Photosynthetic inorganic carbon utilization and growth of Porphyra linearis (Rhodophyta)

Photosynthetic (oxygen evolution) and growth (biomass increase) responses t o ambient pH and inorganic carbon (Ci) supply were determined for Porphyra linearis grown in 0.5 L glass cylinders in the laboratory, or in 40 L fibre glass outdoor tanks with running seawater. While net photosynthetic rates w ere uniform at pH 6.0-8.0, dropping only at pH 8.7, growth rates were signi ficantly affected by pH levels other than that of seawater (c. pH 8.3). In glass cylinders, weekly growth rates averaged 76% at external pH 8.0, 13% a t pH 8.7 and 26% at pH 7.0. Photosynthetic O-2 evolution on a daily basis ( i.e. total O-2 evolved during day time less total O-2 consumed during night time) was similar to the growth responses at all experimental pH levels, a pparently due to high dark respiration rates measured at acidic pH. Weekly growth rates averaged 53% in algae grown in fibreglass tanks aerated with r egular air (360 mg L-1 CO2) and 28% in algae grown in tanks aerated with CO 2-enriched air (750 mg L-1 CO2). The pH of the seawater medium in which P. linearis was grown increased slightly during the day and only rarely reache d 9.0. The pH at the boundary layer of algae submerged in seawater increase d in response to light reaching, about pH 8.9 within minutes, or remained u nchanged for algae submerged in a CO2-free artificial seawater medium. Phot osynthesis of P. linearis saturated at Ci concentrations of seawater (K-0.5 560 mu M at pH 8.2) and showed low photosynthetic affinity for CO2(K-0.5 6 1 mu M) at pH 6.0. It is therefore concluded that P. linearis uses primaril y CO2 with HCO3- being an alternative source of Ci for photosynthesis. Its fast growth could be related to the enzyme carbonic anhydrase whose activit y was detected intra- and extracellularly.