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Lakes are active components of the global carbon cycle and host a range of processes that degrade and modify organic carbon compounds. Degrading DOM molecules and synthesizing new compounds, microbes of the aquatic ecosystems vigorously and continuously influence the chemodiversity of the available organics, subsequently impacting and the diversity of microbial communities. To unravel the underlying biotic and abiotic assembly mechanisms that control the structure and patterns of microbial communities and DOM in lakes, we combined high-throughput sequencing (18S and 16S rRNA genes) and ultra-high resolution mass spectrometry together with a null model approach. The present study includes spatiotemporal signatures of DOM and the microbial community in six lakes contrasting continental and Mediterranean climate during the productive season. The studied lakes were characterized by different environmental conditions and nutrient sources in a relatively small geographical area. Our results showed that microbial communities along with DOM correlated with nitrate concentration in the continental region, while there was a higher correlation with water temperature and sulfates in the Mediterranean region. We found that the differences between climatic regions were mainly reflected in the diversity of DOM at the molecular formula-level and microeukaryota community. Furthermore, using null model approach, these differences in DOM composition were not persisted, whereas deterministic processes dominated remarkably in each lake. In contrast, microbial community assembly processes were highly variable despite trophic status or climatic region. Taken together, our research provides novel insight into understanding DOM-microbial associations and influence on the carbon cycle under the influence of global change.

In the semi-arid Mediterranean region, limited freshwater resources are experiencing increased pressures due to the growing water demand for irrigation purposes, prolonged droughts and short but intense precipitation events caused by climate change. In the shallow coastal water bodies such as Lake Vrana, the consequence of such events is manifested in the form of precipitation- evaporation imbalance which can distort the nutrient flux and eventually increase their trophic state. To estimate the impact of intensified seasonal changes on the lake’s microbial communities, we conducted spatio- temporal research. By applying the high-throughput 16S and 18S rDNA sequencing and the ultra-high resolution mass spectrometry to identify DOM on molecular formula level, we could link fresh DOM production by phytoplankton and its utilization by the free-living and particle-associated heterotrophic bacteria. Increased autochthonous DOM production occurred during the start of the dry season when the enhanced phytoplankton primary production manifested through high Chlorophyll a concentrations and Cyanobacterial relative abundance. The increasing bacterial functional group abundances involved in the organic matter degradation were triggered by the amount of the more bioavailable DOM. The dry period was marked with the slow salinization and the consequent marine species appearance. At the start of the wet period, the introduction of the allochthonous DOM and suspended particles via heavy rains caused the change in the particle-associated bacterial community.

Mediterranean freshwater systems (coastal lakes or springs) as scarce water resources are experiencing an increase in anthropogenic and climate change-caused pressures, leading to increased productivity and consequent deterioration of the trophic state [1]. Coastal lakes are centers of carbon circulation and transformation managed to a large degree by microbial populations intertwined in a network of interactions [2]. The trophic state of the coastal Lake Vrana is under the influence ofseveral counteracting drivers. In the summer months, at least 50% of the surface area is covered by macrophytes [3] that utilize large amounts of nutrients and thereby mitigate eutrophication. On the other hand, the largest agricultural complex in the region with the water discharge system feeding into the lake and excessive water pumping at local springs can considerably enhance the natural eutrophication process[4]. High-throughput sequencing of 16S and 18S rRNA genes revealed the effect of several environmental pressures on the microbial community structural change, including salinity, and dissolved organic carbon (DOC) concentrations. While the connection between microbial primary producers and heterotrophic bacteria was deciphered through DOM perturbation, the lake’s precipitation-evaporation imbalance caused the seawater intrusion and the introduction of marine species and allochthonous DOM input.

