Bioenergetics of membrane transport in Synechococcus R-2 (Anacystis nidulans, S-leopoliensis) PCC7942

Specialized chemical probe techniques need to be used to measure the membrane potential (Delta psi(i,o)) or the intracellular pH (pH(i)) of the cyanobacterium Synechococcus R-2 (PCC7942). The pH(i) of Synechococcus is essentially a set point (approximate to 7.3) over a wide range of extracellular pH (pH(o)) from 7 to 11. Maintenance of the pH(i) is strongly Na+-dependent and the cells cannot tolerate acid pH(o). The Rb-86(+)-valinomycin method of measuring the Delta psi(i,o) has inherent limitations, the most obvious being that the valinomycin treatment itself might alter the membrane potential. Tl-201(+) has been found in Synechococcus to distribute across the plasmalemma passively, and so the accumulation ratio of the ion ([Tl+](i)/[Tl+](o) or Tl-i,o(+)) can be used to calculate the apparent Delta psi(i,o). The two types of probe give comparable results in Synechococcus. Polarizations of the Delta psi(i,o) of cells, because of electrogenic transport of ions, can be detected from its effects upon the uptake rate of permeant cations using both the Rb-86(+)-valinomycin and Tl-201(+) methods. HCO3- hyperpolarized Delta psi(i,o), whereas NH4+, CH3NH3+, and K+ led to depolarization. Most active transport systems, including the HCO3- pump, in cyanophytes appear to be ATP binding cassette (ABC) type ATP pumps. Few cotransport (H+ or Na+) driven mechanisms have been identified. A substantial proportion of the power available from photosynthesis and respiration is used to maintain ionic gradients and the membrane potential and in the light a large part (approximate to 10%) is used to import inorganic carbon.