Differentially Secreted Proteins of Antarctic and Mesophilic Strains of Synechocystis Salina and Chlorella Vulgaris after UV-B and Temperature Stress Treatment

ABSTRACT

Exponential-phase cell cultures from Antarctic and mesophilic strains of the cyanobacterium Synechocystis salina and the unicellular green alga Chlorella vulgaris were subjected to UV-B and temperature stress and investigated for alterations in the spectrum of secreted proteins. In addition to constitutively secreted proteins, strain-specific stress proteins were detected on silver-stained gels. Extracellular protein patterns within a given strain, as well as between strains after exposure to each UV-B and temperature stress were notably different. UV-B exposure induced only moderate changes of the secreted proteins in both Antarctic and mesophilic Synechocystis salina strains. However, a 21 kDa protein band present in Antarctic Synechocystis salina only was identified using LC-MS/MS analysis, to contain two proteins—a peroxiredoxin and Fe-superoxide dismutase (Fe-SOD). Their presence might modulate the level of reactive oxygen species generated in membranes during UV-B stress and, thus, contribute to the higher survival rate of Antarctic Synechocystis salina. In addition, proteolytic activity in the extracellular protein samples of both species was detected on 1D zymography in a polyacrylamide gel containing gelatin as substrate. Antarctic Synechocystis salina secreted cysteine-, serine-, and metalloproteases, while mesophilic Synechocystis salina secreted mainly cysteine proteases. Subtle changes of the protease activity bands in Antarctic Synechocystis salina and Chlorella vulgaris were observed after UV-B and temperature stress. In contrast, mesophilic Synechocystis salina responded with a considerable increase in protease activity and number of activity bands after 30 min and 60 min exposure to UV-B, as well as when grown at 35°C and 40°C. Peptidases are key players in structuring and adapting cells under various stressful conditions. The varying pattern of secreted proteases among the Antarctic and mesophilic strains of Synechocystis salina and Chlorella vulgaris suggests different mechanisms for coping with UV-B and temperature stress. Characterization of extracellular proteins discovered in the present study will provide further understanding of the rich defense arsenal and mechanisms of survival of cyanobacteria and microalgae.

Keywords:

• Antarctic algae
• Antarctic cyanobacteria
• Chlorella
• extracellular proteins
• proteases
• stress response
• Synechocystis