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Description
DOMUSe project is determined to characterize microbial response to dissolved organic matter (DOM) supplied from the sediments to the near-bottom water of Baltic Sea Deeps. Our study will utilize outcomes of a parallel project, funded by Polish National Science Centre which focuses to determine the return DOM flux from sediment pore waters to near bottom waters.This project will supply an initial condition of supply rates of the DOM to the near-bottom waters. In DOMUSe, we will execute several studies, where part of the sediment and near-bottom waters will be enclosed in the anoxic atmosphere for a period of time. This way, we would like to assess the rates of microbial processes occurring in the near-bottom waters. We have chosen several proxies that will allow us to trace changes in quality and quantity of DOM, including chemical composition and optical DOM properties. We will use optical properties of DOM to assess changes in DOM quality. Optical properties of DOM, such as fluorescence, via statistical multivariate analyses, will allow to assess the DOM fractionation between humic-like (microbially reworked and mainly refractory) DOM and amino acid-like DOM, which is mainly associated to rapidly utilizable fresh DOM. We will use siderophores, ferric iron specific small organic molecules, as a proxy of microbial activity. Their production rates were previously shown to be affected by bioavailability of dissolved organic matter. Their production was previously associated to fresh labile organic matter sources, such as phytoplankton blooms, however, they were also found in the sediment pore waters. The measurements of oxygen respiration by bacteria and microbial nutrient utilization will fulfil the measurements of the microbial response to DOM release from sediments pore waters. Bacterial abundance will be also measured overtime to access bacterial growth associated to DOM supply by the sediments.
Summary of project results
Classically, dissolved organic matter (DOM) in sediment pore waters was assumed to be not available for microbial communities to consume. This assumption was based on the fact that DOM is accumulating with sediment depth, and the consumption rates within the sediments should be smaller than the production rates of DOM. However, more recently it was suggested that sediment pore waters might potentially serve as a source of bioavailable DOM to the overlying near-bottom water layer and water column. It was also hypothesized that DOM supplied by sediments might stimulate microbial activity in the near-bottom waters. DOMUSe project aimed to characterize the microbial response to DOM supplied from the sediments to the near-bottom water of the Baltic Sea Deeps. Our study aimed to utilize outcomes of a parallel project, funded by the Polish National Science Centre which focuses on determining the return DOM flux from sediment pore waters to near-bottom waters. Our project was dedicated to utilizing those return fluxes as initial conditions of supply rates of the DOM to the near-bottom waters and also we were aiming to execute several studies, where part of the sediment and near-bottom waters would be enclosed in the anoxic atmosphere for a period of time.
Our main goal was to assess the rates of microbial processes occurring in the near-bottom waters. For this, we have chosen several proxies that were anticipated to allow us to trace changes in the quality and quantity of DOM, including chemical composition and optical DOM properties. We were planning to use optical properties of DOM to assess changes in DOM quality. We were planning to use optical properties of DOM, such as fluorescence, and statistical multivariate analyses, to assess the DOM fractionation between humic-like (microbially reworked and mainly refractory) DOM and amino acid-like DOM, which is mainly associated with rapidly utilizable fresh DOM. We were planning to use siderophores (ferric iron-specific small organic molecules) analyses and trace their accumulation as a proxy of microbial activity. As siderophores are small chained organic molecules, that are produced by microbes to assess ferric iron, their production rates were previously shown to be affected by the bioavailability of dissolved organic matter sources, such as phytoplankton blooms, however, they were also found in the sediment pore waters. Thus, the accumulation of siderophores would give us the estimate of an increase in heterotrophic activity induced by DOM release from sediments. Furthermore, the measurements of oxygen decrease (as a proxy of respiration by heterotrophic bacteria) followed by nutrient utilization were supposed to fulfill the measurements of the microbial response to DOM release from sediments pore waters. Bacterial abundance was planned to be measured over time to assess an increase in bacterial abundance associated with DOM supply by the sediments. Therefore, the measurements of DOM optical properties in conjunction with siderophores accumulation, microbial abundance, oxygen, and nutrient consumption were aimed to provide critical insights into the ability of sediment-released DOM to serve as a substrate for heterotrophic communities to grow on and function.
