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

Abstract

Photosynthetic (oxygen evolution) and growth (biomass increase) responses to ambient pH and inorganic carbon (Ci) supply were determined for Porphyralinearis 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 O₂ evolution on a daily basis(i.e. total O₂ evolved during day time less total O₂ 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. linear is 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 sea water medium. Photosynthesis of P. linearissaturated at Ci concentrations of seawater (K_0.5560 μM at pH 8.2) and showed low photosynthetic affinity for CO₂(K_0.5 61 μM) at pH 6.0. It is therefore concluded that P. linearisuses 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.