Sustainable Large-Scale Aquaculture of the Northern Hemisphere Sea Lettuce, Ulva fenestrata, in an Off-Shore Seafarm

Abstract

The growing world population demands an increase in sustainable resources for biorefining. The opening of new farm grounds and the cultivation of extractive species, such as marine seaweeds, increases worldwide, aiming to provide renewable biomass for food and non-food applications. The potential for European large-scale open ocean farming of the commercial green seaweed crop Ulva is not yet fully realized. Here we conducted manipulative cultivation experiments in order to investigate the effects of hatchery temperature (10 and 15 °C), nutrient addition (PES and 3xPES) and swarmer density (500 and 10,000 swarmers ml−1) on the biomass yield and biochemical composition (fatty acid, protein, carbohydrate, pigment and phenolic content) of off-shore cultivated Ulva fenestrata in a Swedish seafarm. High seedling densities were optimal for the growth of this northern hemisphere crop strain and significantly increased the mean biomass yield by ~84% compared to low seedling densities. Variations of nutrients or changes in temperature levels during the hatchery phase were not necessary to increase the subsequent growth in an open-water seafarm, however effects of the factors on the thallus habitus (thallus length/width) were observed. We found no significant effect of the environmental factors applied in the hatchery on the total fatty acid or crude protein content in the off-shore cultivated Ulva. However, low seedling density and low temperature increased the total carbohydrate content and furthermore, high temperature in combination with high nutrient levels decreased the pigment content (chlorophyll a, b, carotenoids). Low temperature in combination with high nutrient levels increased the phenolic content. Our study confirms the successful and sustainable potential for large-scale off-shore cultivation of the Scandinavian crop U. fenestrata. We conclude that high seedling density in the hatchery is most important for increasing the total biomass yield of sea-farmed U. fenestrata, and that changing temperature or addition of nutrients overall does not have a large effect on the biochemical composition. To summarize, our study contributes novel insights into the large-scale off-shore cultivation potential of northern hemisphere U. fenestrata and underpins suitable pre-treatments during the hatchery phase of seedlings to facilitate a successful and cost-efficient large-scale rope cultivation.

Closed life-cycle aquaculture of sea lettuce (Ulva fenestrata): performance and biochemical profile differ in early developmental stages

“…Assessments of the fatty acid content and composition of lyophilized U. fenestrata juveniles followed a direct transesterification method which was successfully applied in previous studies on Ulva biomass (see also Toth et al, 2020;Steinhagen et al, 2021;Steinhagen et al, 2022) and was described in detail by Harrysson et al (2018). We used C17:0 as internal standard for quantification and identification of fatty acids was done using GLC-463 Reference standards (Nu-Check Prep, Inc.).…”

“…temperature, salinity, eutrophication) (Steinhagen et al, 2019a;Steinhagen et al, 2019b), which offers excellent qualities as crop species for the emerging Blue Economy at a global scale. Cultivation oriented research especially on the foliose shaped sea-lettuce-type of Ulva has increased during the past ten to fifteen years (Califano et al, 2020;Toth et al, 2020;Steinhagen et al, 2021;Steinhagen et al, 2022). Foliose species of Ulva are mostly maintained in free-floating cultures and are cultivated near-or on-shore (Hafting et al, 2015) in tanks (Toth et al, 2020), bioreactors (Sebök et al, 2019), basins (e.g.…”

“…Foliose species of Ulva are mostly maintained in free-floating cultures and are cultivated near-or on-shore (Hafting et al, 2015) in tanks (Toth et al, 2020), bioreactors (Sebök et al, 2019), basins (e.g. paddle-wheel basins) (Amosu et al, 2016) or ponds (Califano et al, 2020) and more recently also off-shore (Steinhagen et al, 2021;Steinhagen et al, 2022). Due to the increasing interest of the European seaweed sector in the aquaculture of Ulva specieswhich is based on the combination of above named compelling traits for cultivation and biomass valorizationupscaling processes, novel large-scale farming techniques, and closed life-cycle crop systems are needed.…”

“…U. fenestrata is a putative crop species in the Northern Hemisphere (Steinhagen et al, 2021;Steinhagen et al, 2022) where it is abundantly present in marine water bodies (Hughey et al, 2019;Steinhagen et al, 2019a). Individuals can be found in the sublittoral and are mainly present during Northern Hemisphere spring to summer conditions, whereas especially during summer conditions fertile margins of mature individuals could be observed.…”

Food-Grade Biorefinery Processing of Macroalgae at Scale: Considerations, Observations and Recommendations

“…If the macroalgae are cultivated, set growth depth [17] can be controlled, as can seedling age and density, leading to a more standardised, higher quality 'crop' than from wild harvesting with potentially few opportunistic other algae present. Hatchery conditions can also affect off-shore macroalgae composition; for example, in off-shore cultivated U. fenestrata, low seedling density increased the proportion of carbohydrate in the harvested material, but high seedling density was the most important factor for producing high total biomass [18].…”