Effects of low-temperature stress on the green and brown strains of Kappaphycus alvarezii

The present study aimed to evaluate the effects of thermal variation on the green and brown strains of Kappaphycus alvarezii cultivated under laboratory conditions. The seaweeds were maintained in two cultivation regimes for 42 days: a control group at 26 ± 0.5°C and an experimental group subjected to a gradual decrease from 24 ± 0.5°C to 14 ± 0.5°C, with a 2°C reduction per week. Growth was monitored weekly and documented with photographic records. Biomass samples were collected at 22, 18, and 14 ± 0.5°C for analyses of photosynthetic pigments and phenolic compounds, while morphological evaluations using optical microscopy were carried out after exposure to 18 and 14 ± 0.5°C. The gradual decrease in temperature impaired daily growth in both strains, with a pronounced reduction from 18 ± 0.5°C and minimum values at 14 ± 0.5°C. The green strain maintained greater growth stability, while the brown strain preserved photosynthetic pigments more effectively under control conditions and exhibited higher levels of phenolic compounds at low temperatures. In addition, significant interactions between strain and treatment were observed for phycobiliproteins (phycoerythrin and phycocyanin), indicating that the response to cold stress results from the combination of these factors. These findings highlight that the green and brown strains exhibited distinct physiological, morphological, and biochemical responses to thermal stress, reinforcing the importance of selecting cold-tolerant strains to optimize productivity and mitigate seasonal risks in regions with harsh winters.