Crustose coralline algae (CCA) are active participants in the carbon cycle of reefs worldwide. However, mechanisms underpinning the flux of carbon into (acquisition), within (partitioning via photosynthesis and calcification) and out (DOC release) of the thalli of reef-building CCA is largely unknown. With the carbonate chemistry and temperature of seawater changing at rapid rates, the quantification of these patterns would provide an essential tool for understanding the underlying physiological strategies and responses to environmental change in CCA. We quantified carbon acquisition, partitioning, and release in two high-light (Porolithon cf. onkodes and Lithophyllum cf. insipidum) and low-light (Lithothamnion proliferum and Sporolithon cf. durum) species of CCA under ambient and elevated (IPCC RCP 8.5) levels of pCO2 and temperature. We found distinct ambient acquisition, partitioning, and release strategies between high- and low-light reef-builders. When faced with global stressors, there was an association in high-light CCA between decreased surficial carbon retention, increased DOC release, and failure to increase bicarbonate uptake for photosynthesis. In low-light CCA, there was an association between maintained or even increased carbon retention, reversal of DOC release (i.e. uptake, likely involving surface microbes), and increased bicarbonate uptake. Our results suggest that the surficial carbon metabolism of CCA occupying low-light reef environments is more robust than that of high-light reef-builders amidst OA and warming.