Acclimation to darkness in the marine chlorophyte Koliella antarctica cultured under low salinity: Hypotheses on its origin in the polar environment

In order to obtain new insights on the origin and physiology of the marine chlorophyte Koliella antarctica, the response of the microalga was studied at a salinity of 0.2 in the light and during a 60-day dark period. In light conditions, the alga grows and maintains a functional cell organization. In darkness, the chloroplast-to-chromoplast transition, previously described during dark-acclimation in K. antarctica under a salinity of 34 was only partially triggered; thylakoid lamellae became re-organized into short bundles, but storage substructures were almost completely missing. Microspectrofluorometry, pigment analyses, and morphological observations revealed dark-induced degradation of photosystem II (PSII) with relative stability in the light-harvesting complex (LHCII). Chromatin condensation, mitochondrion fragmentation and material digestion in vacuoles were similar to morphological hallmarks of programmed cell death (PCD), but only 30% of cells underwent cell death and, at the end of the experiment, only 1-2% of cells were TUNEL-positive. Rapid recovery in culture growth after exposure to light showed that the changes apparent in the rest of cells were reversible. Taking into account the response of the plastid and assuming an adaptive benefit of PCD, our results are consistent with the hypothesis that K. antarctica evolved from an Antarctic freshwater ancestor.