Efficiency of carbon deposition by carbon dioxide assimilation of microalgae and bivalves

Abstract

Effect of CO₂ concentration on photosynthesis efficiency (Fv/Fm) was studied with Isochrysis galbana and Tetraselmis suecica. The results showed that the Fv/Fm of both species in pure air culture was similar to microalgae cultured in 1.5% of CO₂ (Fv/Fm = 0.8) and 1.5% CO₂ clearly enhanced growth of the microalgae. While CO₂ concentration higher than 1.5% inhibited photosynthesis in both species. The maximum cell density of I. galbana and T. suecica cultivated in F/2 medium supplemented with 1.5% CO₂ were 1.87x107 and 2.39x106 cells/ml, respectively. With this study growth rate of oyster blood cockle and hooded oyster could not be detected, biological CO₂ assimilation process using marine microalgae and marine bivalve was evaluated by carbon assimilation of the green mussel Perna viridis fed with Tetraselmis suecica under laboratory condition. Incorporation of carbon dioxide into microalgae biomass was performed through aeration. The experiment consisted of three treatments which were mussels without feeding (Control), mussels fed with T. suecica cultured with air (Treatment 1: T-Air), and mussels fed with T. suecica cultured with 1.5% CO₂ in air (Treatment 2: T-CO₂). The results showed that growth of mussels in T-Air and T-CO₂ was 22.4 ± 4.0 mg/individual/day and 28.9 ± 12.3 mg/individual/day, respectively which was significant higher than control (mussels without feeding). Growth of mussels in T-Air was significantly lower than T-CO₂, (p≤0.05). Growth of mussels in T-Air was significantly lower than in T-CO₂. Carbon content in shell (15.59 ± 0.57 % D.W.) and meat (38.28 ± 1.72 % D.W.) of mussels fed with aerated T. suecica (T-Air) was significantly higher than that found in mussels fed with 1.5% CO₂ T. suecica (14.2 ± 0.47 and 36.61± 0.43 % D.W. in shell and in meat, respectively) (p≤0.05). With T-Air, 1.93±0.28% and 9.28±1.33% of carbon from T. suecica cells was assimilated into shell and meat of the mussel, respectively, while in T-CO₂, carbon assimilation from T. suecica cells in shell and meat was 2.21±0.56% and 11.17±2.81% respectively. Larger carbon deposition was from surrounding water through calcification process. As meat of the mussel can be utilized as food and carbon can be long term stored as shell, hence carbon assimilation using microalgae and bivalve could be considered as an alternative way of carbon dioxide mitigation process.