Recent studies on the diazotrophic cyanobacterium Trichodesmium showed that increasing CO2 partial pressure (pCO2) enhances N2 fixation and growth. We studied the in situ and satellite-derived environmental parameters within and outside a Trichodesmium bloom in the western coastal Bay of Bengal (BoB) during the spring intermonsoon 2009. Here we show that the single most important nitrogen fixer in today’s ocean, Trichodesmium erythraeum, is strongly abundant in high (≥300？μatm) pCO2 concentrations. N？:？P ratios almost doubled (~10) at high pCO2 region. This could enhance the productivity of N-limited BoB and increase the biological carbon sequestration. We also report presence of an oxygen minimum zone at Thamnapatnam. Earlier studies have been carried out using lab cultures, showing the increase in growth rate of T. erythraeum under elevated pCO2 conditions, but to our knowledge, this study is the first to report that in natural environment also T. erythraeum prefers blooming in high pCO2 concentrations. The observed CO2 sensitivity of T. erythraeum could thereby provide a strong negative feedback to rising atmospheric CO2 but would also drive towards phosphorus limitation in a future high CO2 world. 1. Introduction Climate change will significantly alter the marine environment within the next century and beyond. Future scenarios predict an increase from the current ~380 to ~750 to ~1,000？ppm CO2 in the atmosphere towards the end of this century [1, 2]. As the ocean takes up this anthropogenic CO2, dissolved inorganic carbon (DIC) in the surface ocean increases, while the pH decreases [3]. Rising global temperatures are likely to increase the surface ocean stratification, which may affect the light regime in the upper mixed layer as well as nutrient input from deeper waters [4]. Uncertainties remain regarding both the magnitude of the physicochemical changes and the biological responses of organisms, including species and populations of the oceanic primary producers at the base of the food web. In view of potential ecological implications and feedbacks on climate, several studies have examined the sensitivity of key phytoplankton species to pCO2 [5–8]. Significant response to elevated pCO2 was observed in N2-fixing cyanobacteria [9–13], which play a vital role in marine ecosystems by providing a new source of biologically available nitrogen to the otherwise nitrogen-limited regions [14]. Trichodesmium, a colony-forming cyanobacterium, fixes nitrogen in an area corresponding to almost half of Earth’s surface [15] and is estimated to account for more
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