Abstract

Two bacterial species, Chromatium sp. and Macromonas sp. formed dense populations (bacterial plate) at an upper boundary of the H_2S layer of Lake Kaiike, Kamikoshiki island, Kagoshima Profecture. To obtain information on the utilization of inorganic nitrogen by the phototrophic bacterium Chromatium sp. in relation to light and H_2S, field observation was conducted in Lake Kaiike, in October, 1997. A dense population of Chromatium sp. was found from between 5 to 6 m in the lake, and the maximum number, 1.01×10^6cell/ml, was found at 5.5m. Average daytime light intensities at 5 and 5.5 m, expressed as photosynthetically available radiation (PAR), were 66.7 and 1.2 ㎛ole/m^2/s, respectively. The relative abundances of H_2S and NH_4^+ below the bacterial plate of the lake was established on their molar bases. The molar ratio of △H_2S to △NH_4^+ was 12.2 The bacterial N_2 fication is considered to have occurred when the bacterial requirement for nitrogen increased due to an elebated light level. It is suggested that the high H_2 concentration found at the upper part of the bacterial plate might be attrivuted to the bacterial N_2 fixation.

• Keywords: Chromatium sp.; H_2S; inorganic nitrogen; lake Kaiike; light