Abstract
In oceanographic and limnological studies, spectrophotometry combined with extraction of pigments by organic solvents became a routine procedure for chlorophyll (Chl) quantification in algae and cyanobacteria from oxic layers. However, for bacteriochlorophyll (BChl) of phototrophic bacteria inhabiting the oxic-anoxic interface and deeper anoxic zones, photometric determination of pigment becomes complicated because of overlapping absorption peaks from different chlorophylls, bacteriochlorophylls and phaeopigments in the solvent. The work is aimed to develop a new technique of BChl estimation in aquatic environments and to test this method using natural water samples with green sulphur bacteria (GSB, Chlorobiaceae). Absorption measurements were carried out using a laboratory spectrophotometer on monocultures of green-coloured as well as brown-coloured GSB in water (19 strains), along with natural water samples from the White Sea region containing either green-coloured or brown-coloured GSB. The far-red absorption peak with a maximum at 720-725 nm attributed to chlorosomal BChl in intact cells had practically the same position for all studied cultures. Its peak area was determined for each sample and compared with the total chlorosomal BChl content estimated from absorption spectra of extracts prepared from the same specimen. For all samples containing either green-coloured or brown-coloured GSB a good correlation between the peak area and BChl concentrations was found, with different regression slopes, however. Differences between the slopes were related to variations on mass-specific absorption coefficients between BChl. We provide formulas to evaluate the BChl content in intact GSB cells using absorption measurements performed without sample pre-treatment and pigment extraction. The suggested method can be used in communities where one type of microorganisms predominates (either green-coloured or brown-coloured GSB).