Light availability modulates the effects of warming in a marine N 2 fixer

Abstract. As a group of photosynthetic N2 fixers (diazotrophs), Trichodesmium species play an especially important role in the marine biogeochemical cycles of nitrogen and carbon, especially in oligotrophic waters. How ongoing ocean warming may interact with light availability to affect Trichodesmiumis not yet clear. We grew Trichodesmium erythraeum IMS 101 at three temperature levels of 23, 27 and 31 °C under two growth limiting and saturating light levels of 50 and 160 μmol quanta m−2 s−1, respectively, for at least 10 generations, and then measured physiological performances. Light availability significantly modulated the growth response of Trichodesmium to temperature, with the specific growth rate peaking at ~ 27 °C under the light–saturating conditions, while growth of light–limited cultures was non–responsive across the temperature range of 23–31 °C. When the acclimation of N2 fixation to growth temperatures was evaluated by short–term temperature norms, the optimum temperature (Topt) for N2 fixation increased by 0.6–1.4 °C in the cells grown under high levels of temperature and light, and the susceptibility to supra–optimal temperatures (deactivation energy, Eh) was decreased by 56 %–61 %. However, light limitation decreased the Topt by 0.5–1.8 °C and increased the supra–optimal temperature susceptibility by 33 %–71 %. This made all light–limited cultures unable to sustain N2 fixation during short–term exposure to higher temperatures (33–34 °C) that are not lethal for cultures grown under light–saturating conditions. Our results imply that Trichodesmium spp. growing under low light levels while distributed deep in the euphotic zone or under cloudy weather conditions might be more susceptible to ocean warming.