Protein in marine plankton: A comparison of spectrophotometric methods

Measuring protein is critical to many investigations in oceanography and marine biology. Here, we compared seven colorimetric protein assays (Rutter, Rutter-SDS, Markwell, BCA, microBCA, Bradford and microBradford) for measuring protein in a mysid (Leptomysis lingvura), in a jellyfish (Pelagia noctiluca), and in three different size fractions of marine plankton (0.7-50, 50-200 and 200-2000 mu m). Significant differences occurred in all of these samples. However, the mBCA method was the most accurate for all samples except the mysid, the Rutter method was the least accurate in all organisms, and the Bradford and microBradford methods consistently under-estimated protein. The time-dependent behavior of the protein signal was most accurately determined if the analysis was carried out rapidly and consistently. In relation to the limit of detection (LOD), the most sensitive method for low protein levels was the microBCA assay (at 7 mu g mL(-1) protein). The most sensitive method at higher levels of protein was the Bradford method (at 472 mu g mL(-1) protein). The BCA method was the most linear and the microBCA method, the most sensitive method for detecting bovine serum albumin. We recommend the latter two methods for measuring protein under our assay conditions.