Colonisation and bioerosion of marine bivalve shells from the Baltic Sea by euendolithic cyanobacteria: an experimental study

The destructive activity of different euendolithic cyanobacteria on marine bivalve shells typical for the Baltic Sea area, such as Mytilus edulis, Mya arenaria, Cerastoderma glaucum, and Macoma balthica, was studied in experimental settings for four months. Specifically the cyanobacterial strains of Phormidium sp., Microcoleus chthonoplastes, Anabaena flosaquae and Synechococcus sp. – all of which not classically considered as euendolithic – were cultured under controlled conditions in the laboratory to determine their optimal environmental conditions. Each bivalve species was maintained in four experimental settings with different calcium carbonate contents in both sediment and sea water. After four months of exposure, pieces of shells were collected, thin-sectioned, and examined using scanning electron microscopy (SEM) and light microscopy. The cyanobacterial strains of Phormidium sp. and Microcoleus chthonoplastes exhibited the fastest (after two weeks) colonisation of Mya arenaria and Cerastoderma glaucum shells and formed a dense biofilm in systems with decalcified sand and carbonate-free water. Shells of Mytilus edulis were colonised only in sites where the periostracum was damaged suggesting periostracal protective properties against the destruction of calcium carbonate substrate by dissolution and bioerosion. The examination of SEM images and cross-sections revealed Phormidium sp. to be an active euendolith that produces prominent traces under laboratory conditions whereas signs of boring activity for Microcoleus chthonoplastes, Anabaena flosaquae were inconclusive and Synechococcus sp. did not contribute to bioerosion.