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

Abstract

Since productivity and growth of coral-associated dinoflagellate algae is nitrogen (N)-limited, dinitrogen (N2) 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 N2fixation by the coral holobiont. This laboratory study is the first to investigate short-term effects of ocean acidification on N2fixation 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 (pCO21080 μatm) caused a rapid and significant decrease (53%) in N2fixation rates associated with S. hystrix compared to the present day scenario (pCO2486 μatm). In addition, N2fixation 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 CO2conditions may result in decreased N2fixation 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.