BLUE-LIGHT-INDUCED PH CHANGES ASSOCIATED WITH NO3-, NO2- AND CL- UPTAKE BY THE GREEN-ALGA MONORAPHIDIUM BRAUNII

In M. braunii, the uptake of NO3- and NO2- is blue-light-dependent and is associated with alkalinization of the medium, In unbuffered cell suspensions irradiated with red light under a CO2-free atmosphere, the pH started to rise 10 s after the exposure to blue light, When the cellular NO3- and NO2- reductases were active, the pH increased to values of around 10, since the NH4+ generated was released to the medium, When the blue light was switched off, the pH stopped increasing within 60 to 90s and remained unchanged under background red illumination, Titration with H2SO4 of NO3- or NO2- uptake and reduction showed that two protons were consumed for every one NH4+ released, The uptake of Cl- was also triggered by blue light with a similar 10 s time response, However, the Cl--dependent alkalinization ceased after about 3 min of blue light irradiation, When the blue light was turned off, the pH immediately (15 to 30 s) started to decline to the pre-adjusted value, indicating that the protons (and presumably the Cl-) taken up by the cells were released to the medium, When the cells lacked NO3- and NO2- reductases, the shape of the alkalinization traces in the presence of NO3- and NO2- was similar to that in the presence of Cl-, suggesting that NO3- or NO2- was also released to the medium, Both the NO3-- and Cl--dependent rates of alkalinization were independent of mono- and divalent cations.