and temporal variability of primary productivity render the system highly dynamic with respect to DOM composition and microbial community structure. The project was based on an extended sampling campaign with RV Polarstern in austral summer 2011/2012 in the Southern Ocean (cruises ANT-XXVIII/2 and ANT-XXVIII/4) and across the Atlantic Ocean (ANT-XXIII/5). DOM was extracted from a total of 402 water samples, spanning a wide range of geographical locations and climatic regimes, and including water masses with various DOM sources and histories. Ultra-high resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) yielded a most detailed molecular fingerprint of the DOM. This unique data set provided a comprehensive molecular geography of the Southern and Atlantic Ocean. Surprisingly, the molecular DOM composition was highly similar across the major global water masses. In the deep ocean, DOM fingerprints were almost indistinguishable, indicating that ageing of DOM in the deep ocean did not introduce significant changes of the molecular DOM fingerprint. DOM from the surface mixed layer was characteristically different from deep ocean DOM. These differences can be attributed to microbial production of DOM by phyto- and bacterioplankton, and to photochemical alteration in the sunlit surface layers. We developed and successfully applied two new process-related indices that specifically trace the imprint of bioproduction and photodegradation in molecular DOM fingerprints. We found that most of the differences in molecular DOM composition across water masses can be explained by a simple two-source mixing model, where the two universal endmembers are freshly produced microbial DOM and aged deep ocean DOM. The only clear deviation from this rule was a molecular geography of nitrogen-containing compounds in DOM. Apparently there is a close coupling between the pools of available inorganic nitrogen and of nitrogen bound in DOM. In the nutrient replete Southern Ocean, DOM was enriched in nitrogen compared to the Southern Atlantic Gyre, where inorganic nitrogen supply is limited and organic nitrogen might be used more efficiently as nutrient source. This molecular signal was preserved in the Southern Ocean during deep water formation and was found throughout the water column. Consistently, in the Northern Atlantic Gyre, characterized by occurrence of nitrogen-fixing cyanobacteria, we did not observe nitrogen-depletion of DOM.
Publications
- (2012) Dissolved organic matter in the Southern Ocean: Links between molecules and microbes. 3rd Meeting of SCOR Working Group 134 The Microbial Carbon Pump in the Ocean, 26-28 August, Delmenhorst, Germany
Seibt MA, Dittmar T, Niggemann J
- (2012) Dissolved organic matter in the Southern Ocean: Links between molecules and microbes. YouMares3, 12-14 September, Lübeck, Germany. Dittmar T (2013) Invited key note: Towards a molecular (DOM) geography of the sea. Gordon Research Conference on Chemical Oceanography, 4-9 August, Biddeford, Maine, USA
Seibt MA, Dittmar T, Niggemann J
- (2012) Universal molecular features of refractory dissolved organic matter in fresh- and seawater. European Geosciences Union General Assembly, 22-27 April, Vienna, Austria
Dittmar T, Blasius B, Steinbrink C, Feenders C, Stumm M, Christoffers J, Niggemann J, Gerdts G, Osterholz H, Seibt MA, Seidel M and Vähätalo A
- (2013) Dissolved Organic Matter (DOM) – small in size but large in impact: basis of life in the world’s ocean. In: Recent Impulses to Marine Science and Engineering - From coast to deep sea: multiscale approaches to marine sciences (eds. Einsporn MH, Wiedling J, Beilfuss S), DGM publishing company: 18-28
Seibt MA, Stratmann T, Stumm M
- (2014) Invited key note: Explaining a deep-sea paradox: Starving microbes in a sea of dissolved organic substrate. Gordon Research Conference on Marine Microbes, 22-27 June, Waltham, Massachusetts, USA
Dittmar T
- (2014) Molecular geography of dissolved organic matter in the Southern Ocean. ASLO Ocean Science Meeting, 23-28 February, Honolulu, Hawaii, USA
Seibt MA, Dittmar T, Niggemann J
- (2014) Molecular geography of the Atlantic Ocean: The central and southern Atlantic. Workshop Geo-metabolomics: first steps towards a systems biology understanding of organic matter cycling in aquatic systems, 24-28 November, Delmenhorst, Germany
Seibt MA, Dittmar T, Niggemann J
- (2015) Bacterial community dynamics during polysaccharide degradation at contrasting sites in the Southern and Atlantic Oceans. Environmental Microbiology 17(10): 3822–3831
Wietz M, Wemheuer B, Simon H, Giebel H-A, Seibt MA, Daniel R, Brinkhoff T, Simon M
(See online at https://doi.org/10.1111/1462-2920.12842) - (2016) The molecular geography of dissolved organic matter in the Atlantic Ocean can largely be explained by a simple two-source mixing model. ASLO Ocean Science Meeting, 21-26 February, New Orleans, LA, USA
Seibt MA, Niggemann J, Dittmar T
