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Increasing oceanic CO 2 has caused a decrease in oceanic pH, a process termed ocean acidification (OA). OA may benefit fleshy macroalgae due to the increased availability of inorganic carbon sources for photosynthesis since they are tolerant of decreases in pH. In this study, we analyzed multiple physiological responses of Bostrychia montagnei and Bostrychia calliptera from two biogeographic provinces of Brazil (Tropical Southwestern Atlantic [TSA] and Warm Temperate Southwestern Atlantic [WTSA]) after culturing them at a set of bioreactors in three pH levels (7.2, 7.6, and 8.0). Two pH were decreased by CO 2 enrichment into the culture medium. Molecular analyses using plastidial (rbc L-3P) and mitochondrial (COI-5P) DNA markers were also performed to identify genetic divergences between macroalgae from TSA and WTSA. Molecular evidence of COI-5P marker demonstrated that the specimens of both B. montagnei and B. calliptera from TSA and WTSA constitute different haplotypes, with a strong segregation between them. Macroalgae from both localities increased their growth in treatments of decreased pH with increased CO 2 availability. Overall, physiological responses of the algae were not negatively affected by decreased pH. B. montagnei from TSA increased its polysaccharide and allophycocyanin content at pH 7.2, and B. montagnei from WTSA increased its low molecular weight carbohydrate content at pH 7.2 as well. Antioxidant activity — a proxy for physiological stress — was not affected by decreased pH. Our study indicates that haplotypes of B. montagnei and B. calliptera from TSA and WTSA can be relevant to CO 2 sequestration in mangroves once they are tolerant of decreased pH and increase their growth under increased CO 2 availability. [Display omitted] • Bostrychia species show tolerance to decreased pH by CO 2 enrichment of the seawater. • Bostrychia species increase their growth under increased CO 2 availability. • Bostrychia species can be relevant to CO 2 sequestration in mangroves. • Mitochondrial DNA marker reveals genetic divergence between Bostrychia populations from tropical and subtropical regions. • Overall, Bostrychia physiology is not negatively affected by ocean acidification. [ABSTRACT FROM AUTHOR]