Photosynthetic microorganisms as epibionts and euendoliths on biotic substrates in a thermal spring with ferric-iron deposits

Rust coloured shells of the aquatic gastropod Ventrosia ventrosa, a new record for eastern Greece, indicating presence of iron (EDAX analysis) were studied for detection of iron encrusted photosynthetic epibionts in a Greek brackish water thermal spring (38 degrees C). Microscopic analyses (LM. SEM) revealed the presence of a biofilm consisted of mostly facultative micro epibionts, i.e. a) 5 periphytic taxa of coccal and filamentous cyanobacteria, including a taxonomically and ecologically interesting morphospecies, Xenococcus cf. pyriformis, dominated exclusively on the shell surface, and b) pennate diatoms with higher species richness (18 periphytic taxa of the genera Amphora, Brachysira, Cymbella, Diatoma, Encyonema, Navicula, Nitzschia, Pleurosigma, Synedra, Ulnaria; 5 taxa as new records for Greece), most of them emerging only after acid treatment of whole gastropod shells. The abundant diatoms thriving directly or nearby the iron coatings (Cocconeis placentula var. euglypta and Achnanthes brevipes sensu lato) exhibited different modes of attachment ('adnate' and 'pendunculate', respectively). Two euendolithic cyanobacteria (Hyella sp. and Leptolyngbya terebrans; the former with special taxonomic interest) were also found perforating the delicate gastropod shells, with no distinct differentiation in the extent of infestation between live and dead gastropod shells. Moreover, the possible impact of these encrusted photosynthetic assemblages on V. ventrosa was investigated; statistical analysis showed that a) there is no 'drag effect', induced by the epibionts, influencing the gastropod growth (i.e. shell length), b) shell size enlargement provides a favourable space and promotes the intense fouling by both micro epibionts and macro epibionts (egg capsules), and c) the detachment prevention of egg capsules is attributed to the biofilm development.