Temperature affects the partitioning of absorbed light energy in freshwater phytoplankton

Abstract

Summary Temperature affects phytoplankton growth by altering the balance between light energy absorption and consumption. However, the way absorbed light energy (Qphar) is partitioned as a function of temperature has never been compared among phytoplankton groups. Accordingly, light‐energy partitioning in seven freshwater phytoplankton species acclimated to eight temperatures (7–35 °C) was quantified by photosynthetic and bio‐optical measurements. At low temperature, cyanobacteria and green algae received an excess of energy due to a decreased C‐fixation capacity. Consequently, Qphar partitioning was mainly directed towards dissipation mechanisms, such as non‐photochemical quenching and alternative electron pathways. The same photo‐protective mechanisms were stimulated in diatoms, but their ability to maintain stable C‐fixation capacities over the entire temperature range prevented the cells from experiencing excessive excitation pressure. At high temperatures, Qphar was preferentially invested in growth, showing a taxon‐specific positive correlation with growth rate. The cyanobacteria and a diatom required the lowest amount of energy to sustain their growth rate. The higher efficiency of these taxa in using light for growth is therefore a cellular trait that favours their dominance during summer. Our results are in line with literature reports showing temporal changes in species composition that can be attributed to temperature, and may help to explain the seasonal succession of species observed in nature.