Chlorella thermophila (Trebouxiophyceae, Chlorophyta), a novel thermo-tolerant Chlorella species isolated from an occupied rooftop incubator

Open rooftop incubators may provide a cost-effective way for producing microalgal biomass for biodiesel production without requiring large land areas. While cultivating a Chlorella vulgaris strain with high oil content in a rooftop incubator, we identified an invading thermo-resistant green alga, which replaced the original culture. The isolated strain HTA1-65 had the typical Chlorella morphology with spherical cells and a cup-shaped parietal chloroplast. The protoplast contained a single pyrenoid traversed by a double-layered thylakoid and enveloped by a single starch sheath. The oil content of this strain was only 21.5 %, one third of the original strain. This strain grew at 42 A degrees C, with an optimum of 33 A degrees C. Heat treatment of 45 A degrees C for 3 h per day did not decrease its growth rate, suggesting that it has a high potential in biotechnological applications. Phylogenetic analyses revealed that HTA1-65 belongs to the genus Chlorella, but a specific relationship to any species was not indicated by nuclear and chloroplast small subunit rRNA genes. However, based on at least one compensating base change (CBC) and a few hemi-CBCs in the ITS2 secondary structure of closely related species, we describe strain HTA1-65 as a novel, thermo-tolerant Chlorella species, C. thermophila.