Desiccation‐induced changes in the photosynthetic efficiency and nutrient uptake of Eucheuma denticulatum ( Rhodophyta )

SUMMARY Eucheumatoids are the most extensively cultivated seaweeds in the Philippines, yet climate‐driven abiotic stressors increasingly threaten their production. Desiccation during extreme low tides is a major but understudied stressor in shallow‐water farms. This study examined how desiccation affects photosynthetic performance, moisture retention, and nutrient uptake in Eucheuma denticulatum . Thalli ( n = 3 per treatment) were exposed to 2, 4, and 6 h of desiccation, while controls remained immersed. Maximum quantum yield ( F v /F m ) and moisture content were measured at initial, immediately after desiccation, after 1 h of reimmersion in enriched seawater, and after 24 h and 7 d in natural seawater. Nutrient uptake was quantified from changes in seawater concentrations of total ammonia nitrogen (TAN) and phosphate (PO 4 3− ) at 15, 30, 45, and 60 min post‐rehydration. Only the 2‐h treatment retained sufficient moisture to recover after 7 d ( F v /F m = 0.684 ± 0.048), comparable to the control (0.645–0.767), whereas longer exposures lost too much moisture to regain photosynthetic function. TAN uptake peaked at 15 min in the control, 4‐h, and 6‐h treatments, while the 2‐h treatment showed a delayed peak at 45 min. In contrast, PO 4 3− uptake was positive only in the control; all desiccated treatments exhibited negative values, with the magnitude of efflux varying among treatments. The 75% moisture retention under the 2‐h treatment reflects a recoverable hydration range in this study, with greater water loss impairing photosynthetic recovery, rehydration, and PO 4 3− uptake. Prolonged desiccation compromises nutrient acquisition and promotes solute loss, highlighting the vulnerability of E. denticulatum to extended air exposure under increasingly frequent low‐tide extremes.