Long-term preservation of picophytoplankton for counting by fluorescence microscopy

Abstract

Studies off the west coast of the South Island, New Zealand, showed that eucaryotic cells contribute up to 50% of the total picophytoplankton population in the nutrient-rich inshore environment. This highlighted the need for careful preservation of the autofluorescence properties of both phycoerythrin and chlorophyll a to adequately evaluate picophytoplankton population numbers and composition. Six different methods of preservation were compared. Only two of these, both using 0·2% paraformaldehyde as a fixative, were found to preserve phycobilin plus chlorophyll a fluorescence over periods up to 15 weeks. Other methods, including glutaraldehyde treatments, resulted in either lower initial cell counts or a substantial decrease in cell counts over time. Experiments with both natural and cultured populations of larger eucaryotic cells showed that eucaryotic picophytoplankton counts are unlikely to be elevated by the release of free chloroplasts from larger cells during the pre-filtration step routinely used in epifluorescence analysis. However, use of an inappropriate preservation technique could result in a substantial underestimate of the importance of eucaryotes in the picophytoplankton communities of marine and freshwater environments.