Mesophilic, moderately halophilic bacteria were isolated from a silica-rich geothermal lake, the Blue Lagoon in Iceland. The isolates are strictly aerobic, but reduce nitrate to nitrite, and are oxidase- and catalase-positive. The nonsporeforming and nonmotile Gram negative rods are 0.6-0.8 microm in diameter and variable in length (9-18 microm), and contain gas vacuoles. The GC content in their DNA is 66.15%. The minimum, optimum, and maximum temperatures for growth are 22 degrees C, 45 degrees C, and 50 degrees C, respectively. The isolates do not grow without added salt in the medium and can grow at up to 7% NaCl (w/v). The optimal salinity for growth is 3.5%-4% NaCl. The pH range for growth is 6.5-8.5, with the optimal pH at 7.0. At optimal conditions the bacterium has a doubling time of 80 min. The main cytochrome is a membrane-bound cytochrome c with an alpha-peak at 549nm. Sequencing of 16S rRNA from the type strain ITI-1157 revealed it to be a proteobacterium of the alpha-subclass with the closest relatives being Roseobacter litoralis and Paracoccuss kocuri. The new isolates do not contain bacteriochlorophyll a and are considered to represent a new genus and a new species, Silicibacter lacuscaerulensis.