Abstract:

Background and Aims We examined the effect of acid-alkaline concentrations in media on growth rate and nutrient uptake of a red macroalga, Gracilaria lemaneiformis, which has a rapid growth rate, rapid nutrient uptake rate, and high agar content. We address the following questions: 1) can the culture media be kept relatively stabilized, 2) what organic compound best buffers pH, and 3) how does the compound buffer the media and affect growth of the alga?
Methods Utilizing the dark-light bottles method, we selected a suitable zwitterionic organic compound from seven reagents (barbitone, borate, HEPES, TRIS, MOPS, TAPS, POPSO) that did not inhibit photosynthetic production of dissolved oxygen (DO), did not increase depletion of DO in respiration, and adjusted acid-alkaline balance in the culture media. We examined the effects of each buffer on growth rate (SGR) of G. lemaneiformis, measured diurnal and weekly variations of pH and dissolved inorganic carbon (DIC), and determined CO₂ concentrations according to pH and DIC. We integrated these with the growth rate of G. lemaneiformis, to investigate the mechanisms of growth and carbon uptake.
Key Results POPSO was the zwitterionic organic compound that best met our objectives. During the diurnal experiment of POPSO's effect on algal growth, pH rose continuously during the light cycle, contrary to DIC; however, high concentration of POPSO can buffer this fluctuation. During the one-week experiment, according to results of the diurnal experiment, pH values and DIC at the beginning and end of the light cycle indicated that lower concentrations of POPSO (0 and 5 mmol·L^-1) did not buffer pH fluctuations, but higher concentrations (>15 mmol·L^-1) suitably stabilized pH values and leveled off DIC content in the media. Ambient CO₂ concentration determined algal growth rates, with the relationship between SGR of G. lemaneiformis and ambient CO₂ concentration similar to a Michaelis-Menton kinetics equation. The saturated concentration of CO₂ was 5.25 μmol·L^-1, and the maximum growth rate of the alga was 10.11%·d^-1.
Conclusions This study determined that POPSO is a suitable organic compound to buffer G. lemaneiformis culture media. A possible mechanism is that, under relative stabilization of acid-alkaline media, HCO^-₃and CO^2-₃(components of DIC), transformed into CO₂ and accelerated growth rate of the alga.

Key words: Gracilaria lemaneiformis, pH buffer, POPSO, SGR, DIC