ASLO 2021 Aquatic Sciences Meeting, Aquatic Sciences for a Sustainable Future: Nurturing Cooperation, 22–27 June 2021, Knowledge of dynamic interactions between organic matter (both dissolved and particulate) and microbial communities is critical to draw the routes of organic matter degradation or transformation, but also to understand the evolution and succession of microbial communities in ecosystems. Microorganisms are key mediators in the formation, transformation, and storage of organic matter in all aquatic environments such as marine, freshwater, and sediments. At the same time, organic matter provides the energetic foundation for the growth of heterotrophic microorganisms, affecting their community structure and metabolic potential. The combination of ”omic”-based approaches with novel analytical methods has revolutionized our ability to probe the interactions between marine microbial communities and organic matter. However, new interdisciplinary approaches are needed to move from this descriptive level to a better quantitative, process‐level understanding of the roles of microbes in biogeochemical cycles. In this session, we will explore and discuss recent advances and limitations of how organic matter (dissolved molecules, colloids, gels and particles) and microorganisms (bacteria, archaea, protozoa, fungi) can be integrated into our understanding of biogeochemical cycles to better predict functional diversity patterns, fluxes of matter (C/N/S/P…), and the impact of environmental change across microbial aquatic systems. We need to nurture cooperation between organic biogeochemists and microbial ecologists, between freshwater and marine scientists, between lab bench and computational data analysts, and include scientists from different fields who are interested in advancing the progress at the intersection of microbiology and organic matter biogeochemistry

Meereswissenschaftliche Berichte No 119 2021 - Marine Science Reports No 119 2021, Die Arbeit beschreibt die hydrographisch-hydrochemischen Bedingungen in der westlichen und zentralen Ostsee im Jahr 2020. Basierend auf den meteorologischen Verhältnissen werden die horizontalen und vertikalen Verteilungsmuster von Temperatur, Salzgehalt, Sauerstoff/Schwefelwasserstoff und Nährstoffen mit saisonaler Auflösung dargestellt. Für den südlichen Ostseeraum ergab sich im Winter 2019/2020 an der Station Warnemünde für die Lufttemperatur eine Kältesumme von 0 Kd. Im Langzeitvergleich setzt er damit einen neuen Rekord als wärmster Winter seit Beginn der Aufzeichnungen im Jahr 1948 und wird als extrem mild klassifiziert. Der Sommer 2020 nimmt mit einer Wärmesumme von 234,3 Kd den 14. Platz in der 72-jährigen Datenreihe ein und liegt weit unter dem Rekordwert von 2018 (394,5 Kd). Das Langzeitmittel liegt bei 159,7 +/- 75,1 Kd. Die Situation in den Tiefenbecken der Ostsee war weiterhin geprägt durch stagnierende Bedingungen mit ausgedehnten Sauerstoffmangelgebieten. Kleinere Einstromereignisse ereigneten sich im November 2019 sowie Januar und Februar 2020 in der westlichen Ostsee und prägten das Tiefenwasser im Arkona Becken und Bornholm Becken. Ein weiterer schwacher Einstrom folgte Mitte bis Ende Oktober. Anhand der Temperatur und Salinitätsverhältnisse an den Schlüsselstationen Bornholm Tief und Stolper Rinne hat keines dieser Ereignisse die Stolper Schwelle gequert. Das Bodenwasser in der Stolper Rinne blieb mit 9,1 - 9,5 °C im Jahresverlauf deutlich wärmer im Vergleich zur Temperaturentwicklung im Bornholm Becken, das durch die Wintereinströme geprägt war (Jahresmittel 8,4 °C). Das Tiefenwasser im östlichen Gotland Becken war immer noch geprägt von den warmen Einströmen in den Vorjahren und mit Bodenwerten von 7,2 °C deutlich erhöht. Aufgrund der Daten von 9 Referenzstationen wurde ermittelt, dass die Winterkonzentrationen der Nährstoffe Nitrat und Phosphat im