Use of FDA and flow cytometry to assess metabolic activity as an indicator of nutrient status in phytoplankton

This study reports the use of a technique to determine nutrient limitation of cultured and natural phytoplankton. The technique, an FDA-activity assay, which is usually used to assess cell viability, was used to measure metabolic activity in response to nutrient addition; the metabolic activity of phytoplankton was determined as the rate of hydrolysis of fluorescein diacetate (FDA), by intracellular esterases, to fluorescein, which was detected using a flow cytometer. Replacement of the limiting nutrient to nitrogen- or phosphorus-limited cultures and field populations resulted in an increase in metabolic activity that was detectable 24 h after nutrient addition. By flow cytometry, the natural phytoplankton community can be divided into different taxonomic groups; the response of these to FDA could be determined individually to allow identification of the nutrients limiting each type of phytoplankton. This would be more specific than the assessment of a whole-community response, which may mask subtle differences among taxa.