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.
Key words: Alkalinization, blue light, Hydrodictyon, nitrate uptake, stoichiometry nitrate/protons. Nitrate uptake and the medium alkalinization related to it were studied with nets of the coenocytic, giant cell, green alga Hydrodictyon reticulatum. A compar
In Monoraphidium braunii, uptake of NO3−, NO2− and Cl− is associated with proton transport and triggered by blue light (BL). Only 10 s after cells able to reduce NO3− to NH4+ were irradiated with continuous, low‐fluence BL in the presence of NO3−, an alkalinization of the medium began and only became interrupted by switching off the BL with a 60–90 s time lag. With 30 s BL pulses, the NO3−‐dependent alkalinization lasted 3–5 min until it stopped. When the cells were exposed to continuous BL in the presence of Cl−, the alkalinization also started within 10 s but lasted only 3 min. After that, the pH remained constant and decreased when the BL was switched off. With 30 s BL pulses, the Cl−‐dependent alkalinization lasted 3 min and then decreased to its initial value. The NO3−‐dependent alkalinization shown by cells unable to reduce NO3− to NH4+ was similar to that observed in the presence of Cl−. These alkalinization rates fit the Bunsen‐Roscoe reciprocity law. With 2 s pulses of high‐fluence BL, the delay time of the NO3 ‐ or Cl−‐dependent alkalinizations was only 2 s, one of the fastest BL responses reported so far. The action spectra for Cl− and NO3− uptakes proved to be very similar and matched the absorption spectra of flavins, including the 267 nm peak.
Blue light is one of the most important environmental signals regulating monovalent anion transport in plant cells. In the unicellular freshwater chlorophyte Monoraphidium braunii, blue light is essential for the activation of HCO3−, NO3−, NO22 and Cl− transport systems. These anions are taken up when blue light is present but the uptake ceases when this radiation is suppressed, indicating that blue light is a switch signal for the monovalent anion transport system(s) of this alga. Similar results have been obtained in other green algae and higher plants. The action spectra for the uptake of NO3− and Cl− in M. braunii are very similar and resemble the absorption spectra of flavins or a combination of flavins and pterins. It is proposed that both anions share the same transport system(s). The uptake of monovalent anions consists of a cotransport with H+, thus producing alkalinization of the external medium. The time between the onset of blue light and the beginning of alkalinization can be as short as 2 s. Taken together, the results suggest that the photoreceptor mediating the blue light activation of monovalent anion uptake in this green alga is a plasma membrane‐bound flavoprotein.
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