Direct observation of phytoplankton, TEP and aggregates on polycarbonate filters using brightfield microscopy

There has been little use of standard (i.e. non-inverted) microscopes for observing and counting phytoplankton in filtered water samples using brightfield white light illumination due to light interference from the filters. If filters are placed on newly designed frosted slides (Cyto-clear, Poretics Corp.), however, phytoplankton can be viewed directly on the surfaces of polycarbonate filters under brightfield illumination. Lake and seawater samples were used to show that samples stained with alcian blue (to identify the presence of paniculate polysaccharides) and analyzed with white light can also be simultaneously stained with fluorochromes (i.e. DAPI and acridine orange) for additional examination of the sample using fluorescent techniques. Black filters, which are necessary for epifluorescent techniques, did not interfere with brightfield viewing. Using double staining techniques, we found that transparent exopolymer particles (TEP) recently discovered in marine systems are also present in lakes. Not all aggregates in the fresh and seawater systems absorbed the alcian blue stain, however, indicating that not all amorphous particles in these systems are rich in negatively charged polysaccharides.