Photosynthesis and calcification of the stromatolitic freshwater cyanobacteriumRivularia

Planar optodes and microelectrodes have been used to locate photosynthetic activity in calcified Rivularia biasolettiana and R. haematites. Maximum photosynthesis, measured as oxygen concentration and pH, occurred in the upper 1–3 mm of the colony, coinciding with a region of calcification. Stoichiometric calculations indicate that photosynthesis could account for almost all of the calcification within this region but further calcification was identified deeper within the colonies where photosynthesis was negligible. Overall, it was estimated that a maximum of 14% of the calcium carbonate within the colonies could have been formed as a direct result of photosynthesis.