Dissolved organic matter (DOM) contains as much carbon as all living biomass on the continents and oceans combined and plays a major role in global biogeochemical cycles. As bacteria can assimilate and respire only dissolved molecules, DOM represents the main mediator of energy flux in marine and freshwater ecosystems. While most of the molecules released by phytoplankton at the surface are respired in seconds to decades, a fraction of DOM escapes remineralization and can persist in the ocean for millennia. As these molecules carry the signatures of their source and subsequent journey through the marine environment, they parallel the sedimentary record as an information-rich set of tracers. Moreover, the molecular diversity of the marine DOM pool poses a metabolic challenge to microbial communities that rely on its utilization. Using interdisciplinary approaches to couple global scale analyses of DOM with microbial and molecular level research is important to fully understand this important pool of carbon. For this session, we invite contributions from all areas of research on DOM biogeochemistry, including both empirical and modeling studies. We encourage contributions that apply innovative analytical approaches, or identify novel concepts, fundamental challenges and the future directions of this fast growing field of research.
Lead Organizer: Sinikka Lennartz, University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment (sinikka.lennartz@uni-oldenburg.de)
Co-organizers:
Sarah Bercovici, University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment, Germany (sarah.bercovici@uni-oldenburg.de)
Jessika Fuessel, University of Oldenburg, Institute for Chemistry and Biology of the Marine Environment, Germany (jessika.fuessel@uni-oldenburg.de)
Brett Walker, University of Ottawa, Department of Earth and Environmental Sciences, Canada (bwalker3@uottawa.ca)
Taichi Yokokawa, Agency for Marine-Earth Science and Technology, Japan (taichiyokokawa@gmail.com)
Presentations
10:30 AM
Deep-sea microbial activity under in situ hydrostatic pressure conditions (5110)
Tutorial/Invited: Invited
Primary Presenter: Chie Amano, University of Vienna (chie.amano@univie.ac.at)
Deep-sea microbes in the global ocean strive under low temperature and high hydrostatic pressure conditions. Microbial activity, however, have been largely measured under atmospheric pressure conditions. In this study, we determined prokaryotic heterotrophic activity under in situ conditions using an In Situ Microbial Incubator (ISMI) and compared the in situ activity with that under atmospheric pressure conditions. The ISMI was deployed from epi- to bathypelagic depths in the Atlantic, Pacific and Southern Ocean. The bulk activity under in situ conditions was increasingly inhibited with increasing hydrostatic pressure. At 4000 m depth, the bulk heterotrophic prokaryotic activity under in situ hydrostatic pressure was about one-third of that measured in the same community under atmospheric conditions. Single-cell analysis showed that about 85% of bathypelagic prokaryotes were piezotolerant, while piezosensitives and piezophiles comprised rather small fractions. Metaproteomics revealed their taxonomically characteristic survival strategies at meso- and bathypelagic depths. Piezosensitive bacteria likely originating from the upper water column increased their activity more than 100-fold upon depressurization. Thus, the higher bulk activity of the deep-sea prokaryotic community measured upon depressurization is due to a small yet highly active fraction of the deep-sea prokaryotic community. Taken together, the heterotrophic prokaryotic activity in the deep-sea is substantially lower than hitherto assumed with major impacts on the carbon budget of the ocean’s interior.
10:45 AM
QUANTIFYING THE IMPACT OF PROTEORHODOPSIN ON ENERGY AND CARBON FLUXES IN THE NORTH ATLANTIC OCEAN (5369)
Primary Presenter: Carsten Lackner, Technische Universität Berlin (carsten.vick@tu-berlin.de)
Proteorhodopsin enables marine bacteria to gain energy from light. Metagenome surveys have shown that proteorhodopsin is prevalent among bacterioplankton and found in up to 80 % of genomes. Pigment measurements in oligotrophic systems suggest significant energy uptake rates rivaling Chlorophyll based phototrophy. We hypothesize that explicitly accounting for the use of light as energy source for heterotrophic bacteria challenges our understanding of the marine carbon cycle. We developed a model linking the proteorhodopsin phototrophy to cell maintenance of heterotrophic bacteria and parameterized it using lab experiments. The model is implemented as a submodule in the full-scale ecosystem model ERSEM. Physical ocean models GOTM and NEMO simulate results for different regions and light regimes across the North Atlantic. In surface waters, results show relatively higher bacteria concentrations leading to an enhanced microbial carbon pump, producing more refractory organic carbon, and an enhanced bacteria-zooplankton carbon flux than previously predicted (and vice versa in deep layers). Bacteria maintenance energy saved from proteorhodopsin phototrophy is quantified to be one order of magnitude less than the energy gained from photosynthesis in coastal regions while this ratio increases towards the open ocean regions eventually reaching parity and exceeding 1. Our result will offer a possible explanation for high DAPI counts in surface waters of oligotrophic systems and could present a new approach to spatial and temporal patterns of dissolved organic carbon turnover.
11:00 AM
Significant organic carbon acquisition by Prochlorococcus in the oceans (7312)
Primary Presenter: Daniel Sher, University of Haifa (dsher@univ.haifa.ac.il)
Marine phytoplankton are responsible for about half of the photosynthesis on Earth. Many are mixotrophs, combining photosynthesis with heterotrophic assimilation of organic carbon but the relative contribution of these two carbon sources is not well quantified. Here, single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea are obtaining only ~20% of carbon required for growth by photosynthesis. Consistently, laboratory-calibrated evaluations of Prochlorococcus photosynthesis indicate that carbon fixation is systematically too low to support published in situ growth rates in the deep photic layer of the Pacific Ocean. Furthermore, agent-based model simulations show that mixotrophic cells maintain realistic growth rates and populations 10s of meters deeper than obligate photo-autotrophs, deepening the nutricline and Deep Chlorophyll Maximum by ~20 m. Time-series of Prochlorococcus ecotype-abundance from the subtropical North Atlantic and North Pacific suggest that up to 30% of the Prochlorococcus cells live where light intensity is not enough to sustain obligate photo-autotrophic populations during warm, stratified periods. Together, these data and models suggest that mixotrophy underpins the ecological success of a large fraction of the global Prochlorococcus population and its collective genetic diversity.
