Low molecular weight carbohydrate patterns of mangrove macroalgae from different climatic niches under ocean...

Ocean acidification has increased due to the enhanced solubility of CO₂ in seawater. Mangrove macroalgae in tropical and subtropical coastal regions can benefit from the higher availability of CO₂ for photosynthesis and primary production. However, they can be negatively affected by the simultaneously occurring warming and increased salinity in estuaries. Thus, we analyzed the isolated effects of ocean acidification and the interactive effects of increased temperature and salinity on the low molecular weight carbohydrate (LMWC) contents of the mangrove red macroalgae Bostrychia montagnei and Bostrychia calliptera from Brazilian tropical and subtropical populations. Specimens from both climatic niches were tolerant to pH decreased by CO₂ enrichment and enhanced their LMWC contents under increased availability of CO₂. Specimens from both climatic niches also accumulated their dulcitol and sorbitol contents to cope with warming and salt stress. Nevertheless, temperature of 34 °C was lethal for tropical macroalgae, while 29 °C and 31 °C were lethal for subtropical B. calliptera under salinity of 35. Tropical and subtropical B. montagnei synthesized dulcitol (5-110 mmol kg^-1 dry weight) and sorbitol (5-100 mmol kg^-1 dry weight) as osmoregulatory, energy and thermal protection compounds, whereas tropical and subtropical B. calliptera synthesized mainly dulcitol (10-210 mmol kg^-1 dry weight). Although digeneaside has an energy function in Bostrychia spp., it is not an osmolyte or thermal protection compound. Our data demonstrated that both tropical and subtropical Bostrychia spp. benefit from ocean acidification by CO₂ enrichment, increasing their LMWC contents. However, warming and increased salinity in estuaries will be detrimental to them, even they producing protective metabolites. Multifactorial approaches are recommended to investigate whether negative effects of increased temperature and salinity nullify positive effects of ocean acidification on these Bostrychia species/populations.