PHOTOSYNTHETIC PROPERTIES OF PROTOPLASTS, AS COMPARED WITH THALLI, OFULVA-FASCIATA (CHLOROPHYTA)

Protoplasts were prepared from Ulva fasciata Delile, and their photosy nthetic performance was measured and compared with that of thalli disc s. These protoplasts maintained maximal rates of photosynthesis as hig h as those of thalli (up to 300 mu mol O-2.mg chlorophyll(-1).h(-1)) f or several hours after preparation and were therefore considered suita ble for kinetic studies of inorganic carbon utilization. The photosynt hetic K-1/2(inorganic carbon) at pH 6.1 was 3.8 mu M and increased to 67, 158, and 1410 mu M at the pH values 7.0, 7.9, and 8.9, respectivel y, Compared with these protoplasts, thalli had a much lower affinity f or CO2 but approximately the same affinity for HCO3-. Comparisons betw een rates of photosynthesis and the spontaneous dehydration of HCO3- ( at 50 mu M inorganic carbon) revealed that photosynthesis of both prot oplasts (which lacked apparent activity of extracellular/surface-bound carbonic anhydrase) and thalli (which were only 25% inhibited by the external carbonic anhydrase inhibitor acetazolamide) could not be supp orted by CO2 formation in the medium at the higher pH values, indicati ng HCO3- uptake. Since both protoplasts and thalli were sensitive to 4 ,4'-diisothiocyanostilbene-2,2'-disulfonate, we suggest that HCO3- tra nsport was facilitated by the membrane-located anion exchange protein recently reported to function in certain Ulva thalli. These findings s uggest that the presence of a cell wall may constitute a diffusion bar rier for CO2, but not for HCO3-, utilization under natural seawater co nditions.