The Use of Fast Repetition Rate Fluorometry to Determine Phytoplankton Growth Rate and Nutrient Status via the Ratio of Single-Turnover to Multiple-Turnover Fluorescence

Two long-standing goals in phytoplankton research have been the development of a rapid, non-invasive method for determining net primary production, and a method for determining the degree to which a phytoplankton population is nutrient-limited. While chlorophyll fluorescence measurements have been widely used to study phytoplankton photosynthetic physiology, previous attempts to determine net primary production, growth rate or nutrient status via fluorescence have not proven successful. In a previous study, we showed that the ratio of single-turnover fluorescence and multiple-turnover fluorescence (ST/MT) measured via fast repetition rate (FRR) fluorometry varied predictably with growth rate and cellular chlorophyll concentration in the green algae Dunaliella tertiolecta. In addition, ST/MT varied with the growth condition of the algae, such that light-limited cultures could be distinguished from nitrate-limited cultures even when growing at similar growth rates. In the present study, we have extended this research to several other species of phytoplankton (Phaeodactylum tricornutum, Thalassiosira weisfloggi, Synechococcus sp.). Our research shows that, under light-limited conditions, the same basic relationship between ST/MT and cellular growth rate/chlorophyll holds across this diverse range of species. However, the relationship under nitrate limitation appears to be species-specific, with some species exhibiting little to no change in ST/MT across different growth rates. In addition to our laboratory work, we performed an analysis of field data collected from eight cruises over the course of ten years, wherein we examined the relationship in situ between ST/MT and several physical (temperature, light) and biological factors (growth rate, cellular chlorophyll). The implications of these findings will be discussed in regards to the use of ST/MT measurements as a tool in phytoplankton research.