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Surface temperature in the ocean is projected to be elevated and more variable in the future, which will interact with other environmental changes like reduced nutrient supplies. To explore these multiple stressor relationships, we tested the influence of thermal variation on the key marine diazotrophic cyanobacterium Trichodesmium erythraeum GBRTRLI101 as a function of the limiting nutrient phosphorus (P). Two constant temperature treatments represented current winter (22 degrees C) and summer (30 degrees C) mean values. Three variable temperature treatments fluctuated around the constant control values: Mean 22 degrees C, either +/- 2 degrees C or +/- 4 degrees C; and mean 30 degrees C +/- 2 degrees C. Each thermal treatment was grown under both P-replete (10 mu mol/L) and P-limiting conditions (0.2 mu mol/L). Effects of thermal variability on Trichodesmium were mainly found in the two winter variable temperature treatments (22 degrees C +/- 2 degrees C or +/- 4 degrees C). P availability affected growth and physiology in all treatments and had significant interactions with temperature. P-replete cultures had higher growth and nitrogen and carbon fixation rates in the 22 degrees C constant control, than in the corresponding variable treatments. However, physiological rates were not different in the P-replete constant and variable treatments at 30 degrees C. In contrast, in P-limited cultures an advantage of constant temperature over variable temperature was not apparent. Phosphorus use efficiencies (PUE, mol N or C fixed h(-1) mol cellular P-1) for nitrogen and carbon fixation were significantly elevated under P-limited conditions, and increased with temperature from 22 to 30 degrees C, implying a potential advantage in a future warmer, P-limited environment. Taken together, these results imply that future increasing temperature and greater thermal variability could have significant feedback interactions with the projected intensification of P-limitation of marine N-2-fixing cyanobacteria.