Influence of reproductive cell density of Ulva ohnoi (Ulvophyceae) in seeded Algal Turf Scrubbers (sATS): biomass yield and bioremediation implications

The genus Ulva has gained prominence in aquaculture due to its rapid growth, adaptability, and capacity to deliver ecosystem services such as nutrient remediation while producing commercially valuable biomass. This study evaluates the performance of Ulva ohnoi cultivated in a seeded Algal Turf Scrubber (sATS), a land-based system designed to enhance cultivation control through reproductive cell inoculation. Fronds were collected from an aquaculture facility in Florianópolis, southern Brazil, induced to release reproductive cells, and inoculated at five distinct densities into sATS units operated under controlled conditions for 41 days. Growth parameters, including daily growth rate (DGR), frond morphology, and areal biomass productivity, were assessed alongside nutrient uptake kinetics for ammonium (N–NH 4 ⁺) and phosphate (P–PO 4 3 ⁻). Results revealed that lower inoculation densities significantly improved cumulative DGR and frond length due to reduced intraspecific competition, while higher densities enhanced nutrient uptake efficiency and rate, particularly during the initial surge phase. Despite these contrasting responses, overall areal productivity remained stable across treatments (11.41 ± 1.41 g DW m⁻ 2 day⁻ 1 ), while the system’s capacity to respond to different inoculation densities highlights its potential for dual applications, either maximizing biomass yield or enhancing nutrient removal efficiency. When expressed as volumetric productivity, biomass production reached 0.96 g DW L⁻ 1 day⁻ 1 , surpassing most reported values and demonstrating high efficiency in water-limited culture systems. This result highlights the importance of coupling inoculation density with nutrient-supply strategies to overcome resource limitations and optimize system performance.