Seminar series

Nutrient management plans have been successful in reducing nutrient inputs to many coastal ecosystems, but ecosystem responses have been unanticipatedly weak. This lack of recovery has been attributed to a legacy effect of past nutrient inputs, possibly sustaining sediment nutrient release and eutrophication over longer periods. Sediment pools of carbon, nitrogen and phosphorus sampled over 25 years (1999-2023) across 27 Danish estuaries have been analyzed. This period follows substantial reductions in inputs of nitrogen (>50%) and phosphorus (>90%) from land, the majority of these occurring from 1985 to 1997. Variability among sediment cores was high, both on spatial and temporal scale, although it was reduced by normalizing total N and P to loss of ignition. All sediment pools changed significantly with depth, but there was no significant difference among estuaries despite large differences in area-specific nutrient loading, highlighting the large spatial variability within estuaries. No significant changes in sediment pools were observed over the study period, with an almost even distribution of directions of change. Given the relatively large sampling effort (>100 cores), we estimated that it should be possible to detect changes of 20% with a probability of 80%. Combining this with the lack of consistent trends suggests that the legacy effect of nutrient reductions was within a few years rather than decades or that the legacy effect is small. Hence, the lack of coastal ecosystem recovery is most likely due to other factors.

During the evolutionary history of our planet, a set of microbial processes that evolved exclusively in the absence of oxygen changed the chemical speciation of all major elements. In the past 20 years, we aimed to discover and elucidate the role of these anaerobes in the microbial nitrogen cycle in several dynamic coastal ecosystems (Bothnian Sea, Stockholm Archipelago & Marine Lake Grevelingen) where terrestrial and marine systems converge. However, these ecosystems face increasing challenges, including eutrophication and oxygen depletion, potentially causing shifts in the nitrogen transformation processes. We explored the intricate microbial pathways governing microbial nitrogen cycling by integrating metagenomic analyses, porewater profiling, sediment incubations, and bioreactor enrichment strategies. Molecular studies showed that anammox bacteria occur in many oxygen-limited ecosystems, and that they can make the rocket fuel hydrazine by novel protein complexes. Depending on the nutrient status, the anammox process can contribute significantly to the loss of reactive nitrogen. Using sediment of the Gullmarsfjord, we were able to enrich and characterize marine anammox bacteria. Recently it appeared that these Scalindua bacteria are much more tolerant to oxygen than previously assumed. Lastly, we are exploring the removal of ammonium from anoxic sediments with solid electron acceptors, in an attempt to find and enrich one of the remaining elusive impossible chemolithotrophic N cycle microbes missing in nature.

The Black Sea is the largest meromictic marine basin characterised by a well-oxygenated surface layer and a deep sulfidic anoxic core, separated by a ca. 50 m suboxic zone. In other oxygen-depleted marine environments, nitrogen cycling at oxic-anoxic boundaries has been identified as a major source of nitrous oxide (N₂O). Yet, there appears to be negligible net production of N₂O within the suboxic zone of the Black Sea, and the region is considered only a minor source of N₂O. So far, the sources and sinks of this powerful greenhouse gas in the Black Sea are poorly constrained, as are the microorganisms which mediate N₂O turnover.

We combined stable isotope rate measurements with metagenomic and metatranscriptomic analyses to investigate N₂O turnover in the suboxic zone of the western Black Sea. We show persistent N₂O formation from ammonia oxidation (up to 0.2 nmol N L^-1 d^-1) and sporadic rates of high N₂O production from denitrification (up to 20 nmol N L^-1 d^-1). Molecular analysis of the amoA gene revealed that the Nitrososphaerales were the dominant ammonia oxidisers, while the genetic potential to form N₂O from nitrite was mainly encoded by Alpha- and Gammaproteobacteria. Interestingly, the measured N₂O reduction rates (up to 24 nmol N L^-1 d^-1) could exceed N₂O formation rates from both nitrite and ammonia, with the potential for N₂O reduction mainly encoded and expressed by Gammaproteobacteria and Marinisomatia. Our results suggest that, despite the diverse pathways of N₂O production in the Black Sea, emissions are low due to tight balancing of N₂O production and consumption, which is facilitated by the presence and activity of specialised N₂O reducers.

Oxygen (O₂) concentrations are declining in coastal and open ocean systems with detrimental ecological and socioeconomic effects. Deoxygenation has far reaching impacts on element cycling, for instance ~30% of bioavailable nitrogen (N) loss occurs in O₂-depleted waters. As further warming-related expansion of low O₂ regions is predicted and expected to enhance N loss, with feedbacks to ocean productivity and climate, it is critical that we understand how N loss-controlling microbial communities respond to deoxygenation. While current paradigms are contested by observation of aerobic metabolism in O₂-depleted waters, further progress has been hampered by inevitable O₂ contamination in shipboard experiments and inadequate detection limits and spatial resolution of O₂ dynamics. Here we will look across low O₂ systems at our recent insights into O₂ dynamics and elucidation of controls and pathways at relevant O₂ concentrations using novel custom-made instrumentation, as we build towards a quantitative mechanistic framework of microbial responses to ocean deoxygenation.

Nitrous oxide (N₂O) stands out among other climatically active trace gases because of its central role in stratospheric ozone depletion and its effectiveness in enhancing the Earth’s warming on centennial time scales. The ocean is a net source for atmospheric N₂O, albeit the still large uncertainties in the extent and spatio-temporal variability of the emissions. Because of this, considerable efforts to quantify the emissions and improve our understanding of the driving mechanisms of the observed variability have been undertaken over the past decades.

