Blue‐light‐induced pH changes associated with NO₃⁻, NO₂⁻ and Cl⁻ uptake by the green alga Monoraphidium braunii

Abstract

In M. braunii, the uptake of NO3" and NO2~ is blue-lightdependent and is associated with alkalinization of the medium. In unbufTered cell suspensions irradiated with red light under a COj-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 NOa" or NO2" uptake and reduction showed that two protons were consumed for every one NH/ released. The uptake of CI" was also triggered by blue light with a similar 10 s time response. However, the Cr-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 cr, 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.

Extracellular pH Is Under Circadian Control inGonyaulax polyedraand Forms a Metabolic Feedback Loop

“…1A). A light-dependent alkalization has also been shown for other micro-algae (Aparicio et al, 1994;Eisele and Ullrich, 1975;Rybova´et al, 1980;Ullrich et al, 1998;Witt and Aparicio, 1995). It is mainly due to photosynthesis since it can be completely inhibited by the photosynthesis inhibitor DCMU (3-(3,4-dichlorphenyl)-1,1-dimethylurea) (Kamiya, 1997;Rybova´et al, 1980).…”

EFFECTS OF SHORT PULSES OF BLUE LIGHT ON THE ALKALINIZATION ASSOCIATED WITH THE UPTAKE OF NO3−AND CL−BY THE GREEN ALGAMONORAPHIDIUM BRAUNIIAND RELATED ACTION SPECTRA

“…Algal growth and general experimental conditions. Monoraphidium braunii (Legnerová, 202–7d) from the Universität Göttingen (Germany) were grown with either NO 3 or NH 4 + as the only N source under a 14/10 light/dark photoperiod as previously described 10,12 . After 2 h in the light, the cells were harvested, washed and resuspended in 2 m M MgSO 4 .…”

“…These results were interpreted as a BL activation of the NO 3 − and NO 2 − uptakes. Recently, it was shown that, in M. braunii , the uptakes of NO 3 − , NO 2 − and Cl − were associated with protons as counterions to maintain isoneutrality 12 . This allowed us to study the monovalent anion uptakes by following the pH changes in the external medium of the cell suspensions.…”

“…This allowed us to study the monovalent anion uptakes by following the pH changes in the external medium of the cell suspensions. When cells illuminated with relatively strong photosynthetically active red light (RL) in the absence of any carbon source were additionally irradiated with relatively weak BL in the presence of NO 3 , NO 2 or Cl, an immediate alkalinization (10 s) of the medium was observed 12 …”

“…When the cells were unable to reduce NO 3 to NH 4 + , but retained the monovalent anion transport system, the alkalinization features were similar for these monovalent anions, i.e. the pH leveled off under BL and decreased when the BL was switched off 12 …”