Ocean acidification rapidly reduces dinitrogen fixation associated with the hermatypic coral Seriatopora hystrix

Since productivity and growth of coral-associated dinoflagellate algae is nitrogen (N)-limited, dinitrogen (N-2) fixation by coral-associated microbes is likely crucial for maintaining the coral-dinoflagellate symbiosis. It is thus essential to understand the effects future climate change will have on N-2 fixation by the coral holobiont. This laboratory study is the first to investigate short-term effects of ocean acidification on N-2 fixation activity associated with the tropical, hermatypic coral Seriatopora hystrix using the acetylene reduction assay in combination with calcification measurements. Findings reveal that simulated ocean acidification (pCO(2) 1080 mu atm) caused a rapid and significant decrease (53%) in N-2 fixation rates associated with S. hystrix compared to the present day scenario (pCO(2) 486 mu atm). In addition, N-2 fixation associated with the coral holobiont showed a positive exponential relationship with its calcification rates. This suggests that even small declines in calcification rates of hermatypic corals under high CO2 conditions may result in decreased N-2 fixation activity, since these 2 processes may compete for energy in the coral holobiont. Ultimately, an intensified N limitation in combination with a decline in skeletal growth may trigger a negative feedback loop on coral productivity exacerbating the negative long-term effects of ocean acidification.