11:15 AM
FLUORESCENT ORGANIC MATTER DYNAMICS DURING MESOPELAGIC PARTICLE DEGRADATION: EXPERIMENTAL INSIGHTS USING NATURAL PHYTO- AND BACTERIOPLANKTON ASSEMBLAGES (6407)
Primary Presenter: Miguel Cabrera-Brufau, Institute of Marine Sciences (ICM-CSIC) (cabrera@icm.csic.es)
Phytoplankton-derived particles represent one of the main sources of organic matter (OM) for marine systems, fuelling heterotrophic life and sequestering C in the ocean interior. The solubilization of sinking particles and the production of recalcitrant dissolved organic matter (RDOM) linked to microbial activities are two of the main processes influencing the ocean C storage capacity. These processes depend on the seasonally-variable makeup of phytoplankton assemblages, however, our understanding of this relationship is hindered by the difficulty of relating the processing of OM at depth with its phytoplanktonic source. To better understand the effects of the phytoplanktonic origin of particles on their degradation process, we performed three experiments using particles from the surface NW Mediterranean covering a wide range of phytoplankton assemblage compositions. The particulate material was added to mesopelagic water containing natural microbial communities and incubated in the dark during 25 days while monitoring particulate and dissolved organic carbon, and microbial concentrations. In addition, using fluorescence spectroscopy, we were able to track the labile and recalcitrant fluorescent fractions of both the dissolved and total (particulate + dissolved) OM pools. We will describe the fluorescent OM dynamics during the degradation process and discuss the results in relation to the source phytoplankton composition, with special focus on the processes of particle solubilization and production of RDOM within the context of the biological and microbial pump efficiencies.
11:30 AM
Characterization of coral reef primary producer exometabolites (7352)
Primary Presenter: Andreas Haas, Royal Netherlands Institute for Sea Research (andreas.florian.haas@gmail.com)
Marine DOM constitutes one of the most complex chemical mixtures on earth containing hundreds of thousands of different compounds. In nearshore systems like coral reefs metabolites exuded by primary producers comprise a significant fraction of this marine DOM pool. To learn more about the chemical composition we performed untargeted molecular analysis of exudates released by coral reef primary producers (corals and algae) using liquid chromatography–tandem mass spectrometry. Of 10,568 distinct ion features recovered from reef waters, 1,667 were primary producer exudates; the majority (86%) of these exudates were organism specific, reflecting a clear divide between coral and algal exometabolomes. The stoichiometric analyses of the exudates revealed a significantly reduced nominal carbon oxidation state of algal- compared to coral exometabolites, illustrating an ecological mechanism by which algal phase shifts engender fundamental changes in the biogeochemistry of reef biomes. Coral exometabolomes were enriched in diverse sources of nitrogen and phosphorus, including tyrosine derivatives, oleoyltaurines, and acyl carnitines. In contrast DOM released by algae was dominated by nonnitrogenous compounds, including diverse prenol lipids and steroids. Additional experiments indicate that exudates, specifically exudates unique to the respective treatment, were the main substrate used by heterotrophic microbes exposed to the respective exometabolome. This data provides molecular-level insights into biogeochemical cycling on coral reefs and illustrates how changing benthic cover on reefs influences reef water chemistry with implications for microbial metabolism.
11:45 AM
Correspondence between DOM molecules and microbial community in the ocean (6246)
Primary Presenter: Qiang Zheng, Xiamen University (zhengqiang@xmu.edu.cn)
The dissolved organic matter (DOM) pool and microbial communities are highly diverse in the aquatic environment, and their interactions play critical roles in regulating biogeochemical cycles. Heterotrophic bacteria are the important conduit for shaping the marine DOM pool via assimilation, transformation and release of organic molecules. Meanwhile the changes in bioavailable organic substrates can also affect the composition of bacterial community. Both microbial communities and organic substrates could influence the microbial produced DOM, and the experiments reveal that the microbial complexity was more important than the substrate in shaping the molecular composition of DOM. Further, the DOM composition played a larger role in shaping the microbial compositions than the concentrations of DOM and inorganic nutrients. The co-occurrence network analysis revealed that complex substrates resulted in higher molecular diversity of the microbial produced DOM, sustained a higher microbial diversity, and maintained a more diverse association between microbial communities and DOM molecules. In addition, the chemical composition of microbial produced DOM demonstrated that CHO, CHON and CHOS containing molecules were enriched in the glucose, GBT and DMSP experiments, respectively. The spatiotemporal signatures of microbial communities and DOM composition suggested microorganisms likely transformed the DOM from a relatively high (>400 Da) to a low (<400 Da) molecular weight, corresponding to an apparent increase in overall aromaticity. K- and r-strategists exhibited different correlations with two-size categories of DOM molecules owing to their different lifestyles and responses to environmental nutrient conditions. A comparison of the environmental variables and DOM composition with the microbial communities showed that the environmental/DOM variations played a more important role in shaping the microbial communities than vice versa. These results shed light on the interactions between microbial populations and DOM compounds at the molecular scale, improving our understanding of microbial roles in marine biogeochemical cycles.
SS089B The Biogeochemistry of Dissolved Organic Matter
Description
Time: 10:30 AM
Date: 5/6/2023
Room: Sala Palma