A Preliminary Study On CO₂ Bio-fixation By Microalgae Coupled With Production Of High Value-added Substances

The large number of CO₂ released from the consumption of fossil fuels is a major source of greenhouse gases,which exacerbates the global warming greatly.Therefore,the emission reduction and fixation of CO₂ has become a problem need to be solved urgently in the field of industry and science.The biological CO₂ fixation,represented by microalgae carbon sequestration,is considered as one of the most potential method which has the advantages of green,safe and recyclable.However,the research on the microalgal carbon sequestration is still at the initial stage,and facing the followingserious problems:Most of the microalgal CO₂ fixation studies were reported only in laboratory,there is no data on large-scale culture,especially on open raceway pond;The evaluation system for CO₂ fixation of microalgae in open culture is missing,and this missing restricted the screening of microalgae and the improvement of the reactors;Few microalgae strains suitable for CO₂ fixationin open large-scale cultivation were reported;The high cost limits the application of microalgal CO₂ fixation in industry.To solve the above problems,the following points were studied in present research:Isolation,cultivation and screening of microalgal strains with high CO₂ fixation efficiency and value-added products;Establishment of evaluation system for CO₂ fixation of microalgae in open culture;Large-scale cultivation of microalgae for CO₂ fixation coupled with the production of carotenoid and phycocyanin.The main results are summarized as follows:1.In this study,we research the effects of three main factors,including pH,the concentration of inorganic carbon source and the aeration depth of CO₂,on the CO₂ absorption rate;we research the effects of the pH and the concentration of inorganic carbon source on the CO₂ evaporation rates.We establish a system to evaluate the utilization efficiency of exogenous CO₂,this system uses two indicators to reflect the ability of microalgae CO₂ bio-fixation quantitatively:?1?the rate of carbon fixation?Rc?of microalgae,indicating the rate of microalgae convert the exogenous CO₂ into biomass;?2?the utilization efficiency of exogenous C02 representsthe proportion of external CO₂ which converted to biomass.The significance and innovation of the evaluation system for CO₂ fixation of microalgae in open culture are embodied in three aspects:?1?the utilization efficiency of exogenous CO₂ is determined mainly by microalgae biomass yield,CO₂ absorption and evaporation rates in culture,and elaborating the principles of how these 3 factors work;?2?this system could provides theoretical basis for the screening of CO₂ fixation microalgae;?3?this system could provides a simple and reliable method of how to evaluate the utilization efficiency of exogenous CO₂ of microalgae.2.One hundred and thirty-one microalgal strains with the potential of CO₂fixation were isolated from different habitats in China from 2014 to 2016.Thirty-two strains,including twenty-two green algae strains and ten Arthrospira?Spirulina?strains,were selected for outdoor 40 L and 5 m2 open raceway pond culture,after evaluated in indoor 200 mL photo bioreactor.The fixation efficiencies of exogenous CO₂ and carotenoid production of Arthrospira strains reached0.154 g L^-1d^-1-0.187 g L^-1d^-1 and 0.019 mg L^-1d^-1-0.032 mg L^-1d^-1 respectively in 40 L reactors,0.154 g L^-1d^-1-0.186 g L^-1d?1 and 0.019 mg L^-1d^-1-0.027 mg L^-1d^-1 in 5 m2 raceway pond.The phycocyanin contents of all Arthrospira strains are more than 12%of the dry weightat the end of culture in 5 m2 raceway ponds,and the highest 14.5%and 15.8%contents were observed in strains GT-2 and GT^-1 respectively.These data shows the great potential of Arthrospira in the CO₂ fixation coupled with phycocyanin production.For those green algal strains,0.047 g L^-1d^-1-0.261 g L^-1d^-1 and 0.009 mg L^-1d^-1-0.043 mg L^-1d^-1 were observed for the fixation efficiencies of exogenous CO₂ and carotenoid production respectively in 40 L reactors,and 0.047 g L^-1d^-1-0.164 g L^-1d^-1 and 0.009 mg L^-1d^-1-0.039 mg L^-1d^-1 respectively in 5 m2 raceway ponds.Compared to green algae,Arthrospira strains are more suitable for CO₂ fixation in open culture due to their strong pollution toleranceand high optimum pH.3.On the basis of 40 L and 5 12 culture test,two strains with excellent comprehensive performance,which are the green algae A419?Pediastrum sp.?and GT^-1?Arthrospira sp.?,were selected and cultured in 200 m2 open raceway pond.The fiixation efficiencies of exogenous CO₂ and carotenoid production were 27.02 g m-2 d'1 and 33.71 g m-2 d^-1,53.94 mg m-2 d^-1 and 72.29 mg m-2 d^-1,respectively for these two strains in this scale.For the GT^-1,2.79 g m-2 d^-1 phycocyanin production was observed.This pilot scale culture further indicated that the CO₂ fixation coupling of value-added products by microalgae is feasible in technology and has great application potential.