Global declines in oceanic nitrification rates as a consequence of ocean acidification

Abstract

Ocean acidification produced by dissolution of anthropogenic carbon dioxide (CO 2 ) emissions in seawater has profound consequences for marine ecology and biogeochemistry. The oceans have absorbed one-third of CO 2 emissions over the past two centuries, altering ocean chemistry, reducing seawater pH, and affecting marine animals and phytoplankton in multiple ways. Microbially mediated ocean biogeochemical processes will be pivotal in determining how the earth system responds to global environmental change; however, how they may be altered by ocean acidification is largely unknown. We show here that microbial nitrification rates decreased in every instance when pH was experimentally reduced (by 0.05-0.14) at multiple locations in the Atlantic and Pacific Oceans. Nitrification is a central process in the nitrogen cycle that produces both the greenhouse gas nitrous oxide and oxidized forms of nitrogen used by phytoplankton and other microorganisms in the sea; at the Bermuda Atlantic Time Series and Hawaii Ocean Time-series sites, experimental acidification decreased ammonia oxidation rates by 38% and 36%. Ammonia oxidation rates were also strongly and inversely correlated with pH along a gradient produced in the oligotrophic Sargasso Sea (r 2 = 0.87, P < 0.05). Across all experiments, rates declined by 8-38% in low pH treatments, and the greatest absolute decrease occurred where rates were highest off the California coast. Collectively our results suggest that ocean acidification could reduce nitrification rates by 3-44% within the next few decades, affecting oceanic nitrous oxide production, reducing supplies of oxidized nitrogen in the upper layers of the ocean, and fundamentally altering nitrogen cycling in the sea.A tmospheric carbon dioxide (CO 2 ) concentrations are projected to double over the next century as human societies continue to burn fossil fuels and biomass (1), yet a large proportion of emitted anthropogenic CO 2 will dissolve in the ocean rather than accumulate in the atmosphere (1-4). Dissolution of CO 2 in seawater produces a weak acid that has decreased surface ocean pH by ∼0.1 below preindustrial levels, and an additional 0.3-0.4 decline is expected by the year 2100 (2, 4). This more than twofold increase in surface ocean hydrogen ion concentrations [H + ] will be accompanied by increasing CO 2 partial pressures (pCO 2 ), increasing bicarbonate ion concentrations [HCO 3 − ], decreasing carbonate ion concentrations [CO 3 2− ], and multiple shifts in trace metal and nutrient chemistry (2-4). Predicting the responses of marine organisms, ecosystems, and biogeochemical processes to these fundamental changes in ocean chemistry is consequently a major scientific challenge (5-9). 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 (10), we lack a clear understanding of how they will react to ocean acidification (9, 11).In an acidifying ocean, microbial comm...

Impact of ocean acidification on benthic and water column ammonia oxidation

“…Of significant concern is the potential impact of OA on the processes involved in the biogeochemical cycling of nitrogen (N) and, in particular, nitrification; a key microbial process through which remineralized N in the form of ammonia (NH 3 ) is oxidized to inorganic nitrite (NO 2 − ) and subsequently to nitrate (NO 3 − ). A number of recent and past studies have demonstrated an inhibitory effect of decreasing pH on water column nitrification activity [Beman et al, 2011;Huesemann et al, 2002;Jones, 1992;Stein et al, 1997]. The implications of such an effect on ecosystem function could be significant with modeling studies suggesting that substantial reductions in water column nitrification over the next century would affect nutrient stoichiometry, denitrification and by extension marine productivity and the biological carbon pump [Beman et al, 2011;Blackford and Gilbert, 2007].…”

“…A number of recent and past studies have demonstrated an inhibitory effect of decreasing pH on water column nitrification activity [Beman et al, 2011;Huesemann et al, 2002;Jones, 1992;Stein et al, 1997]. The implications of such an effect on ecosystem function could be significant with modeling studies suggesting that substantial reductions in water column nitrification over the next century would affect nutrient stoichiometry, denitrification and by extension marine productivity and the biological carbon pump [Beman et al, 2011;Blackford and Gilbert, 2007]. However, the impact of OA on nitrification within sediments has not been examined.…”

Major impacts of climate change on deep-sea benthic ecosystems

“…Ocean acidification is also predicted to reduce microbial production of nitrate from ammonium (Beman et al, 2011), which could have major consequences for oceanic primary production because a significant fraction of the nitrate used by phytoplankton is generated by nitrification at the ocean surface (Yool et al, 2007). Major consequences of such changes over regional scales will probably include (1) reductions in primary production combined with (2) shifts from diatom-dominated (low SA:V ratio) phytoplankton assemblages with high POC-export efficiencies to picoplankton communities (high SA:V ratio) characterized by low export efficiencies Morán et al, 2010;Morán et al, 2015).…”

Long-term legacy of massive carbon input to the Earth system: Anthropocene versus Eocene

“…A second impact of anthropogenic CO 2 emissions is ocean acidification, which refers to the ongoing decline in ocean pH and the reduction in the ocean's carbonate mineral saturation state, with possible negative consequences for marine life [3][4][5]. Other geochemical and physical consequences of an increasingly acidic ocean include effects on metal speciation, reduced NH 3 /NH 4 + ratios (probably affecting ammonia oxidation rates), the marine source of atmospherically active trace gases and alteration of underwater sound absorption [6][7][8][9].…”