HELIOBACTERIUM MODESTICALDUM, SP-NOV, A THERMOPHILIC HELIOBACTERIUM OF HOT-SPRINGS AND VOLCANIC SOILS

Enrichment cultures for heliobacteria at 50 degrees C yielded several strains of a thermophilic heliobacterium species from Yellowstone hot spring microbial mats and volcanic soils from Iceland. The novel organisms grew optimally above 50 degrees C, contained bacteriochlorophyll g, and lacked intracytoplasmic membranes. All isolates were strict anaerobes and grew best as photoheterotrophs, although chemotrophic dark growth on pyruvate was also possible. These thermophilic heliobacteria were diazotrophic and fixed N-2 up to their growth temperature limit of 56 degrees C. Phylogenetic studies showed the new isolates to be specific relatives of Heliobacterium gestii and, as has been found in H. gestii, they produce heat-resistant endospores. The unique assemblage of properties found in these thermophilic heliobacteria implicate them as a new species of this group, and we describe them herein as a new species of the genus Heliobacterium, Heliobacterium modesticaldum.