Multiparameter-based photosynthetic state transitions of single phytoplankton cells

Phytoplankton are a major source of primary production. Their photosynthetic fluorescence uniquely reports on their type, physiological state and response to environmental conditions. Changes in phytoplankton photophysiology are commonly monitored by bulk fluorescence spectroscopy, where gradual changes are reported in response to different perturbations such as light intensity changes. What is the meaning of such trends in bulk parameters if their values report ensemble averages of multiple unsynchronized cells? To answer this, we developed an experimental scheme that enables acquiring multiple fluorescence parameters, from multiple excitation sources and spectral bands. This enables tracking fluorescence intensities, brightnesses and their ratios, as well as mean photon nanotimes equivalent to mean fluorescence lifetimes, one cell at a time. We monitored three different phytoplankton species during diurnal cycles and in response to an abrupt increase in light intensity. Our results show that we can define specific subpopulations of fluorescence parameters for each of the phytoplankton species and in response to varying light conditions. Importantly, we identify the cells undergo well-defined transitions between these subpopulations that characterize the different light behaviors. The approach shown in this work will be useful in the exact characterization of phytoplankton cell states and parameter signatures in response to different changes these cells experience in marine environments, which will be useful in monitoring marine-related effects of global warming. Significance StatementUsing three representatives of red-linage phytoplankton we demonstrate distinct photophysiological behaviors at the single cell level. The results indicate cell wide coordination into discrete cell states. We test cell state transitions as a function of light acclimation during diurnal cycle and in response to large intensity increases, which stimulate distinct photoprotective response mechanisms. The analysis was made possible through the development of flow-based confocal detection at multiple excitation and emission wavelengths monitoring both pigment composition and photosynthetic performance. Our findings show that with enough simultaneously recorded parameters per each cell, the detection of multiple phytoplankton species at their distinct cell states is possible. This approach will be useful in examining the response of complex natural marine populations to environmental perturbations.