Abstract:
The unicellular cyanobacterium Synechocystis sp. PCC 6803, a model organism known for
its unique combination of highly desirable molecular genetic, physiological and morphological
characteristics, was employed in the present study. The species was cultured in BG11 liquid medium
contained various initial concentrations of Pb2+ and Cd2+ (0, 0.5, 1, 2, 4, 6 and 8 mg/L). The experiment
was conducted for six days and the metal induced alterations in the ultrastructure, growth and pigment
contents were assessed. Alterations in the ultrastructure of the Synechocystis sp. PCC 6803 cells became
evident with the increased (>4 mg/L Pb2+) metal concentration. The photosynthetic apparatus (thylakoid
membranes) were found to be the worst affected. Deteriorated or completely destroyed thylakoid
membranes have made large empty spaces in the cell interior. In addition, at the highest concentration
(8 mg/L Pb2+), the polyphosphate granules became more prominent both in size and number. Despite the
initial slight stimulations (0.2, 3.8 and 6.5% respectively at 0.5, 1 and 2 mg/L Pb2+), both metals inhibited
the growth in a dose-dependent manner as incubation progressed. Pigment contents (chlorophyll a, β
carotene and phycocyanin) were also decreased with increasing metal concentration. Cells exposed to
6 mg/L Pb2+, resulted in 36.56, 37.39 and 29.34% reductions of chlorophyll a, β carotene and
phycocyanin respectively over the control. Corresponding reductions for the same Cd2+concentrations
were 57.83, 48.94 and 56.90%. Lethal concentration (96 h LC50) values (3.47 mg/L Cd2+ and 12.11 mg/L
Pb2+) indicated that Synechocystis sp. PCC 6803 is more vulnerable to Cd2+ than Pb2+.
