Characterization of salt stress-induced palmelloids in the green alga, Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a model, free-living, soil and freshwater alga with unicellular vegetative cells under favourable growth conditions. When exposed to certain stress agents, however, C. reinhardtii forms multicellular aggregates known as “palmelloids.” This study characterizes palmelloid formation in response to sodium chloride (NaCl) salt stress and probes the role of proteins from the spent medium in this process. Detailed morphological and biochemical analysis of NaCl-induced palmelloids of C. reinhardtii were carried out along with quantitative mass spectrometry-based protein analysis of stress and post-stress spent media. Exposure to NaCl concentrations of 100 and 150 mM (equivalent in saline soils) induces palmelloid formation and withdrawal of this stress causes rapid dissociation of these clusters. Heterogeneity in cell size and cells with excised flagella remnants within palmelloid clusters suggests heterogeneous cell cycle arrest during palmelloid formation. Additional hallmarks of palmelloidy include: intracellular starch and lipid accumulation and the presence of an extracellular polysaccharide envelope. Analysis of proteins from the spent media of stressed and post-stress conditions implicated involvement of cell wall proteins in palmelloid formation and maintenance and peptidase and flagellar proteins in palmelloid dissociation. With this report, the relevance of NaCl, as a probable environmental cue for driving unicellular, free-living C. reinhardtii cells into a multicellular, palmelloid stage is discussed.