Palmelloid formation in the Antarctic psychrophile, , is photoprotective.

Cultures of the obligate, Antarctic psychrophile, grown at permissive low temperature (8°C) are composed of flagellated, single cells, as well as non-motile, multicellular palmelloids. The relative proportions of the two cell types are temperature dependent. However, the temperature dependence for palmelloid formation is not restricted to psychrophilic but appears to be a general response of mesophilic Chlamydomonas species ( and ) to non-permissive growth temperatures. To examine potential differences in photosynthetic performance between single cells versus palmelloids of the psychrophile, a cell filtration technique was developed to separate single cells from palmelloids of grown at 8°C. Flow cytometry was used to estimate the diameter of isolated single cells (≤5 μm) versus isolated palmelloids of varying size (≥8 μm). Compared to single cells, palmelloids of showed a decrease in the abundance of light-harvesting complex II (LHCII) proteins with a 2-fold higher Chl a/b ratio. A decrease in both lutein and β-carotene in palmelloids resulted in carotenoid pools which were 27% lower in palmelloids compared to single cells of the psychrophile. Chlorophyll fluorescence analyses of the isolated fractions revealed that maximum photochemical efficiency of PSII (F/F) was comparable for both single cells and palmelloids of . However, isolated palmelloids exhibited lower excitation pressure, measured as 1 - qL, but higher yield of PSII (Φ) and 50% higher rates of electron transport (ETR) than single cells exposed to high light at 8°C. This decreased sensitivity to high light in isolated palmelloids compared to single cells was associated with greater non-regulated dissipation of excess absorbed energy (Φ) with minimal differences in Φ in in response to increasing irradiance at low temperature. The ratio Φ/Φ observed for isolated palmelloids of developed at 8°C (1.414 ± 0.036) was 1.38-fold higher than Φ/Φ of isolated single cells (1.021 ± 0.018) exposed to low temperature combined with high light (1,000 μmol m s). The differences in the energy quenching capacities between palmelloids and single cells are discussed in terms of enhanced photoprotection of palmelloids against low-temperature photoinhibition.