During the project, the measurements of the pore waters, obtained by the collaborative project were analyzed by the DOMUSe PI, and the incubation setup, which was necessary for the ex-situ incubations was designed according to the DOMUSe PI’s drawings and built at the institute''s workshop. During the test cruise and 4 research cruises, we performed two test incubations and six actual incubation experiments which took place during two seasons (Spring and Fall) at Bornholm Deep, Gdansk Deep, and the eastern Gotland Deep. So, at each of the deeps, the incubation experiments were performed once in Spring and once in Fall. The incubation setup was kept gas-tight as much as possible in the dark and under the constant temperature, close to the in-situ temperature of the Baltic Sea bottom waters, oxygen concentrations were controlled by optical sensors, while samples of nutrients, colored and fluorescent DOM, dissolved organic carbon, total dissolved nitrogen, dissolved metals and bacterial abundance were subsampled over the course of each incubation 5-6 times (from T0 – incubation setup to T6 – ~80hrs of incubation – termination of the incubation). At T0 and T6 also samples for siderophore analyses and molecular DOM composition analyses were collected along with samples dedicated to microbial community composition analyses.
Thus, from pore water sample analyses, we examined the diffusive DOM release by sediments and from samples obtained during incubations we evaluated the net DOM release (meaning that the Net rates of DOM release consist of release and consumption). We also traced the quality alterations of the sedimentary-derived DOM in the ex-situ incubations.
Our findings indicate highly variable, but, overall, slightly higher than previously reported diffusive DOM fluxes across the study areas, suggesting greater input of sediment DOM release to the carbon budget of the Baltic Sea (at least during the stagnation period) than previously thought. We found the highest sediment release (~1 mmol m-2 d-1) in the Gdansk Basin, while the lowest diffusive DOM fluxes (0.02 mmol m-2 d-1) were associated with the Gotland Deep. Notable differences in pore water DOM quality indicating varying degrees of organic matter reworking among investigated areas were observed across studied depositional areas, suggesting Gdansk Deep accumulates the freshest/or the most bioavailable DOM among all investigated areas.
The ex-situ incubation experiments revealed sedimentary DOM as a reliable substrate for heterotrophic communities. It was indicated by negative dissolved organic carbon Net fluxes and accumulation of humic-like fluorescent DOM over the incubation period. Although heterotrophic response to sedimentary DOM supply and chemical DOM composition displayed seasonal variability, we found compositional shifts in DOM and an increase in the abundance of heterotrophic bacteria over the incubation period. This indicated microbial DOM processing in the incubations at all the investigated areas, the magnitude of the response of heterotrophic bacteria (i.e., bacterial abundance) responded also to the initial quality differences of pore water DOM between the investigated areas.
Additionally, our analyses highlighted the stoichiometry of trace metals bound to DOM, indicating a consistent pattern across investigated areas and during incubations despite the alteration in DOM composition during the incubation period. This suggests that trace metals attached to organic matter are influenced rather by their chemical properties, than the quality changes of DOM.
By revealing the complex interactions between sedimentary DOM and the overlying water column, our study contributes to our current understanding of the cycling of sedimentary-derived DOM at the near bottom waters and the biogeochemical cycling of carbon in the Baltic Sea. We shared our data in the open-source repository. Although our results are mainly dedicated to the scientific community, to people living around the Baltic Sea our results provide an understanding of what are the biogeochemical processes and what mechanisms may influence the quality of water they see every day, as well as an important “positive” role that heterotrophic bacteria can play at the seafloor (and therefore, change the only negative perception of bacteria as only disease inducers). The understanding of processes taking place in the water that we have at our “doorstep” is especially important, when the Baltic Sea serves as a source of income.