Trade-offs between photosynthetic capacity and structural economy underlie the rapid proliferation of floating Ulva prolifera.

The green tide alga Ulva prolifera shifts from an attached to a floating state during bloom formation, yet the physiological basis of this transition remains unclear. Here, using attached and floating thalli derived from a single parental clonal line and cultured under identical controlled conditions, we combined physiological measurements, protein-fraction analysis, and transcriptomics to compare growth and resource-allocation patterns between the two states. Despite similar bulk carbon and nitrogen contents, floating thalli reached significantly greater thallus length than attached thalli. Attached thalli showed higher maximum photosynthetic rate (Pmax), dark respiration (Rd), effective PSII quantum yield (Y(II)), and photosynthetic nitrogen-use efficiency (PNUE), whereas floating thalli showed higher maximum PSII efficiency (Fv/Fm) but lower Pmax, Rd, and PNUE. Together, these physiological patterns suggest that floating thalli maintain PSII efficiency while operating with lower photosynthetic capacity and respiratory demand. Floating thalli also exhibited higher levels of water-soluble (Pr) and SDS-soluble (Pp) proteins, together with lower levels of the SDS-insoluble fraction (Pw), consistent with reduced investment in detergent-insoluble structural pools. Transcriptomic analysis further showed enrichment of chloroplast- and photosynthesis-related functions in attached thalli, but cytoskeletal, cell-cycle, and DNA-repair functions in floating thalli. Together, these results support a lower-cost, resilience-oriented state in floating U. prolifera, in which rapid proliferation is associated with reduced structural investment rather than maximized instantaneous photosynthetic capacity. This resilience-associated strategy may help explain how floating thalli sustain rapid proliferation and contribute to the persistence of large-scale green tides under the fluctuating light and physical stress encountered at the sea surface.