Abstract
The use of germling clusters derived from spontaneous sporulation offers a promising strategy to stabilize Ulva cultivation, particularly during reproductive events that compromise biomass yield. This study assessed the growth, photophysiological responses, and biochemical composition of unattached Ulva sp. germling clusters cultivated in flat-bag photobioreactors under two different nutrient supplement rates: nitrate additions of 50 or 500 µmol L⁻ 1 every 2 days, with phosphate supplied to maintain an N:P molar ratio of 10:1, over a 14-day period. Biomass density remained stable across treatments, suggesting limited growth. Higher nitrogen availability enhanced photosynthetic performance, as indicated by increased F v /F m , and promoted greater accumulation of chlorophyll a , b , and lutein. In contrast, nutrient-limited conditions stimulated the accumulation of phenolic compounds and ulvan, potentially as part of a stress-induced metabolic shift. Total protein content, estimated from total nitrogen, was higher under nitrogen-enriched conditions. These results demonstrate that photobioreactors can support the early-stage cultivation of Ulva germling clusters and enable modulation of biomass composition through nutrient management. This approach may contribute to improved flexibility and efficiency in land-based macroalgae production systems.