Effects of spectral light quality and carbon dioxide on the physiology of Micractinium inermum: growth, photosynthesis, and biochemical composition

Abstract

Growth, photosynthetic parameters, and biochemical composition of the chlorophyte Micractinium inermum were determined under conditions of different light-emitting diode (LED) wavelength distributions and carbon dioxide (CO₂) levels of 1, 5, and 10% v/v. Increasing the inflowing CO₂ concentration from 1 to 5 to 10% had negative effects on maximum growth rate (μ_max), relative growth rate (RGR), biomass production, and CO₂ fixation rate. Cultures subjected to a spectral distribution containing a blue wavelength peak were less negatively affected than red as CO₂ stress increased, and evidently there was a quantitative effect depending on the peak area percentage. Under non-stressful conditions (1% CO₂), there were no significant differences in μ_max or RGR among light treatments; however, blue light resulted in decreased lag phases (0.75 less days than red). Biomass production was significantly higher in red wavelength peak treatments at 1% CO₂ (i.e., monochromatic red treatment = 0.772 ± 0.07 g L⁻¹) which demonstrates that although blue light allows for an early growth advantage, cultures grown with red light are able to catch up and result in more biomass. At a 10% CO₂ level, RGR was higher in treatments with blue peaks; red peak treatments were no longer able to overcome the stress and demonstrated lag phases 4.87 days longer than blue peak treatments. Inhibition of photosystem II function was evident as CO₂ concentrations increased. Evaluation of biochemical composition revealed that protein content was significantly greater in blue peak treatments at 10% CO₂, pigment content was up to 2× greater than red at all CO₂ levels, and total FAME and fatty acid profiles did not exhibit major changes.