Diversity of the nitrogen starvation responses in subarctic Desmodesmus sp. (Chlorophyceae) strains isolated from symbioses with invertebrates

We report on common and strain-specific responses to nitrogen (N) starvation recorded in four closely related symbiotic Desmodesmus strains from taxonomically very distant animals (hydroids, a sponge, and a polychaete) dwelling in the White Sea. A number of common for the studied strains and free-living microalgae as well as some specific patterns of acclimation to the N starvation were documented. The common responses included a slowdown of cell division, a reduction of photosynthetic apparatus, and a vast expansion of storage subcompartments of the cell. Although these responses were qualitatively similar to those known in free-living chlorophytes, in the studied strains they occurred in a strain-specific manner. The specific N starvation responses comprised formation of chloroplast envelope membrane twirls, thinning of the appressed thylakoid membranes and a loss of the luminal depositions, and channeling of the fixed carbon to cell wall polysaccharide layer. Desmodesmus sp. from a hydroid featured a unique, among the studied strains, capability of ‘emergency’ degradation of Rubisco, apparently to salvage the N contained in this protein. The obtained results are discussed in view of the remarkable physiological plasticity of the symbiotic Desmodesmus spp. and their survival under the harsh conditions of the subarctic sea habitat.