Prevalence of Ca²⁺-ATPase-mediated carbonate dissolution among cyanobacterial euendoliths

Recent physiological work has shown that the filamentous euendolithic cyanobacterium Mastigocoleus testarum (strain BC008) is able to bore into solid carbonates using Ca²⁺-ATPases to take up Ca²⁺ from the medium at the excavation front, promoting dissolution of CaCO₃ there. It is not known, however, if this is a widespread mechanism or, rather, a unique capability of this model strain. To test this, we undertook a survey of multispecies euendolithic microbial assemblages infesting natural carbonate substrates in marine coastal waters of the Caribbean, Mediterranean, South Pacific, and Sea of Cortez. Microscopic examination revealed the presence of complex assemblages of euendoliths, encompassing 3 out of the 5 major cyanobacterial orders. 16S rRNA gene clone libraries detected even greater diversity, particularly among the thin-filamentous forms, and allowed us to categorize the endoliths in our samples into 8 distinct phylogenetic clades. Using real-time Ca²⁺ imaging under a confocal laser scanning microscope, we could show that all communities displayed light-dependent formation of Ca²⁺-supersaturated zones in and around boreholes, a staple of actively boring phototrophs. In 3 out of 4 samples, boring activity was sensitive to at least one of two inhibitors of Ca²⁺-ATPase transporters (thapsigargin or tert-butylhydroquinone), indicating that the Ca²⁺-ATPase mechanism is widespread among cyanobacterial euendoliths but perhaps not universal. Function-community structure correlations point to one particular clade of baeocyte-forming euendoliths as the potential exception.