Marine production and consumption of N₂O are mostly microbially-driven and in tight coupling with environmental oxygen concentrations. Yet, there is a wide range of physical and chemical processes which, in turn, play a significant role in shaping the distribution of N₂O, its fluxes across interfaces and ultimately, its overall budget. The Baltic Sea is an unparalleled system to investigate these dynamics as it encompasses environmental features that make up for several of the uncertainties in current emission estimates in close-coastal systems, which include: marked seasonality in e.g. stratification, salinity gradients, strong oxyclines, sea-ice cycles, eutrophication, upwelling and storms, among others.

During this seminar, I will discuss the current status of the N₂O emissions from the Baltic Sea, as well as the main drivers of regional variability in fluxes across the sediment-water-air interfaces. I will also provide perspectives regarding future research directions.          

The Baltic cyanobacterial community is composed of filamentous, N₂ fixing forms of the Nostocales order, and picocyanobacteria. Both forms have a significant effect on the functioning of the ecosystem. However, the presence of filamentous cyanobacteria usually attracts more attention, due to their massive occurrence and nitrogen input to the already over-fertilized system. In addition, some of the filamentous species produced toxins and other N-rich metabolites. These compounds supply an additional pool of nitrogen and make an important contribution to the nitrogen budget in the sea. The majority of these compounds have potent biological activity, but their effect on co-occurring organisms and the functioning of the ecosystem is not clear cut. In this presentation, the variety and significance of N-containing cyanobacterial metabolites will be discussed.

An overview of the Baltic Sea Nitrogen Cycle Network (BSNCN), funded by the Swedish Institute.

Marine sediments regulate the availability of nitrogen in the ocean, which affects primary production and ecosystem health. Macrofauna (animals >1 mm) and meiofauna (<1 mm) play critical roles in shaping physical structures and geochemical properties of sediments globally. Macrofauna organisms generally promote the formation of oxic zones within sediments, enhancing nitrification processes. However, in the Baltic Sea, invasive polychaetes of the genus Marenzelleria spp. have the opposite effect, suppressing nitrification and denitrification as they promote anaerobic rather than aerobic microbial processes. Marenzelleria spp. and the clam Macoma balthica also have the potential to produce nitrous oxide.

Meiofauna organisms, albeit less studied due to challenges in isolation and manipulation, are enormously abundant as they make up approximately 60% of total metazoan abundance on our planet. They have been shown to significantly stimulate benthic aerobic processes. Current research suggests that intense bioturbation by meiofauna can double denitrification rates in Baltic Sea sediments. This presentation will explore the intricate networks between biota and environmental matrices in Baltic sediments, highlighting both the causes and possible ecological consequences, and outline critical directions for future research.

Biological nitrogen fixation is considered the major input of reactive nitrogen in low latitude open ocean regions, fuelling primary productivity and carbon export. For decades, nitrogen fixation had been assumed to be absent in the temperate shelf seas and high latitude ocean. However, there is now growing evidence that nitrogen-fixers are active in these systems, but the magnitude, environmental drivers, players and ecological implications are poorly known. This evidence is accumulating at a time when these systems are rapidly changing, underscoring the need for a focused effort in the scientific community to accelerate understanding of nitrogen fixation beyond the low latitudes. Shelf seas and high latitude oceans are cold and nutrient-rich, conditions that seem to favour ‘non-cyanobacterial diazotrophs’ (NCDs) over cyanobacterial diazotrophs more prevalent in low latitudes. The ecology and significance of NCDs for nitrogen cycling in temperate shelf seas and high latitudes remains elusive due to their fundamentally different metabolic profiles: contrary to their cyanobacterial counterparts NCDs cannot photosynthesize, depending external organic matter sources to meet their carbon and energy demands. In this talk, I will explore the associations of NCDs with dissolved and particulate matter associations and provide outlooks on how to constrain their contribution to nitrogen supply in shelf seas and high latitude ocean regions.

Sixteen percent of the Baltic Sea are shallower than 10 m and 65% shallower than 50 m. These waters are characterized by a strong connection of water – including currents and wave action – and sediments. In addition, shallow waters are connected to the land including peatlands, estuaries, archipelagos. The coastline is shaped by bays, bights, lagoons and estuaries. This variety of land- and seascapes makes coastline unique and diverse. And they need a lot of dedicated studies to be fully understood. At the IOW we work on this challenge since recent years but the work is also based on numerous previous projects.

In the lecture I will discuss the special characters of coastal oceans in comparison to deep waters and describe the differences in nutrient cycles. While the northern Baltic Sea is plagued by cyanobacteria blooms in summer, the central part by oxygen minimum zones, the coasts of the southern Baltic Sea appear to be in reasonably good ecological shape, however not lagoons. Many studies indicate that the warming of the water and oxygen deficiency could also impact shallow waters. Factors favoring this critical situation are the residence time of the water and the pollution with nutrients. Which sediments are present also influences the nutrient turnover and sands appear to fulfill an important filter function, while mud increases the residence time of nitrogen compounds. Transport processes regulate the quantity and composition of nutrients that reach deeper waters, so that changes on the coast also affect the entire Baltic Sea. The lecture will provide an overall view on the processes our knowledge gaps and look into the future.

Biological dinitrogen (N₂) fixation is performed solely by specialized prokaryotes termed diazotrophs, introducing new reactive nitrogen into aquatic environments. Conventionally, cyanobacteria are here considered the major diazotrophs. However, accumulating evidence indicates that diverse non-cyanobacterial diazotrophs (NCDs) inhabit a wide range of aquatic ecosystems, including temperate and polar latitudes, coastal environments and the deep ocean. In this talk, I will review our efforts to decipher the importance and ecology of marine NCDs, with particular focus on their particle-associated lifestyle.

The presenters will provide general information on the role of HELCOM, including nutrient management, inputs and pressures, and the nutrient input reduction scheme. Further, also insight to nitrogen related state of the sea and assessments, and currently used indicators.