Rather than try to measure the total mass of organisms present, sometimes it is more practical or desirable to measure the mass of a particular cell constituent. For example, it may be possible to treat a sample to extract the microbial protein or DNA that is present and to quantify it by weight or by a chemical means. The amount of such fundamental cell constituents present may then be correlated to the biomass present (see Table 11.4, for example). In some cases these measures may be even more useful then total biomass estimates, since the amounts per cell vary less than for some other constituents, such as exocellular polymer or food storage products.
Another cell constituent of interest for estimating biomass is ATP. It is present in all organisms, and since it disappears quickly upon death of a cell, it can serve as a useful indicator of viable (living) biomass. However, the amount of ATP present in a cell can vary considerably with its physiological state.
Planktonic algae concentrations can often be estimated by measurement of chlorophyll a. This method also can be used for cyanobacteria, and in fact includes them in the estimates obtained. The sample is prepared by grinding the cells and extracting the pigment with acetone. The chlorophyll is then quantified by measuring absorbance at 664 nm with a spectrophotometer, or fluorescence with a fluorometer, or by high-performance liquid chromatography (HPLC). Corrections may be necessary due to the presence of other pigments.
Some methods have been developed to extract and identify cell constituents that are characteristic of a particular group of microorganisms, including cell wall constituents, fatty acids, and DNA or RNA sequences that are specific for particular taxa. So far these are used primarily to help identify what types of microorganisms are present, but quantitative methods are also being developed that can be used to estimate biomass.
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