Impact of bubble size on growth and CO 2 uptake of Arthrospira (Spirulina) platensis KMMCC CY-007 Kisok Kim a , Jaeho Choi a , Yosep Ji a , Soyoung Park a , Hyungki Do b , Cherwon Hwang a , Bongju Lee a , Wilhelm Holzapfel a,⇑ a Advanced Green Energy and Environment Institute (AGEE), Handong Global University, Pohang 791-708, Gyeongbuk, Republic of Korea b School of Life Sciences, Handong Global University, Pohang 791-708, Gyeongbuk, Republic of Korea highlights  CO 2 bubble size influenced carbon uptake in Arthrospira platensis KMMCC CY-007.  mRNA expression levels varied according to uptake complexes and CO 2 bubble size.  Low affinity NDH-I 4 complex was strongly expressed under fine bubble.  NDH-I 3 and also bicA and sbtA systems were weakly expressed under fine bubble. article info Article history: Received 12 June 2014 Received in revised form 4 August 2014 Accepted 5 August 2014 Available online 12 August 2014 Keywords: Arthrospira (Spirulina) platensis CO 2 uptake Bubble size Cyanobacteria abstract Optimisation of cyanobacterial cell productivity should consider the key factors light cycle and carbon source. We studied the influence of CO 2 bubble size on carbon uptake and fixation, on basis of mRNA expression levels in Arthrospira platensis KMMCC CY-007 at 30 °C (light intensity: 40 lmol m 2 s 1 ; 1% CO 2 ). Growth rate, carbon fixation and lipid accumulation were examined over 7 days under fine bubble (FB) (100 lm Ø) bulk bubble (BB) (5000 lm Ø) and non-CO 2 (NB) aeration. The low affinity CO 2 uptake mRNA (NDH-I 4 complex) was stronger expressed than the high affinity NDH-I 3 complex (bicA and sbtA) under 1% CO 2 and FB conditions, with no expression of bicA1 and sbtA1 after 4 days. The high affinity CO 2 uptake mRNA levels corresponded to biomass, carbon content and lipid accumulation, and increase in NDH-I 3 complex (9.72-fold), bicA (5.69-fold), and sbtA (10.61-fold), compared to NB, or BB conditions. Ó 2014 Published by Elsevier Ltd. 1. Introduction Photosynthetic microorganisms, microalgae and cyanobacteria, use inorganic carbon for growth by CO 2 capturing, while cyanobac- teria may also fix atmospheric nitrogen and are able to grow in freshwater, marine and terrestrial habitats. Phytoplankton, with cyanobacteria as major group, is responsible for an estimated 50% of photosynthesis, worldwide, and thereby dominating CO 2 global conversion (Fuhrman, 2003). Up to the industrial era the level of atmospheric carbon dioxide has ranged between 180 and 260 ppm, but during the last 100 years its concentration has risen to between to 380 ppm in 2004, according to the CNRS (French National Centre for Scientific Research) (Siegenthaler et al., 2005), and increased to about 401 ppm, 10 years later (end of May 2014), as was measured at Mauna Lua Observatory (http://co2now.org/ Current-CO2/CO2-Now/weekly-data-atmospheric-co2.html). It is generally accepted that the growing industrialisation is closely related to the rise in atmospheric CO 2 levels, which, in turn, are a result of a constant increase in fossil fuel consumption (Kumara et al., 2011). Simulating regional pCO 2 growth trends, Tjiputra et al. (2014) have predicted a higher oceanic pCO 2 trend of more than 40% by the year 2100, which, by implication, would be the result of a reduced atmospheric uptake rate. Hughes and Benemann (1997) already suggested to strive for a balance between carbon emission and consumption by mass production of photosynthetic microorganisms on an industrial scale. The potential of biological approaches for CO 2 sequestration lies in the produced biomass (and its contents) with diverse appli- cations including production of biofuels (Mata et al., 2010; Pruvost et al., 2011), pharmaceuticals, aquaculture feeds, and human food supplements (Chen et al., 2012; Singh et al., 2011). Moreover, exogenous production of chemicals from cyanobacteria (e.g., 2,3-butane-diol by Synechococcus) holds great promise for the future (Oliver et al., 2013). http://dx.doi.org/10.1016/j.biortech.2014.08.034 0960-8524/Ó 2014 Published by Elsevier Ltd. ⇑ Corresponding author. Tel.: +82 10 8845 1360; fax: +82 54 260 1319. E-mail address: wilhelm@woodapple.net (W. Holzapfel). Bioresource Technology 170 (2014) 310–315 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech