Atmospheric carbon dioxide (CO2) concentrations are projected to double over the next century as human societies continue to burn fossil fuels and biomass, yet a large proportion of emitted anthropogenic CO2 will dissolve in the ocean rather than accumulate in the atmosphere.  Dissolution of CO2 in seawater produces a weak acid that has decreased surface ocean pH by ca. 0.1 below pre-industrial levels, and an additional 0.3-0.4 decline is expected by the year 2100.  This more than twofold increase in surface ocean hydrogen ion concentrations [H+] will be accompanied by increasing CO2 partial pressures (pCO2), increasing bicarbonate ion concentrations, decreasing carbonate ion concentrations, and multiple shifts in trace metal and nutrient chemistry. Predicting the responses of marine organisms, ecosystems, and biogeochemical processes to these fundamental changes in ocean chemistry is consequently a major scientific challenge. Although marine bacteria and archaea constitute the majority of biomass in the sea, sustain a large percentage of global primary production, and govern biogeochemical cycling of carbon and nitrogen, we lack a clear understanding of how they will react to ocean acidification.

In an acidifying ocean, microbial community responses to reduced seawater pH and elevated pCO2 may act as positive or negative feedbacks to perturbation of the carbon and nitrogen cycles, and guilds that carry out critical biogeochemical processes may be affected in unanticipated ways.  Our study examined how ocean acidification will alter the cycling of nitrogen in the sea.  Nitrogen is an essential nutrient for all organisms, but some microorganisms use nitrogen as an energy source; of these groups, ammonia oxidizers play a pivotal role in determining which forms of nitrogen are present—hence changes in their behavior may affect other marine organisms.  We experimentally reduced seawater pH in six experiments in the Atlantic and Pacific Oceans, and in every case, ammonia oxidation rates decreased.  These declines were similar throughout the sea, indicating that ocean acidification may produce global changes in nutrient cycles that may, in turn, alter marine ecology and chemistry.  


NSF press release:

Scientific American article:

Discovery News article:

Wired article (love the comments, but the author’s statement that this is ‘just a modeling study’ is flatly incorrect):


Top left: Sonoran coast from the GOC; photo by Mike Beman

Lower left: bow-wake surfing porpoises; photo by Angel White

Right: Sunrise at Bahia del Tobari, Mexico; photo by Mike Beman

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Ocean Acidification and the Nitrogen Cycle