The measurement of bacterial chlorophyll and algal chlorophyll a in natural waters

Several groups of chlorophyll-bearing photosynthetic organisms often coexist in anoxic waters. Because of the close similarity between the absorbance spectra of chlorophyll a (in cyanobacteria and algae) and the chlorophylls of green sulfur bacteria (Chl-GSB) it is difficult to distinguish these two groups of organisms where they co-occur. We have developed a simple method to measure both chlorophyll a and Chl-GSB and used it in a meromictic coastal pond. In anoxic bottom waters Chl-GSB was the major pigment present (averaging 20 mg m-)), but chlorophyll a accounted for from 5 to > 50% of the total pigment present throughout the year. Without our method large errors in calculated Chl-GSB and chlorophyll a concentrations would have resulted. The distribution of photosynthetic sulfur bacteria is restricted by their requirement for both anoxic conditions and light (Fenchel and Blackburn 1979; Pfennig 1967). Where light reaches anoxic waters, however, these bacteria can be important; in some such systems the biomass of photosynthetic bacteria is often as large as the algal biomass (e.g. Cloern et al. 1983; Indrebo et al. 1979; Parker et al. 1983). The biomass of photosynthetic bacteria is frequently calculated from bacterial chlorophyll measured from the absorbance of 90% acetone extracts of particles (Lawrence et al. 1978; Parkin and Brock 198 1; Takahashi and Ichimura 1968, 1970), but this method of chlorophyll determination can lead to errors in calculation. The two major groups of photosynthetic sulfur bacteria, green and purple, are distinguished largely by the type of chlorophyll they possess (Pfennig 1967, 1977). Acetone extracts of purple sulfur bacteria have a peak absorbance at 772 nm due to a dominance of bacterial chlorophyll a (Takahashi and Ichimura 1970). Acetone extracts of green sulfur bacteria have a peak absorbance near 653 nm due to a mixture of bacterial chlorophylls c, d, and e (Gloe et al. 1975; Parkin and Brock 198 1). In waters containing both purple and green sulfur bacteria, these two groups are relatively easily distinguished be-