Diazotrophs provide a significant reactive nitrogen source in the ocean. Increased warming and stratification may decrease nutrient availability in the future, forcing microbial communities toward using dissolved organic matter (DOM). Not depending on reactive nitrogen availability, diazotrophs may be “winners” in a nutrient-depleted ocean. However, their ability to exploit DOM may influence this success. We exposed cultures of the widespread Crocosphaera to low (26°C, pH 8.1), moderate (28°C, pH 8.0), and extreme (30°C, pH 7.9) climate change scenarios, under control or DOM-amended conditions. Growth was suboptimal in the low and extreme treatments and favored in the moderate treatment. DOM was preferred as a carbon source regardless of the treatment and promoted N2 fixation in extreme conditions. This was reflected in the increased expression of photosynthesis genes to obtain energy. DOM provides Crocosphaera with a key ecological advantage, possibly dictating diazotroph-derived nitrogen inputs in the future ocean.