LAUSR

Synechocystis  sp. PCC 6803 and  Synechococcus elongatus PCC 7942 exhibit dissimilar tolerance to Cr(VI) with a tenfold difference in their EC 50 value for Cr(VI). This contrasting tolerance was attributed to the difference in the ability to transport Cr(VI) and to detoxify ROS. The present study used biochemical assays and chlorophyll fluorescence to investigate the effect of growth with Cr(VI) on photosynthesis in the two cyanobacteria. In absence of Cr(VI), all the measured parameters  viz., rates of CO 2 fixation, PSII and PSI activities were higher in Synechocystis  in comparison to  Synechococcus,  suggesting intrinsic differences in their photosynthesis. Growth in the presence of Cr(VI) reduced the pigment content and photosystems’ activities in both cyanobacteria. It was further observed that photosynthetic functions were more adversely affected in  Synechocystis in comparison to  Synechococcus, in spite of exposure to tenfold lower Cr(VI) concentration. The effective quantum yield of PSII and PSI obtained by chlorophyll fluorescence measurements increased in the presence of Cr(VI) in Synechococcus whereas it decreased in  Synechocystis.  However, the overall CO 2 fixation remained unchanged. These results indicated that, in addition to the intrinsic difference in photosynthetic rates, the two cyanobacteria exhibit differential modulation of photosynthetic machinery upon Cr(VI) exposure and  Synechococcus could adapt better it’s photosystems to counter the oxidative stress.