Marine macroalgae are key components of coastal ecosystems, acting as primary producers and habitat structurers. Along the Brazilian coast, Gracilaria caudata (Gracilariales, Rhodophyta) populations have been heavily impacted by overharvesting for agar extraction and are exposed to global and local stressors, such as rising ocean temperatures and increased nutrient pollution from urbanization, respectively. We tested the hypothesis that these abiotic factors modulate sex determination in G. caudata , and that exposure to higher temperature and nutrient concentration impairs reproductive and somatic performance, regardless of the thallus color (red or greenish-brown phenotype). Sex ratio, biomass, growth, photosynthesis, and fertility were evaluated at two temperatures (25ºC and 33ºC) and three von Stosch (VS) nutrient concentrations (6.25%, 25%, and 50%). Our results suggest that reproductive aspects of G. caudata are influenced by both factors. Specifically, higher temperatures inhibit male reproductive structure formation (spermatangia), while elevated nutrient levels (VS 50%) reduce male fertility and shift the sex ratio of juvenile plants toward a male-biased composition, resulting in a higher proportion of male gametophytes relative to females (3♂:1♀ and 2♂:1♀ sex ratios for red and greenish-brown strains, respectively). Overall, the red specimens showed similar or higher physiological performance than greenish-brown ones. However, at 33ºC, greenish-brown specimens showed photoprotection and, in some conditions, higher biomass and growth rates than red ones, while the reverse pattern was not observed. These results suggest that rising temperatures may induce changes in the phenotypic composition of natural populations, leading to a shift from red to greenish-brown specimens.
Marchi et al. (Wed,) studied this question.