The effect of a graded concentration (0.05-0.6 mmol/l) of sodium diethyldithiocarbamate (NaDDC) on the growth rate of Anacystis nidulans was found to be concentration-dependent. A 50% inhibition (Iso) of the growth rate was observed at 0.28 mmol/l of NaDDC. A sudden decrease in cellular absorbance at 435,625 and 675 nm on the third day, and followed by a gradual decrease up to the 6th day indicated an NaDDC-induced bleaching of the chlorophyll a and phycocyanin pigments. Furthermore, an NaDDC-induced increase in absorbency at 340 nm after 48 hrs of light incubation was indicative of the reduced state of the carriers. The result on the reduction of 1,6-dichlorophenol indophenol (DCPIP), cytochrome c and NADP was an NaDDC-induced stimulation of all the reductive processes, with saturating rates at approximately 0.4 mmol/l of NaDDC. Furthermore, the rate of DCPIP reduction in the presence of NaDDC was inhibited by rotenone (100 pmol/l). SHAM (salicyl hydraxamic acid, 500 pmol/l) and DCMU (100 pmol/l). Similarly, the addition of 10 pmol/l of DCMU resulted in the inhibition of both cytochrome and NADP reductions. Therefore, the results suggested that NaDDC could bring about the over-reduction of carriers by interacting with the electron transport chain on or before the'DCMU sensitive site. An NaDDC-dependent general increase in the fluorescence of chlorophyll a was further stimulated by the electron transport inhibitors. The result on oxygen evolution was a 50 %inhibition (Iso) of H,O and H,O, supported 0, evolution at 0.12 and 0.19 mmol/l of NaDDC, respectively. Taken together, these could suggest that the highly reduced state of carriers in the presence of NaDDC might be preventing the uphill electron flow from the light-harvesting pigment complex, H,O and H,Oz. Thus, the foregoing results inferred that the bleaching of pigments and over-reduction of electron carriers ultimately accounted for the death of the organism.
Alkyl derivatives of thiocarbamates are a class of pesticide which not only kill the target organisms (ANDERSON 1978), but are equally toxic to the other organisms including bacteria (SUD and GUPTA 1972), cyanobacteria (VENKATARAMAN and RAJYALAKSHMI 1972, LAZAROFF 1967) and green algae (AUDUS 1970). Relative toxicity of thiocarbamates is found to be dependent on the biochemical nature of the organisms (CORDEN and YOUNG 1965). Sodium diethyldithiocarbamate (NaDDC) is a well-known metal chelator (JACKSON 1958, Yu andWAI 1984), but its detailed toxicological properties still remain unexplored.
Cyanobacteria, by virtue of their cosmopolitan habitat, (CARR and WHITTON 1973) may encounter a wide range of chemical toxicants. Some of the important parameters used to study the effect of pesticides on photosynthetic organisms include growth, pigments, photosynthesis, protein synthesis etc. (MORELAND 1980, SUMLDA and UEDA 1973, KRINSKY 1966, ASTIER et af. 1979). The present study is mainly concerned with the effect of NaDDC on growth, pigments and photosynthetic electron transport in the unicellular cyanobacterium Anacystis nidufans. In addition, attempts are also made to make a preliminary characterization of NaDDC's mechanism for the inhibition of cyanobacterial growth.
Methods
Organism and growth conditions: Wild type Anacystis nidulans IU 625 (ATCC 27144) was obtained from the U.S. Environmental Protection Agency, Cincinnati, Ohio (U.S.A.). The cultures were grown in ALLEN'S medium (ALLEN 1968) and supplemented with NaN03 (17.6 mmol/l). The cultures were routinely grown in