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

Photosynthetic (oxygen evolution) and growth (biomass increase) responses to 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 fibreglass outdoor tanks with running seawater. While net photosynthetic rates were uniform at pH 6.0-8.0, dropping only at pH 8.7, growth rates were significantly 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% at 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, apparently due to high dark respiration rates measured at acidic pH. Weekly growth rates averaged 53% in algae grown in fibreglass tanks aerated with regular air (360 mg L-1 CO2) and 28% in algae grown in tanks aerated with CO2-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 reached 9.0. The pH at the boundary layer of algae submerged in seawater increased in response to light reaching, about pH 8.9 within minutes, or remained unchanged for algae submerged in a CO2-free artificial seawater medium. Photosynthesis 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 61 mu M) at pH 6.0. It is therefore concluded that P. linearis uses primarily CO2 with HCO3- being an alternative source of Ci for photosynthesis. Its fast growth could be related to the enzyme carbonic anhydrase whose activity was detected intra- and extracellularly.