Assessing the potential for sea-based macroalgae cultivation and its application for nutrient removal in the Baltic Sea

Assessment of Ecosystem Services Provided by Macrophytes in Southern Baltic and Southern Mediterranean Coastal Lagoons

The ecological importance of macrophytes is well known and reflected in nature protection law, for example, as a key biological quality element. However, the socio-economic role, such as the impact of macrophyte presence on recreational activities, is often overlooked. The purpose of this study was to assess the human benefits (or ecosystem services) provided by macrophytes. We developed a list of 25 macrophyte ecosystem services and 79 assessment indicators based on expert knowledge and literature data. First, hypothetical scenarios of coastal lagoons were developed to assess the impact of different ecological states (i.e., macrophyte coverage) and management measures (i.e., fisheries) on the ecosystem service provision. Scenario assessments were carried out by stakeholder workshops and literature search. Second, the ecosystem service potential of submerged and emergent macrophyte habitats were assessed by macrophyte experts and literature data. Results showed that cultural services are most important in terms of the overall actual provision of ecosystem services (scenario assessment) but also showing highest potential of the hypothetical ecosystem service provision (habitat assessment). Highest overall potential is shown for reeds and tall forb communities (83 out of maximum 125), followed by seagrass beds (71) and seaweed communities (61). Our ecosystem service assessment approaches (i.e., scenario and habitat-based) using socio-cultural data (i.e., stakeholders and experts-based) and biophysical data (i.e., indicators-based) can serve as supportive tools for coastal management and policy implementation visualizing the benefits of macrophytes to humans.

Modelling and mapping carbon capture potential of farmed blue mussels in the Baltic Sea region

This study applies a regional Dynamic Energy Budget (DEB) model, enhanced to include biocalcification processes, to evaluate the carbon capture potential of farmed blue mussels (Mytilus edulis/trossulus) in the Baltic Sea. The research emphasises the long-term capture of carbon associated with shell formation, crucial for mitigating global warming effects. The model was built using a comprehensive pan-Baltic dataset that includes information on mussel growth, filtration and biodeposition rates, and nutrient content. The study also examined salinity, temperature, and chlorophyll a as key environmental factors influencing carbon capture in farmed mussels. Our findings revealed significant spatial and temporal variability in carbon dynamics under current and future environmental conditions. The tested future predictions are grounded in current scientific understanding and projections of climate change effects on the Baltic Sea. Notably, the outer Baltic Sea subbasins exhibited the highest carbon capture capacity with an average of 55 t (in the present scenario) and 65 t (under future environmental conditions) of carbon sequestrated per farm (0.25 ha) over a cultivation cycle - 17 months. Salinity was the main driver of predicted regional changes in carbon capture, while temperature and chlorophyll a had more pronounced local effects. This research advances our understanding of the role low trophic aquaculture plays in mitigating climate change. It highlights the importance of developing location-specific strategies for mussel farming that consider both local and regional environmental conditions. The results contribute to the wider discourse on sustainable aquaculture development and environmental conservation.

Carbon sequestration from refractory dissolved organic carbon produced by biodegradation of Saccharina japonica

Using macroalgae cultures to sequester carbon has been proposed in recent years. Yet the key mechanism of carbon sequestration-how carbon in degrading biomass is converted into refractory dissolved organic carbon (RDOC) remains poorly understood. The process of producting RDOC via biomass degradation of Saccharina japonica, the most productive algae in China, was thus studied in the laboratory. Most of the carbon in the kelp biomass was converted to dissolved inorganic carbon (DIC) by bacterial respiration. Only 7.8% of the carbon in the kelp biomass was converted into labile DOC, semi-labile or semi-refractory DOC, and refractory DOC in turn. The enrichment of DIC resulted in hypoxic conditions in the water. For the hypoxia in the experiment, the sulfur-degrading bacteria Campylobacteria and Bacteroidia became the dominant bacterial classes, which were the key drivers for the transformation of labile DOC to semi-labile or semi-refractory DOC. Then, semi-labile or semi-refractory DOC was converted to RDOC, driven by the sulfite-reducing bacteria Clostridia and Kapabacteria. Finally, 0.3% of the carbon content in kelp was transformed into RDOC. The resulting RDOC, which was rich in sulfur and nitrogen elements, increased the molecular diversity and average molecular weight in the water. An important finding was that the production of RDOC may be accompanied by the environmental risk of hypoxia.

Brown seaweed hydrolysate as a promising growth substrate for biomass and lipid synthesis of the yeast yarrowia lipolytica

Biomass of the brown algae Fucus vesiculosus and Saccharina latissima is a promising, renewable feedstock because of the high growth rate, accessibility and content of glucose and mannitol. Saccharification of seaweeds is a simple process due to the lack of lignocellulose in the cell wall. The high content of glucose and mannitol makes these seaweeds an attractive feedstock for lipid production in the yeast Yarrowia lipolytica. This study demonstrated that hydrolysates of brown algae biomass can be applied as a substrate for synthesis of yeast biomass and lipids without any supplementation. To increase the lipid titer in yeast biomass, we employed an engineered strain of Y. lipolytica overexpressing DGA1/DGA2. In consequence, the C/N ratio has a lower impact on lipid synthesis. Moreover, the applied substrates allowed for high synthesis of unsaturated fatty acids (UFA); the level exceeded 90% in the fatty acid pool. Oleic (C18:1) and linoleic acids (C18:2) achieved the highest content. The study showed that Y. lipolytica is able to grow on the seaweed hydrolysate and produces a high content of UFA in the biomass.