Changes in the Levels of Ribulose-l,5-bisphosphate Carboxylase/Oxygenase (Rubisco) in Tetraedron minimum (Chlorophyta) during

Exponentially growing cultures of the chlorophyta Tetraedron minimum were allowed to photoadapt to low (50//mole quanta m~2s~') and high (500//mole quanta m~2s~') irradiance levels. In these cultures, various aspects of the organization of the photosynthetic apparatus and related differences in its performance were studied. In this organism, the observed five-fold increase in pigmentation of low-light adapted cells was due to increases in the numbers of PSU's, while their sizes remained constant. Using radioimmunoassay technique, we found that high-light adapted algae had over five times more Rubisco per PSU than their low-light adapted counterparts. The high-light adapted algae also exhibited far higher (x 2.3) light saturated photosynthetic rates per chl a. This increase was the result of a reduction of tau, r, the turnover time of PS II reaction centers. We propose that the increase in Rubisco per PSU in high-light adapted algae explains the re-duction in T, which results in the higher Pmax rates per chl a in these algae. The relationship is non linear, since the increase in Rubisco per PSU was x 5.3 whereas that in PmM per chl a was only x 2.3