Oberflächenwasser der westlichen und zentralen Ostsee 2020 etwas niedriger als im Jahr 2019 lagen. Eine Ausnahme bildete die Mecklenburger Bucht, die eine höhere Nitratkonzentration als in 2019 aufwies. Ein klarer Trend über die letzten Jahre zeichnete sich aber nicht ab. Auffällig war noch das Verhältnis des gelösten anorganischen Stickstoffs und Phosphors im Oberflächenwasser im Winter, das in der Mecklenburger Bucht über 11 und in der westlichen Gotlandsee bei immerhin 8 lag, deutlich höher als in den vergangenen Jahren. Dies könnte die Bedeutung der Cyanobakterien in den Seegebieten etwas zurückgedrängt haben. Die euxinischen Bedingungen im Tiefenwasser der zentralen Ostsee verschärften sich im Jahr 2020 weiter. Dies bestimmte auch die Nährstoffsituation in den Baltischen Tiefs entlang des Talwegs. Im Bornholmtief nahmen die Phosphat und Ammonium Konzentrationen noch leicht ab. Im Gotlandtief, Landsorttief und Karlsötief akkumulierten diese Nährstoffe weiter, sogar bis auf über 5 ?mol/l Phosphat und etwa 20 ?mol/l Ammonium im Gotlandtief in 2020. Eine leichte Erholung zeigte das Fårötief, das in 2020 einen Schub sauerstoffhaltiges Wasser erhielt. Dadurch nahmen die Schwefelwasserstoffkonzentration und die Nährstoffkonzentrationen leicht ab. Unter den herrschenden euxinischen Bedingungen wurde im Tiefenwasser kein Nitrat mehr vorgefunden. Eine Ausnahme bildete das Bornholmtief, das im Jahresmittel noch 0,9 ml/l Sauerstoff und damit auch eine hohe Nitratkonzentration von etwa 8 ?mol/l aufwies. So konnte hier noch kein Ammonium nachgewiesen werden und die Phosphatkonzentration lag mit etwa 3?mol/l in einem normalen Bereich für oxisches Tiefenwasser. In diesem Bericht sind die während des Ostsee-Umweltmonitorings im Januar/Februar 2020 ermittelten Oberflächenwasserkonzentrationen und -sedimentgehalte für chlorierte Kohlenwasserstoffe (CHC) und polyzyklische aromatische Kohlenwasserstoffe (U.S. EPA PAH), sowie Oberflächensedimentgehalte für Organozinnsubstanzen (OT) zusammengefasst. Für alle im Oberflächenwasser untersuchten Schadstoffe zeigt sich ein Konzentrationsgradient von der westlichen Ostsee im Bereich der Kieler Bucht/Fehmarnbelt (?DDTsum: 5,72 pg/L,?PCBICES,SUM: 7,20 pg/L, HCBSUM: 4,90 pg/L, ?PAKSUM: 4660 pg/L) bis zur östlichen Gotlandsee (?DDTsum: 1,96 pg/L, ?PCBICES,SUM: 1,22 pg/L, HCBSUM: 3,43 pg/L, ?PAKSUM:1344 pg/L) mit zudem auffälligen Konzentrationen im Bereich der Pommerschen Bucht(?DDTsum: 13,06 pg/L, ?PCBICES,SUM: 5,15 pg/L, HCBSUM: 9,30 pg/L, ?PAKSUM: 5626 pg/L).Die Daten lassen darauf schließen, dass die Oder eine Quelle für Schadstoffe in der Ostsee ist, besonders für partikulär gebundene. Die höchsten CHC- und PAH-Gehalte im Oberflächensediment wurden für das Arkonabecken nachgewiesen (?DDT: 90,7 ng/g TOC,?PCBICES: 9 1,3 n g/g T OC, H CB: 5,0 n g/g TOC, ?PAH: rd. 37000 ng/g TOC), während höchste Organozinngehalte in der Mecklenburger Bucht detektiert wurden (?OT: 275 ng/g TOC). Die Bewertung der Daten auf Grundlage der UQN der Wasserrahmenrichtlinie zeigt, dass eine schädliche Wirkung auf marine Organismen durch die Konzentrationen des hochmolekularen PAK Benzo(b)fluoranthen für die Bereiche Kieler Bucht/Fehmarnbelt, östliche und westliche Gotlandsee zu erwarten sind. Die Gehalte für Anthracen im Oberflächensediment überstiegen den Grenzwert des HELCOM-Indikators PAH an der Station N1 im Fehmarnbelt. Die Oberflächensedimentgehalte von Tributylzinn überstiegen an allen untersuchten Stationen den Grenzwert des HELCOM-Testindikators TBT and imposex.Die Zeitreihenanalysen der Oberflächenwasserdaten zurückliegend zum Teil bis zum Jahr 2001 zeigen abnehmende Trends der Konzentrationen für PCBICES sowie DDT und seine Metabolite; die der Oberflächensedimentdaten zeigen keine Trends im betrachteten Zeitraum., The article summarizes the hydrographic-hydrochemical conditions in the western and central