What was done
The authors cultured asexually-produced individuals of Marginopora kudakajimensis -- a large calcifying microorganism that contributes to both organic and inorganic carbon production in coral reefs -- under carefully controlled laboratory conditions for a period of 71 days in glass jars containing approximately 110 ml of filtered natural seawater (control pH of about 8.2) and two less basic pH conditions of about 7.9 and 7.7, which lower values were created by additions of 0.1 N HCl.

What was learned
In declining from the control pH of 8.2 to a pH of 7.9, the mean maximum shell diameter of the large foraminifer actually rose by 8.6%, while its mean shell weight rose by a much smaller and insignificant 0.7%. As the seawater's pH declined all the way to 7.7, however, the organism's mean maximum shell diameter fell by 12.1%, while its mean shell weight fell by 49.3%.

What it means
Kuroyanagi et al. conclude that if oceanic pH remains within the range of 8.2 to 7.9, the "large foraminifers should be able to maintain present calcification rates," but they note that any further drop in pH could lead to reduced rates of calcification. We hasten to add, however, that although the IPCC's A2 scenario predicts a maximum pH decline of approximately 0.5 pH units by about AD 2270, the more recent analysis of Tans (2009) suggests a maximum pH drop of only about 0.14 unit at about AD 2090, after which pH begins to rise to asymptotically return to its current value after several hundred years. This latter projection, therefore, suggests that oceanic pH will not even -- or ever -- come close to creating a major decline in M. kudakajimensis calcification rate.

Reference
Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future. Oceanography 22: 26-35.