Acclimation to environmental changes involves a modification of the expressed proteome and metabolome. The reproductive advantage associated with the higher fitness that acclimation provides to the new conditions more than compensates for the costs of acclimation. To exploit such an advantage, however, the duration of the perturbation must be sufficiently long relative to the growth rate. Otherwise, a selective pressure may exist in favour of responses that minimize changes in carbon allocation and resource use and do not require reversal of the acclimation after the perturbation ceases (compositional homeostasis). We hypothesize that the choice between acclimation and homeostasis depends on the duration of the perturbation relative to the length of the cell cycle. To test this hypothesis, we cultured the green alga Tetraselmis suecica at two growth rates and subjected the cultures to three environmental perturbations. Carbon allocation was studied with Fourier transform infrared (FTIR) spectroscopy; elemental stoichiometry was investigated by total reflection X-ray fluorescence (TXRF) spectroscopy. Our data confirmed that growth rate is a crucial factor for C allocation in response to external changes, with a higher degree of compositional homeostasis in cells with lower growth rate.

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.

Microalgae biofilms have been proposed as an alternative to suspended cultures in commercial and biotechnological fields. However, little is known about their architecture that may strongly impact biofilm behavior, bioprocess stability, and productivity. In order to unravel the architecture of microalgae biofilms, four species of commercial interest were cultivated in microplates and characterized using a combination of confocal laser scanning microscopy and FTIR spectroscopy. In all the species, the biofilm biovolume and thickness increased over time and reached a plateau after seven days; however, the final biomass reached was very different. The roughness decreased during maturation, reflecting cell division and voids filling. The extracellular polymeric substances content of the matrix remained constant in some species, and increased over time in some others. Vertical profiles showed that young biofilms presented a maximum cell density at 20 μm above the substratum co-localized with matrix components. In mature biofilms, the maximum density of cells moved at a greater distance from the substratum (30–40 μm), whereas the maximum coverage of matrix components remained in a deeper layer. Carbohydrates and lipids were the main macromolecules changing during biofilm maturation. Our results revealed that the architecture of microalgae biofilms is species-specific. However, time similarly affects the structural and biochemical parameters.