Baltic Sea in 2020. Based on the meteorological conditions, the horizontal and vertical distribution of temperature, salinity, oxygen/hydrogen sulphide and nutrients are described on a seasonal scale. For the southern Baltic Sea area, the Warnemünde station recorded in the winter 2019/2020 a “cold sum” of the air temperature of 0 Kd leading to a classification of an extreme mild winter season, setting a new record as warmest winter since the beginning of the time-series in 1948. The summer “heat sum” of 234.3 Kd ranks on the 14th position over the past 72 years and is far below the record of 394.5 Kd during 2018. The long-term average is 159.7 +/- 75.1 Kd. The situation in the deep basins of the Baltic Sea was mainly characterized by stagnation and widespread hypoxic to euxinic areas. In wintertime 2019/2020 three weak inflow phasesoccurred in November, January and February which dominated the situation in the deep water of Arkona Basin and Bornholm Basin. A next weak inflow pulse occurred from mid to end of October. Comparing of temperature and salinity measurements at the key stations Bornholm Deep and Slupsk Channel, none of these events crossed the Slupsk Sill. The deep water temperature in the Slupsk Sill stayed warm between 9.1 to 9.5 °C whereas an annual mean of 8.4 °C was found the Bornholm Deep, which was influenced by colder water of the winterly inflow pulses. The deepwater at the eastern Gotland Basin was still influenced by the warm inflows of previous years and with bottom temperatures of 7.2 °C above average. The winter nutrient concentrations of nitrate and phosphate in surface water in the western and central Baltic Sea were slightly lower in 2020 compared to 2019 according to 9 reference stations. An exception was the Mecklenburg Bight that showed an elevated nitrate concentration in 2020. However, a clear trend over the last years could not be registered. It may be emphasized that the dissolved inorganic nitrogen/phosphorus ratio in winter in surface water was above 11 in the Mecklenburg Bight and almost 8 in the western Gotland Sea, significantly higher compared to recent years. This might have reduced the cyanobacteria abundance in 2020 in these areas. The euxinic conditions in the deep water of the Baltic proper continued to intensify in the year 2020. This determined the nutrient situation in the Baltic Deeps along the Thalweg. In the Bornholm Deep phosphate and nitrate concentrations still slightly declined. The Gotland, Landsort and Karlsö Deeps showed ongoing accumulation, with a maximum in the Gotland Deep of about 5 ?mol/l phosphate and 20 ?mol/l ammonium in 2020. A slight improvement was documented for the Fårö Deep that received some oxygenated water. So hydrogen sulphideconcentration and nutrient accumulation were reduced there. Under the prevailing euxinicconditions nitrate was depleted in deep waters. An exception was the Bornholm Deep, that showed an annual average of 0.9 ml/l oxygen and consequently a high annual average nitrateconcentration of 8 ?mol/l. So, no ammonium could be detected and the phosphateconcentration was determined at 3 ?mol/l, a quite normal value for oxic deep water. This report summarizes surface water concentrations and sediment contents for chlorinatedhydrocarbons (CHC) and polycyclic aromatic hydrocarbons (U.S. EPA PAH), as well as surface sediment contents for organotin substances (OT) which were determined during the Baltic Sea monitoring in January/February 2020. Seawater concentration gradients for the analysed contaminants ranging from the western Baltic Sea in the area of the Kiel Bight/Fehmarn Belt (?DDTsum: 5.72 pg/L, ?PCBICES,SUM: 7.20 pg/L,HCBSUM: 4.90 pg/L, ?PAKSUM: 4660 pg/L) to the eastern Gotland Sea (?DDTsum: 1.96 pg/L,?PCBICES,SUM: 1.22 pg/L, HCBSUM: 3.43 pg/L, ?PAKSUM: 1344 pg/L) with noticeable concentrations in the Pomeranian Bight (?DDTsum: 