A high-throughput method to measure photosynthesis-irradiance curves of phytoplankton

Abstract

Photosynthesis-irradiance (P-E) curves are an important tool used to characterize the physiology of phytoplankton. In most marine environments, low biomass and photosynthetic rates make the carbon-14 tracer technique, used in conjunction with photosynthetrons to create the light gradient, the method of choice to measure P-E curves. However, conventional photosynthetrons are bulky and require large chilled-water recirculators to maintain temperature. In addition, setting the light levels is difficult. Here we describe a new type of photosynthetron, the microphotosynthetron, which avoids these problems. The microphotosynthetron is based on the standard 96-well plate format and allows precise light control using tunable light-emitting diodes (LEDs). Temperature is regulated using a Peltier device. Using monotypic cultures of several phytoplankton strains grown at different light levels and mixed field populations from different sampling sites and depths, we evaluated the utility of the microphotosynthetron by comparing the measured P-E curves to curves from standard photosynthetrons. The microphotosynthetron P-E curves were identical in initial slope (α) (P > 0.1) and light-saturated photosynthesis (Pmax) (P > 0.1) to P-E curves measured using conventional photosynthetrons. In addition, because the microphotosynthetron is based on a standard 96-well plate format, set-up and processing is greatly expedited in comparison to the conventional photosynthetron technique. Although the microphotosynthetron utilizes smaller sample volumes (200 μL) and therefore might not be suitable for highly heterogeneous samples, it has numerous advantages over the conventional protocol. We recommend the microphotosynthetron approach to measure P-E curves as a method for the rapid screening of photophysiological properties of phytoplankton, in particular, monotypic cultures.