Abstract
The green sulfur bacteria (GSB) are obligately anaerobic photosynthetic bacteria found in photic zone under euxinic condition with the presence of hydrogen sulfide. Photosynthetic pigments indigenous to GSB are bacteriochlorophylls c, d, and e. The most significant structural difference from chlorophylls a and b and from bacteriochlorophylls a and b is the side-chain alcohol other than phytol, primarily composed of farnesol. Hence the detection of farnesol in sediments, for example, implies that the sediments were deposited under the regime of photic zone anoxia. In the present study, alcohols were analyzed by GC and GC/MS for sediment core samples collected from six meromictic brackish water lakes with the different salinities. A strong positive linear correlation with a regression through the origin was observed among farnesol, and n-C14:0, n-C15:0, and n-C16:1 alcohols for individual lake sediments. These results suggest that farnesol and those aliphatic alcohols in the sediments are commonly originated from GSB or their bacteriochlorophylls. Moreover, except for one case, the abundance ratio of farnesol: n-C16:1 was large (16-17:1) in highly saline but small (8-11:1) in lower saline lake sediments. Hence a tentative hypothesis is proposed that the farnesol/n-C16:1 ratio may employed as a potential proxy reflecting the salinity of the anoxic bottom water in meromictic brackish water lakes.
