An updated method for the calibration of transparent exopolymer particle measurements

Transparent exopolymer particles (TEP) are biologically derived, polysaccharide-rich, gel-like particles that play important roles in ocean carbon cycling. The original spectrophotometric method for TEP quantification published in 1995 by Passow and Alldredge describes staining TEP with the dye Alcian Blue (AB), and calibration of the AB staining solution using a commercially available polysaccharide xanthan gum (XG) powder as the reference material. In the original method, a XG solution is prepared to create gel-like particles that resemble TEP. The solution is filtered to collect the XG gels onto a filter, and the filters are stained with AB solution. However, recently, it has been found that the XG powder commercially available today has higher solubility and forms negligible amounts of gel particles in solution, which precludes its use as described in the original method. Here, we present an updated calibration method to generate reproducible XG calibration curves using the currently available XG powder, whereby a XG dilution series is stained with AB solution prior to filtration resulting in AB-stained XG gels that are retained on a filter. Subsequent extraction and spectrophotometric analysis are performed as described in the original method. The updated calibration method described here yielded calibration curves consistent with those reported in the original method. The updated method should only be used in the preparation of a calibration curve when the new soluble XG is available, whereas samples for TEP quantification should be processed as described in the original method.