Seaweed Cultivation as a Climate Positive Solution

Title: Seaweed Cultivation as a Climate Positive Solution - The role of seaweed farms in the production of recalcitrant organic carbon with implications for carbon sequestration and carbon markets By Luiza Neves, NTNU Department of Chemistry and SINTEF Ocean This PhD project investigated how seaweed cultivation can be used for carbon capture and storage, by cultivating seaweed and storing it in such a way that the carbon in the seaweed is not released back into the atmosphere. Seaweed grows very quickly and efficiently incorporates carbon from CO2 into biomass (leaves, stems) through photosynthesis (sunlight), much like grass, trees, and other land plants do. Seaweed also produces individuals that are easy to harvest, making seaweed cultivation a very efficient way to produce biomass. Norway has very favorable natural conditions for seaweed cultivation with a long coastline and nutrient-rich waters, as well as an active marine industry. While seaweed grows in cultivation facilities, dissolved substances, small particles, and larger pieces that can sink to great ocean depths will leak out, thus contributing to carbon storage, provided that this material ends up at depths sufficient for permanent storage. At ocean depths below 1000 meters, seawater and sediments are not stirred up by currents and mixed with surface water, and carbon-rich deep water and sediments represent a permanent storage of carbon captured by seaweed—indirectly from the atmosphere—through photosynthesis. This storage occurs regardless of how we use the grown seaweed biomass, but we do not know the proportion of carbon that will actually be stored in this way, as it depends on how much is lost from the seaweed facility and how much of this is recalcitrant and can be "stored" for very long periods—1000 years or more. This was a main goals of the PhD project. This knowledge is crucial for understanding the carbon dynamics that occur when seaweed is cultivated on an industrial scale and is critical for establishing a future methodology for CO2 removal using seaweed cultivation. Such a methodology could be used as a climate measure. The experimental activity in the PhD work was carried out at a test facility for seaweed cultivation in Hitra (SINTEF) and at one of the cultivation facilities of Seaweed Solutions in Frøya, as well as in mesocosms at Trondheim Biological Station (NTNU). Studies have been conducted to quantify leakage, erosion, and loss of biomass in the form of dissolved carbon, fragments, and particles. The studies also looked at how multiple harvests from the same plants affect the total biomass yield from the facility. Experiments were conducted on seaweed that had been planted as seedlings in October or January, as it has previously been shown that the planting time significantly affects seaweed growth and thus how much can be harvested. The experiments showed that the timing of planting and harvesting significantly impacts how much carbon can be captured and removed using seaweed cultivation. This is important knowledge in developing new carbon removal methods. The quality of seaweed growing in cultivation facilities deteriorates rapidly over the summer due to biofouling, making it unusable for food. By doing partial harvesting of the biomass early in the spring, the seaweed can grow back and be used for carbon capture with harvesting in the summer when the seaweed is fully grown. Simultaneously with the continued growth of seaweed, passive carbon removal also occurs. Therefore, the impact of partial harvesting on the quality and yield of seaweed was studied. Such a strategy could utilize the production potential of seaweed farms much better than complete harvesting early in the spring, and biomass suitable for various products can be achieved, e.g., food from seaweed harvested early in the spring and plastic, feed, and biochar from seaweed harvested in the summer. Furthermore, in vitro studies of the production and degradation of seaweed carbon (POC and DOC) over 4 and 188 days were conducted. It is important to obtain quantitative data on such carbon capture and loss in the seaweed facility to make accurate calculations of how much seaweed is actually removed by seaweed cultivation. Analysis of POC, DOC, TOC, DIC, pH, dissolved oxygen, inorganic nutrients, temperature, and microbiota were performed from water and seaweed. Several master's students from NTNU have been involved in the work. The PhD work is expected to be completed in the spring of 2025.

This PhD-project has investigated the dynamics of carbon in seaweed farming, aiming to quantify CO2-removal (CDR) potentials in this industry. Seaweed (macroalgae) can be cultivated in large seafarms in to produce biomass with many interesting applications. Especially the kelp grows very quickly and efficiently incorporates carbon from CO2 into biomass (leaves, stems) through photosynthesis (sunlight), much like grass, trees, and other land plants do, but without the use of fresh water, artificial fertilizers or pesticides. Kelp also produces individuals that are easy to harvest within less than one year, making seaweed cultivation a very efficient and environmentally friendly way to produce large amounts of biomass. Due to large CO2-uptake it is possible to cultivate kelp for CO2-removal purposes if the captured carbon is not lost to the atmosphere again but incorporated into products or used for solutions that keep the carbon out of the short carbon cycle for centuries. The PhD-project has contributed with quantification of the different carbon fractions that arise during seaweed farming and increased the understanding of the loss of carbon from the kelp into the seawater. It has been revealed that the timing of planting out and harvesting significantly impacts how much carbon can be captured through the cultivation. These numbers and new knowledge are crucial for calculation of the CO2-removal potentials in industrially scaled kelp farming and critical for establishing a future methodology for CO2 removal using seaweed cultivation. Such a methodology could be used as a climate measure. Further, the PhD-project has contributed with new knowledge about cultivation strategies that ensures both high quality food grade and large biomass for industrial purposes and possible carbon storage products. Norway has a long coastline with nutrient-rich waters and thus very favourable for seaweed cultivation, as well as a significant marine industry and world leading expertise, not only in ocean technologies but also in seaweed farming. This gives us a big opportunity to use the ocean for nature-based CO2-removal, as a significant climate action. Therefore, the most important impact of the PhD- project is the verification and quantification of CO2-removal during seaweed farming, as this can contribute to a future registration of a possible new CDR-methodology. Such a methodology is crucial for establishing seaweed farming as a climate action, enabling the sale of carbon credits in the carbon market.