13.06 pg/L, ?PCBICES,SUM: 5.15 pg/L,HCBSUM: 9.30 pg/L, ?PAKSUM: 5626 pg/L). The data suggest that the Odra River is a source of pollution in the Baltic Sea, especially for particulate contaminants. The highest CHC and PAHlevels in the surface sediment were detected for the Arkona Basin (?DDT: 90.7 ng/g TOC,?PCBICES: 9 1.3 n g/g T OC, H CB: 5 .0 n g/g T OC, ?PAH: a bout 37000 ng/g TOC); while highest organotin levels were detected in the Mecklenburg Bight (?OT: 275 ng/g TOC).The assessment of the data based on the EQS of the Water Framework Directive shows that a harmful effect on marine organisms must be expected from concentrations of the high molecular weight PAH benzo(b)fluoranthene for the Kiel Bight/Fehmarn Belt, eastern and western Gotland Sea areas. Contents for anthracene in surface sediment exceeded the threshold value of the HELCOM indicator PAH at station N1 in the Fehmarn Belt. Surface sediment contents of tributyltin exceeded the threshold value of the HELCOM test indicator TBT and imposex at all sites studied. Time series data for surface water back to year 2001 show decreasing trends for concentrations of PCBICES and DDT and its metabolites. No trends can be observed for surface sediment timeseries data within the studied time period.

Fresh surface water resources, as karst lakes in the coastal region of the Mediterranean, represent a fragile hydrological system. Rising sea levels make previously freshwater coastal ecosystem partially saline. Seasonal changes in salinity and water temperature greatly influence the biological and nutrient structure of the lake. In freshwater lakes, bacteria are a key player in organic carbon processing and nutrient re-mineralization, forming the base of the microbial food web. Therefore, an increase of ionic content and water temperature potentially affect the nutrient fluctuation, which may lead to changes in the diversity of aquatic microbial communities in food webs and result in a shift of the trophic state. To evaluate the shifts in microbial community composition driven by salinity and temperature variation, we sampled the largest natural freshwater lake in Croatia, Vrana Lake Biograd na Moru, in the period from April to September in 2017. The lake is a karst cryptodepression connected to the nearby Adriatic Sea through the karst subsoil and a channel. Current trends suggest higher salinity and seawater intrusions into the lake which places this lake as a model system to explore changes of microbial community structure along with dissolved organic matter (DOM) molecular composition. To track changes during the seawater intrusion, the lake was sampled at two sites: nearby the channel connected to the sea and in the inner part of the lake. We used molecular approaches of Nextgeneration sequencing (Illumina) amplifying 16S and 18S rRNA genes to evaluate prokaryotic and eukaryotic composition, and ultrahigh resolution mass spectrometry for molecular DOM composition. Species richness (α diversity) of the microbial community and DOM molecular composition showed a high correlation between two sampling sites. Cluster analysis showed a higher difference between the sites based on eukaryotic composition. Through the season, from lower to higher ionic content, the chemoorganotrophic bacteria - Actinobacteria, followed by Gammaproteobacteria and Verrucomicrobiae - were dominant. This may indicate their ability to persist completely independent of salt concentration and ionic ratios. We found evident seasonal changes of the eukaryotic community with the highest relative abundance in September of Cryptophyta and Stramenopiles (Bacillarophyta), when the concentration of sodium, potassium and chloride was the highest in the season. This study highlights the changes in the microbial community and DOM composition in response to seawater intrusion and high-water temperature.

