Cadmium toxicity and uptake as influenced by different pH values have been studied in the freshwater cyanobacterium Anabaena flos-aquae, using the techniques of morphometric analysis, x-ray energy dispersive analysis and atomic absorption spectrophotometry. A general reduction in cell dimension, thylakoid surface area, number and volume of polyhedral bodies, polyphosphate bodies, cyanophycin granules, lipid bodies, membrane limited crystalline inclusions, volume and number of wall layers and mesosomes was observed. These reductions were more pronounced in both acidic and alkaline medium than at pH 7.2. At 0.12 microM Cd, the uptake increased with alkaline pH values, and uptake was greater at pH 7.2 than at either acid or alkaline pHs. Lysis of cell wall at 1.18 microM Cd showed the following decreasing trend: pH 4.0 greater than pH 5.5 greater than pH 10.0 greater than pH 9.0 greater than pH 7.2. There was a total loss of lipid bodies at 1.18 microM Cd at all pH values listed. It is suggested that these techniques can be successfully employed for bioassay studies of metal toxicity to algae. In particular, cell wall lysis and loss of lipids by algae are good indicators of pH effects and metal toxicity in the aquatic ecosystem.

This paper presents the inhibition of photosynthetic electron transport chain of Nostoc muscorum by divalent Ni2+ and monovalent Ag+. PS I (DCPIP/ASC-MV) and PS II (H20-*PBQ) activities were markedly inhibited by Ni2+ and Ag+ in a concentration-dependent fashion but PS II was more susceptible than PS I to Ni2+. Ago was more toxic to PS I than PS II. The greater sensitivity of PS II to Ni2+ was further confirmed by the inhibition of DCPIP photoreduction and Chl a fluorescence. Restoration of Agtinduced inhibition of DCPIP photoreduction and Chl a fluorescence by artificial electron donors (DPC, NH2OH, MnCl2) and their failure to restore Ni2t induced inhibition suggests that Ni24 inactivates PS II by causing alteration and destruction of photosynthetic membranes, but Ag+ inhibits the electron flow at the oxidizing side of PS II. Nevertheless, the suppression of the fluorescence intensity at low concentrations of both metal cations points to the involvement of phycobilisomes in the inhibition of PS II activity.Though a considerable amount of work has been done on the metabolic processes of algae (12,22), compared to higher plants, the photosynthetic system of algae and cyanobacteria as affected by heavy metals are least explored. Interest in work on metal toxicity in cyanobacteria stems from: (i) their prokaryotic nature but higher plant type photosynthesis, (ii) their role in the nitrogen economy of the biosphere, and (iii) the ease of experimentally manipulating them.The effects of Hg2+, Cd2+, Zn2+ and Ni2+ on the electron transport of isolated chloroplasts to ascertain the modes of action of these cations on the photosynthesis have been documented (3,7,18,19). These studies indicate that the metal ions specifically affect the carriers and reaction centers thereby interrupting the flow of