Crustose coralline algae (CCA) are key reef-building primary producers that are known to induce the metamorphosis and recruitment of many species of coral larvae. Reef biofilms (particularly microorganisms associated with CCA) are also important as settlement cues for a variety of marine invertebrates, including corals. If rising sea surface temperatures (SSTs) affect CCA and/or their associated biofilms, this may in turn affect recruitment on coral reefs. Herein, we report that the CCA Neogoniolithon fosliei, and its associated microbial communities do not tolerate SSTs of 32 1C, only 2-4 1C above the mean maximum annual SST. After 7 days at 32 1C, the CCA exhibited clear signs of stress, including bleaching, a reduction in maximum quantum yield (F v /F m ) and a large shift in microbial community structure. This shift at 32 1C involved an increase in Bacteroidetes and a reduction in Alphaproteobacteria, including the loss of the primary strain (with high-sequence similarity to a described coral symbiont). A recovery in F v /F m was observed in CCA exposed to 31 1C following 7 days of recovery (at 27 1C); however, CCA exposed to 32 1C did not recover during this time as evidenced by the rapid growth of endolithic green algae. A 50% reduction in the ability of N. fosliei to induce coral larval metamorphosis at 32 1C accompanied the changes in microbiology, pigmentation and photophysiology of the CCA. This is the first experimental evidence to demonstrate how thermal stress influences microbial associations on CCA with subsequent downstream impacts on coral recruitment, which is critical for reef regeneration and recovery from climate-related mortality events.