INORGANIC CARBON UTILIZATION AND GROWTH ABILITIES IN THE MARINE RED MACROALGA GELIDIOPSIS SP.

Abstract

We studied the effects of NH4+, photosynthetic photon flux (PPF), and temperature on growth rates, together with inorganic carbon (Ci) utilization properties of Gelidiopsis sp. cultivated in tanks. At 25% sunlight, weekly growth rates and dry weight yields increased up to 6-fold with increasing NH4+(0–2 mM); however, at 5% or 100% sunlight the effects were much lower. Contents of photosynthetic pigments (chlorophyll a and phycoerythrin) increased in correlation with increases of NH4+. Maximal chlorophyll a concentrations occurred under high PPF, while phycoerythrin concentrations were higher at low PPF. Ash amounts in Gelidiopsis sp. did not vary significantly with different NH4+ or PPF levels. Optimal temperatures and PPFs for growth were 20–25 °C and 170–320 μmol m−2 s−1, respectively, correlating with short-term photosynthetic O2 evolution measurements. The pH of both thallus surface and bulk medium increased during photosynthesis, reaching average values of 8.75, and resulting in low rates of O2 evolution. Activities of carbonic anhydrase (CA) were detected inside and outside the cells and were apparently involved in the Ci uptake system of Gelidiopsis sp. since both acetazolamide (membrane-impermeable) and ethoxyzolamide (membrane-permeable) inhibited photosynthetic O2 evolution by 89% on average. Half-maximal rates of photosynthetic O2 evolution (K0.5) were reached at 17 μM CO2 at pH 5.0 and 2–3 mM Ci at pH 8.0, indicating high affinity for CO2 and close to saturated photosynthesis at Ci levels of seawater. Thus the Ci uptake system of Gelidiopsis sp. probably uses an extracellular CA catalyzed conversion of HCO3− to yield CO2, which could diffuse into the cells, and an intracellular CA catalyzed HCO3− ↔ CO2 interconversion which may assure CO2 for Rubisco. Direct uptake of HCO3− may also be required based on the K0.5 (CO2) estimated for Gelidiopsis sp. and the pH generated at the thallus surface at which CO2 concentrations would only be approximately 10 μM. Therefore, in addition to limitations of low NH4+ concentrations and high temperatures during the summer, growth of Gelidiopsis sp. from the Israeli Mediterranean may also be restricted by its limited Ci utilization system and the low CO2 concentrations prevailing in seawater.