A novel cyanobacterial mat type is characterized from near-neutral pH, low sulphide geothermal springs of 45-60 °C in the Philippines. Mats were free floating, several metres in diameter and several cm in thickness. The upper surface of mats was covered in a waxy scytonemin-like layer, solvent extracts of which absorbed light strongly at 384nm. Light microscopy revealed mats to posses highly ordered layers of air spaces at both the macroscopic and microscopic level, apparently as an adaptation to buoyancy. Morphospecies composition was exclusively filamentous, with Fischerella-like and Oscillatoria-like taxa closely associated throughout mats. Abundant heterocystous cells were observed in Fischerella filaments, suggesting nitrogen fixation occurs in these mats. Morphological structure did not vary among mats from pools of different temperature, but several 16S rDNA-defined genotypes were resolved by DGGE with some displaying greater thermophily than others. Sequencing of fourteen DGGE bands (Genbank accession numbers: AY236467-AY236480) yielded nine novel Fischerella sequences, whilst the five Oscillatoria sequences showed high similarity to other thermophilic Oscillatoria sequences. These data are relevant to astrobiology in that they expand our knowledge of oxygenic photosynthetic community diversity in geothermal environments, which serve as modern analogues for early life on Earth and other planets. Acknowledgements The authors are extremely grateful to the Philippine Institute of Volcanology and Seismology (PHIVOLCS) for advice and assistance with fieldwork. This work was supported by grants awarded by The University of Hong Kong CRCG Seed Funding for Basic Research and Small Projects programmes.published_or_final_versio
