Physiological constraints on the global distribution of <I>Trichodesmium</I> ? effect of temperature on diazotrophy

International audienceThe cyanobacterium Trichodesmium is an important link in the global nitrogen cycle due to its significant input of atmospheric nitrogen into the ocean. Incorporating Trichodesmium in ocean biogeochemical circulation models relies on field-based correlations between temperature and Trichodesmium abundance. The observed correlation of Trichodesmium abundance with temperature in the ocean may result in part from a direct effect on Trichodesmium growth rates through the control of cellular biochemical processes, or indirectly through its influence on mixed layer depth, light and nutrient regimes. Here we present results indicating that the observed correlation of Trichodesmium with temperature in the field reflects primarily the direct physiological effects of temperature on diazotrophic growth of Trichodesmium. Trichodesmium IMS-101 (an isolate of T. erythraeum) could acclimate and grow at temperatures ranging from 20 to 34°C. Maximum growth rates (?max=0.25 day?1) and maximum nitrogen fixation rates (0.13 mmol N mol POC?1 h?1) were measured within 24 to 30°C. This empirical relationship and global warming scenarios derived from state-of-the-art climate models set a physiological constraint on the future distribution of Trichodesmium that could significantly affect nitrogen input into oligotrophic waters by this diazotroph