IMBeR Future Oceans 2, 17-21 June 2019, Brest, France, Interactions between microbial communities and dissolved organic matter (DOM) include microbial uptake, transformation and release, affecting the molecular composition of the heterogeneous DOM pool. Conversely, DOM composition can select for specific microbial taxa. Here, we explore the diversity of microbial (both Bacteria and Archaea) communities (Illumina tag sequencing of 16S rRNA genes) and the chemodiversity of dissolved organic molecules (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS). Sampling was conducted in the Eastern North Atlantic Ocean off the Galician coast (43ºN, 9º-15ºW) from 100 m to >5000 m, thereby encompassing a wide variety of water masses with contrasting origins and different aging. Eastern North Atlantic Central Water (ENACW) displayed lower values of AOU (35.3-77.1 µmol/kg) than those measured in the Mediterranean Water (MW, 98.8 µmol/kg) and the underlying North East Atlantic Deep Water (NEADW, 95.7 µmol/kg) and the Lower Deep Water (LDW, 107.2 µmol/kg). FT-ICR-MS average molecular composition of Solid Phase Extracted (SPE) DOM samples was C871H1087O415N18S2P1, with a mean molecular mass of 432.1 Da. Highly unsaturated molecular formulae were the most abundant (90.1%) followed by unsaturated (5.1%) and aromatic molecular formulae (3.0%). Potential peptides, carbohydrates and polycyclic aromatics accounted, each, for less than 1% of the total composition. There was no significant relationship between the FT-ICR-MS elemental composition and molecular mass with depth or AOU values, except for the H/C ratio, which correlated negatively with AOU. Highly unsaturated molecular formulae correlated significantly and positively with AOU whereas unsaturated aliphatics and potential peptides also correlated significantly, but negatively, with AOU. Accordingly, the relative abundance of some bacterial phylotypes (such as Acidobacteria, SAR202 and Chlamydiales) increased their relative contribution to community composition with depth or AOU, while some others (such as Acidimicrobiia, Bacteroidia and some Verrucomicrobia) showed the opposite trend. Thaumarchaeota was the dominant archaeal group and tended to increase in relative abundance with depth or AOU. Applying ecologic diversity metrics to relative abundances of both, organic compounds and microbial operational taxonomic units, we found that microbial (both Bacteria and Archaea) diversity and richness were negatively correlated to DOM diversity and richness, respectively, and also negatively correlated to the organic carbon concentration. Besides, our results demonstrate that this trend is linked to the water mass aging, which speeds biosphere taxonomic diversity but reduces the chemodiversity. DOM diversity decreasing along the AOU gradient would likely reflect the persistence of the most refractory molecules, as result of the organic refractory compounds produced as subproduct of the microbial carbon pump. We further speculate that the observed increase in DOM homogenization with water mass aging can be associated to the functional diversity of microbial communities. We are currently analyzing functional diversity (i.e. genes encoding enzymes involved in organic matter degradation) by using the metagenomic technique

Dissolved organic matter (DOM) represents one of the largest actively cycled carbon pools on earth. Water is the driving force moving dissolved organic matter through earth reservoirs and across the continuum of the atmosphere, soils, inland waters and oceans. To a significant extent, the DOM in freshwater systems is derived from terrestrial vegetation and soils with varying contributions of autochthonous production. In order to study the interplay of allochthonous versus autochthonous factors driving lake DOM molecular composition, we sampled six lakes of different trophic status across Croatia over the productive season from April to September 2017. We determined pro- and eukaryotic community compositions based on 16S and 18S rRNA genes, respectively, and molecular DOM composition via ultrahigh resolution mass spectrometry. We found that the abundance and average molecular weight of detected molecular formulas decreased with bulk DOC concentration. The influence of terrigenous input declined as lakes became more eutrophic in favor of higher potential lability of the DOM. Lake Vrana on Cres island was an exception regarding most analyzed DOM properties showing exceptionally low terrigenous influence, likely as it represents an isolated cryptodepression without river in- or outflows. Overall, catchment characteristics were the major factor influencing DOM molecular composition. Inter-lake dissimilarity was driven by trophic status while in-lake dissimilarity was not. Seasonal changes were generally small and preliminary results show no significant correlation between the composition and diversity of the eukaryotic community and the DOM molecular composition across lakes.