Evidence of Substrate Specialization among Euendolithic Cyanobacteria from Mona Island, Puerto Rico

Abstract:

Euendolithic Cyanobacteria are able to actively bore into mineral carbonates. They are of global significance because they colonize any exposed marine, freshwater or terrestrial carbonate surface. They are responsible of shaping coastal limestomes, damaging mollusk shells, coral skeletons and historical landmarks. Despite their global importance, little is known about the boring mechanism, more specifically how Cyanobacteria realize the tour de force of combining oxygenic photosynthesis to carbonate dissolution. In fact there is only one cultured boring strain, which only bores into calcium carbonate while microborers are found on various types of substrates including magnesium carbonates. The boring mechanism proposed for the model strain may not be general highlighting the need of going back to the field to interrogate eudolithic Cyanobacterial communities boring on other substrates. We collected samples from marine intertidal limestones (mostly calcite CaCO₃) and dolostones (mostly dolomite CaMg(CO₃)₂) from the Mona Island, Puerto Rico that were all infested by endolithic microbial communities. After determination of the samples mineralogy using powder X-ray diffraction, we analyzed the endolithic microbial communities using 16S rDNA library next generation sequencing. Our results indicate that the substrate type does not drive any significant difference in community composition at the phylum level but affects the distribution of Cyanobacteria. We found that while close relatives to the model strain Mastigocoleus testarum BC008 are prominent in limestones, 2 OTUs accounting for up to 26% of the Cyanobacteria, they are in low abundance in dolostones. These latter being dominated by members of the Pleurocapsales closely related to the known boring strain Hyella sp. This study brings the first evidence of substrate specialization among euendolithic Cyanobacteria and stresses the need of studying dolostone boring strains in more details.