Anomalous Enhanced Calcification in Irregular Archaeocyaths in a High-CO2 World: Evidence for Photosymbiosis in Irregular Archaeocyaths

Archaeocyaths are reef-building calcareous sponges that are restricted to the Cambrian Period. Carbon mass-balance modeling (e.g., Berner and Kothavala, 2001) indicate that the level of atmospheric CO₂ rose very fast during the Cambrian, and it peaked in the Furongian at perhaps 18 times the present atmospheric level.

Experiments in which extant species of calcium-carbonate-secreting organisms, such as corals and pteropods, were subjected to rising levels of atmospheric CO₂ have led to concerns that ocean acidification may eventually prevent such organisms from secreting their skeletons.

To test the hypothesis that Cambrian reef-building irregular archaeocyaths suffered just this fate, we measured the thickness of archaeocyath skeletal elements in reefs of Esmeralda County, Nevada (Poleta and Harkless formations) and South China (Xiannudong and Tianheban formations). These reefs span the terminal 8 m.y. of the Early Cambrian.

Our hypothesis was that skeletal elements would exhibit a progressive thinning toward the end of the Early Cambrian, in response to acidification caused by rising levels of CO₂. In fact, we found just the opposite. Chi-square analysis of thickness data show a statistically significant thickening of skeletal elements through time.

These surprising results are in agreement with a recent study of living and fossils coccolithophores by Iglesias-Rogriguez et al. (2008). These phytoplankton increase their rate of calcification as the level of atmospheric CO₂ rises. In the case of coccolithophores, photosynthesis certainly plays a role in their ability to increase their rate of calcification in spite of increased acidification. We interpret the thickening skeletons of irregular archaeocyaths throughout the Early Cambrian to similarly record a photosynthetic response. These results support the interpretation that irregular archaeocyaths harbored photosynthesizing endosymbionts. Ocean acidification apparently was not the cause of the extinction of archaeocyaths.