The diatom Chaetoceros socialis: spore formation and preservation

Planktonic diatoms thrive in the water column, and several species can transform into resting stages – spores or resting cells – that sink to the bottom of the sea. Resting stages are generally produced when environmental conditions are not optimal for growth and can remain viable in sediments for a long time. We tested different aspects related to the formation of spores in Chaetoceros socialis, one of the dominant diatoms in the Gulf of Naples (Mediterranean Sea, Italy). Observations in time lapse and confocal laser scanning microscopy allowed illustration of the formation of endogenous spores, in which valve deposition is accompanied by an acytokinetic nuclear division. The complete transition from vegetative cell to spore takes about 8–10 h and, if exposed to the light, spores can germinate again after a few days. The depletion of nitrogen source in the culture medium induced the formation of spores, with very similar dynamics among different sympatric strains; extremely high percentages of spores (up to >95%) were produced after 4–5 days. Once formed, spores can remain viable for up to nine months, and anoxic conditions favour their preservation. Identical dynamics of spore formation were detected in freshly established cultures and in cultures produced by the germination of spores kept dormant for different lengths of time. Our results suggest that spores of C. socialis can, in principle, rapidly shift between vegetative cells and resting stages that may explain the success of this species in coastal water. Our results also demonstrate that storage of resting spores may represent an alternative to cryopreservation.