New approaches for understanding group-specific phytoplankton photosynthesis in mixed populations

The marine phytoplankton represent globally important components of the Earth's biogeochemical cycles. Phylogenetically diverse, they exhibit wide variations in physiological strategies in order to maximize resource utilization in complex and dynamic communities. Chlorophyll fluorescence-based photosynthetic efficiency measurements are widely used to assess the physiological status of phytoplankton populations, providing critical information on resource use that underpins population growth, seasonal succession and competition. Photosynthetic efficiency is normally monitored in "bulk" natural population samples by the use of fluorometry (pulse amplitude modulated (PAM) fluorometry or fast repetition rate fluorometry (FRRF)). However these approaches suffer a major drawback in that they do not provide information on species- or group-specific photosynthesis that that is needed to understand community dynamics in mixed populations. Such limitations can be overcome by the use of microscopy-based PAM imaging approaches that can monitor the photosynthetic efficiency of single identified cells. However, this approach is severely limited in natural phytoplankton populations by the number of cells (typically tens of cells) that can be analyzed in a typical microscopic field of view. The Mesolens is a highly sophisticated unique microscope that produces aberration-free wide field low magnification (x4) images but with the resolving power and light collecting properties of a x40 objective, allowing the simultaneous imaging of many thousands of cells, each with sub-cellular resolution. We propose to adapt our existing Mesolens to carry out single cell PAM imaging of mixed natural populations, providing a new tool to gain unprecedented species- or group- specific information on phytoplankton photosynthetic performance. Together with a novel method for concentrating viable phytoplankton cells for in situ physiological studies we will aim to provide better mechanistic understanding of phytoplankton succession and competition. Study sites will include: 1). The Western Channel Observatory station L4 Weekly fresh samples will allow detailed studies of seasonal succession. 2). An exciting opportunity to place this equipment on the newly commissioned Tara International Polar Ice Station that is planned to carry out annual winter north-south Arctic ice drifts through the Arctic winters over a 25 year period. We expect that the technological developments achieved with this project will have far broader application in a range of coastal, oceanographic and freshwater phytoplankton research.