Significant amelioration of Cr and Pb toxicity was observed in Nostoc muscorum at high concentrations of phosphate. The effect of Cr and Pb on growth, photosynthetic oxygen evolution, carbon and nitrogen fixation of N. muscorum was less in cultures pre-and post-treated with high (80 /2M) phosphate concentration than in those treated with a lower (20 /IM) orthophosphate concentration. The uptake of Cr and Pb in the cyanobacterial cells was not affected by phosphate concentration. Likewise phosphate uptake was not disturbed either by Cr or Pb. A rapid accumulation of polyphosphate bodies in the cells treated with Cr and Pb indicates that uptake and accumulation of phosphate was not inhibited. N. muscorum cells exposed to both the metals registered a notable decrease in the breakdown of polyphosphate bodies and acid and alkaline phosphatase activities. Cr and Pb competitively inhibited the activity of partially purified acid and alkaline phosphatases. It is concluded that Cr and Pb affect intracellular phosphate metabolism rather than the uptake and incorporation of phosphate in N. muscorum.Phosphorus is one of the key elements responsible for regulating algal distribution and cyanobacterial blooms (13). Algae, the first link of aquatic ecosystem, are known to suffer frequently from phosphorus limitation. A consequence of low phosphorus availability is the development of high phosphatase activities in microorganisms including cyanobacteria. Phosphatases are known to efficiently hydrolyze organic and polyphosphates, producing inorganic phosphate and thereby regulating the phosphate turnover in biota (10).The adaptation of plants to exceptionally high concentrations of heavy metals appears to be associated inter alia with their ability to either use phosphorus or to
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
This study presents information on the mechanism of inhibition of the photosynthetic electron transport of Nostoc muscorum by chromium (Cr) and lead (Pb). Photosystem II (PS II) was found to be more sensitive both to low and high concentrations of test metals used. A considerable inhibition of photosystem I (PS I) was, however, observed at high concentrations only. Although Cr‐induced inhibition of DCPIP photoreduction and lowering of chlorophyll a (Chl a) fluorescence intensity (F685) could not be reversed by artificial electron donors (diphenyl‐carbazide (DPC), NH2OH, MnCl2 and benzidine) of PS II, these electron donors did substantially reverse the Pb‐induced inhibition of DCPIP photoreduction as well as the lowering of Chl a fluorescence. Nevertheless, an increase in Chl a fluorescence at high concentrations of Pb suggested that this metal also arrests electron flow on the reducing side of the PS II reaction centre. Besides this, the suppression of fluorescence intensity of phycocyanin at low concentrations of both metals points to the involvement of phycobilisomes in the inhibition of PS II activity. The present study demonstrates that the modes of action of Cr and Pb on PS II are quite different.
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