The identity of an endolithic alga: Ostreobium brabantium Weber-van Bosse is recognized as carbonate-penetrating rhizoids of Acetabularia (Chlorophyta, Dasycladales)

In the study of microbial endoliths of shallow subtropical waters of the Bahamian carbonate platform, the endolithic siphonalean chlorophyte Ostreobinm brabantium WEBER-VAN BOSSE was recognized as identical with endolithic rhizoids of the holdfast of Acetabularia crenulata LAMOUROUX. Complete plantlets of Acetabularia together with the carbonate substrate were dehydrated and embedded in polymerizing resin allowing resin penetration into pores of the substrate. Casts of endolithic lower parts with rhizoids were exposed following carbonate removal by acid, and the boring patterns visualized using SEM. In parallel fixed samples, the organisms were extracted completely by dissolving the carbonate matrix, and mounted on slides for light microscopic examination. The rhizoids of Acetabularia were found to penetrate several millimeters deep into carbonate substrate firmly anchoring the organism and creating characteristically ramified, terminally rounded boreholes. The external morphology as well as cell content of the endolithic parts of Acetabularia were found to be indistinguishable from those described as a separate organism Ostreobium brabantium, and identified in the literature under this name. The latter taxon is now recognized as invalid.