Cyanophyte calcification and changes in ocean chemistry

Abstract

Cyanophytes range from at least 2,200 Myr (ref. 1) to the Recent, but they only produced common marine shelly fossils during the Palaeozoic and Mesozoic (570–80 Myr) (Fig. 1). This contrasts with the pattern of metazoan evolution in which rapid diversification near the Precambrian–Cambrian boundary was closely accompanied by skeletonization² which has been retained in marine environments to the Recent. Attempts to explain this unusual geological distribution of marine calcareous cyanophytes cannot be made solely by reference to biological processes because these algae are mainly dependent on environmental conditions for their calcification³. Thus, the presence or absence of calcified cyanophytes may be a general indication of long-term changes in seawater chemistry. This likelihood has been recognized previously⁴ but has not been explored in any detail. Here I outline some possible explanations and suggest that cyanophyte calcification was facilitated by enhancement of marine CaCO₃ precipitation rates in the late Precam-brian because of decrease in the Mg²⁺/Ca²⁺ ratio, linked to falling $P_{\text{C}{\text{O}}_{\text{2}}}$ levels and extensive dolomite formation. Scarcity of calcareous cyanophytes in Cenozoic marine environments again implicates the Mg²⁺/Ca²⁺ ratio, inferred from oöid mineralogy to have increased in the late Mesozoic⁵, as a factor influencing cyanophyte calcification in the sea.