Ocean acidification and warming are current global challenges that marine diazotrophs, i.e., bacteria capable to reduce gas N 2 to ammonium, must cope with. Barely nothing is known about the effects of changes in pH and temperature with increased CO 2 levels in combination with different nutrient regimes in N 2 -fixers, especially in heterotrophic bacteria, and those found in association with seagrasses. Here, we selected four cultivable diazotrophs, i.e., cyanobacteria and heterotrophic bacteria, found in association with the endemic Mediterranean seagrass Posidonia oceanica . We tested different pH (from pH 4 to 8) and temperature levels (from 12 to 30 ºC), both under different nutrient levels of phosphorus, P (0.1 µM and 1.5 mM) and iron, Fe (2 nM and 1 µM). We also tested different CO 2 concentrations [410 and 1000 particles per million (ppm)] under different P/Fe and temperature levels (12, 18 and 24 ºC). We reveal the sensitivity of the diazotrophs tested to lower pH (pH < 7-8) and warmer temperatures (ºC > 12-24). Growth responses were dependent on their nutrient status, as was evidenced for the heterotrophic bacteria, which displayed a higher susceptibility to changes in pH, temperature, and CO 2 than the cyanobacterial species. High CO 2 levels (1000 ppm) decreased heterotrophic growth only when cultures were nutrient-limited, regardless of the temperature. On the other hand, cyanobacteria were insensitive to CO 2 increased levels, independently of the nutrient and temperature levels. Changes in N 2 -fixation were mainly controlled by changes in growth. Our findings show that diazotrophic responses against climate change factors can be dependent on their nutrient status and their mode of life.