CO2 tweaks nitrogen cycle

IN WHAT could have ramifications on the ocean food web, scientists have found that rising acidity of seawaters slows down marine nitrogen cycle.

Ocean acidification is the result of CO₂ dissolving in seawater and lowering its pH. Scientists know that a drop in ocean pH affects carbon cycle, reducing carbonate ions that corals use to build shells. Now a study has shown that CO₂- induced increase in acidity disrupts aquatic nitrification rate. The finding was published in the December 21 issue of PNAS. The nitrification process is central to nitrogen cycle in which marine microrganisms convert one form of nitrogen—ammonium —into nitrate, a form other aquatic organisms require.

Marine biologist Michael Beman, from University of California and his team collected five seawater samples from Atlantic and Pacific oceans from depths ranging from 45m to 240m. They modified the pH from 8.09-7.99 to 7.99-7.85—either by mixing CO₂ through the bottles or adding dilute acid.

Nitrification rate was measured and watched over 11 to 120 hours. They contended that acidification could reduce nitrification rate by 3-44 per cent within next few decades. They noted that if at pH 8 oceanic ammonia constitutes five per cent of total ammonia and ammonium, by 2100, acidification could reduce available ammonia by 50 per cent.

“The decreased pH levels can impact marine nitrogen cycle by reducing nitrification rate that directly affects microbial productivity and marine food web,” said Richard Feely, chemical oceanographer at Pacific Marine Laboratory, Seattle. But there could be a silver lining—reduction of nitrous oxide, a greenhouse gas.

The nitrification process produces nitrous oxide through oxidation of ammonium by aquatic nitrifier bacteria. “It is possible that decrease in nitrification will reduce nitrous oxide,” Beman said.

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