Background
With respect to the effects of ocean acidification on the marine nitrogen cycle, the authors write that "the current hypothesis, based on the manipulation of water column pH in laboratory studies, states that decreasing pH will impact the nitrogen cycle by decreasing nitrification," and this decrease in the microbial conversion of ammonium to nitrate would likely negatively impact both marine phytoplankton composition and production.

What was done
Fulweiler et al. record that they "compiled an existing unique data set of concurrent water column nitrification rates and water column pH values from a temperate New England estuary (Narragansett Bay, Rhode Island, USA)," which had been obtained and reported previously by Berounsky (1990) and Berounsky and Nixon (1985a,b, 1990, 1993).

What was learned
The four researchers say they "found the exact opposite trend to the current hypothesis: water column nitrification rates were highest at low pH and decreased significantly as pH increased," and they note that "these results are in direct contradiction to some of the more recently published studies examining the impact of ocean acidification on marine nitrification (Huesemann et al., 2002; Beman et al., 2011)." However, they indicate that their findings "are consistent with previous studies from three decades ago," citing the work of Anthonisen et al. (1976) and Focht and Verstraete (1977).

What it means
Fulweiler et al. emphasize that their results "highlight that nitrifying organisms in coastal systems tolerate a wide range of pH values," adding that "the degree of negative correlation with pH may depend on site-specific environmental conditions." And in a grand understatement, they conclude by saying their findings indicate that "the current hypothesis of the negative impacts of ocean acidification on nitrification, at least for the coastal ocean, might need reevaluation."

References
Anthonisen, A.C., Loehr, R.C., Prakasam, T.B. and Srinath, E.G. 1976. Inhibition of nitrification by ammonia and nitrous acid. Journal of Water Pollution Control Federation 48: 835-852.

Beman, J.M., Chow, C.-E., King, A.L., Feng, Y., Fuhrman, J.A., Andersson, A., Bates, N.R., Popp, B.N. and Hutchins, D.A. 2011. Global declines in ocean nitrification rates as a consequence of ocean acidification. Proceedings of the National Academy of Sciences USA 108: 208-213.

Berounsky, V.M. 1990. Rates of Nitrification and Their Importance to the Nitrogen Cycle of Narragansett Bay. University of Rhode Island, Rhode Island, USA.

Berounsky, V.M. and Nixon, S.W. 1985a. Eutrophication and the rate of net nitrification in a coastal marine ecosystem. Estuarine, Coastal and Shelf Science 20: 773-781.

Berounsky, V.M. and Nixon, S.W. 1985b. The role of nitrification in contributing to low oxygen conditions in the Providence River Estuary (RI). Estuaries 8: A102.

Berounsky, V.M. and Nixon, S.W. 1990. Temperature and the annual cycle of nitrification in waters of Narragansett Bay. Limnology and Oceanography 35: 1610-1617.

Berounsky, V.M. and Nixon, S.W. 1993. Rates of nitrification along an estuarine gradient in Narragansett Bay. Estuaries 16: 718-730.

Focht, D.D. and Verstraete, W. 1977. Biochemical ecology of nitrification and denitrification. Advances in Microbial Ecology 1: 135-214.

Huesemann, M.H., Skillman, A.D. and Crecelius, E.A. 2002. The inhibition of marine nitrification by ocean disposal of carbon dioxide. Marine Pollution Bulletin 44: 142-148.