Calcification and Productivity in Symbiont-Bearing Foraminifera

Introducing their study, authors Vogel and Uthicke (2012) write that "several studies highlight the importance of large benthic foraminifera (LBF) as biological indicators for water quality and ecosystem health," citing Hallock et al. (2003), Uthicke and Nobes (2008) and Uthicke et al. (2010); but they say that "impacts of ocean acidification on LBF physiology are not well studied." And so they decided "to investigate the influence of increased CO₂ on calcification and photobiology of LBF hosting different types of photosynthetic endosymbionts." For their investigation, the authors collected samples of the foraminiferal Amphistegina radiata, Heterostegina depressa and Marginopora vertebralis from two reefs of the Great Barrier Reef on several field trips between October 2010 and April 2011; and back in the laboratory they exposed them over a period of six weeks to four different seawater conditions (467, 784, 1169 and 1662 ppm CO₂), while periodically measuring a number of physiological parameters and processes. So what did they find?

The two researchers' results revealed no negative effects of elevated CO₂ on the calcification of any of the LBF species investigated compared to control conditions. And in the case of M. vertebralis, they found that its growth rate was actually increased with elevated CO₂, with mean calcification rates at the two highest CO₂ treatments (1169 and 1662 ppm) being some 63% greater than those at the two lowest treatments (467 and 784 ppm). With respect to chlorophyll a content, they also found that increased CO₂ had no effect on either A. radiata or H. depressa; but they observed that "chlorophyll a content approximately doubled in M. vertebralis from initial to final measurements." In addition, they report that "the maximum quantum yield of A. radiata, H. depressa and M. vertebralis did not vary significantly between different CO₂ treatments." And neither did they find any significant negative impacts on photosynthetic production and respiration in the three LBF species.

In stating their conclusions, Vogel and Uthicke write that their study "illustrated that the species investigated were still able to build up their calcite skeletons in CO₂ conditions predicted for the year 2100 and beyond," while highlighting the fact that "contrary to expectations, M. vertebralis showed significantly increased growth rates in elevated CO₂."

Additional References
Hallock, P., Lidz, B.H., Cockey-Burkhard, E.M. and Donnelly, K.B. 2003. Foraminifera as bioindicators in coral reef assessment and monitoring: the FORAM Index. Environmental Monitoring and Assessment 81: 221-238.

Uthicke, S. and Nobes, K. 2008. Benthic Foraminifera as ecological indicators for water quality on the Great Barrier Reef. Estuarine, Coastal and Shelf Science 78: 763-773.

Uthicke, S., Thompson, A. and Schaffelke, B. 2010. Effectiveness of benthic foraminiferal and coral assemblages as water quality indicators on inshore reefs of the Great Barrier Reef, Australia. Coral Reefs 29: 209-225.