Abstract
Threat of global warming due to carbon dioxide (CO2) emissions has stimulated research into carbon sequestration and emissions reduction technologies. Alkaline scrubbing allows CO2 to be captured as bicarbonate, which can be photochemically fixed by microalgae. The carbon concentrating mechanism (CCM), of which external carbonic anhydrase is a key component, allows organisms to utilise this bicarbonate. In order to select a suitable strain for this application, a screening tool is required. The current method for determining carbonic anhydrase activity, the Wilbur and Anderson assay, was found to be unsuitable as a screening tool as the associated error was unacceptably large and tests on whole cells were inconclusive. This paper presents the development of a new, whole cell assay to measure inorganic carbon uptake and external carbonic anhydrase activity, based on classical pH drift experiments. Spirulina platensis was successfully used to develop a correlation between the specific carbon uptake (C) and the specific pH change (dpH). The relationship is described by the following: C[mmol C (g dry algae)−1 h−1] = 0.064 × (dpH). Inhibitor and salt dissociation tests validated the activity and presence of external carbonic anhydrase and allowed correlation between the Wilbur and Anderson assay and the new whole cell assay. Screening tests were conducted on S. platensis, Scenedesmus sp., Chlorella vulgaris and Dunaliella salina that were found to have carbon uptake rates of 5.76, 5.86, 3.86 and 2.15 mmol C (g dry algae)−1 h−1, respectively. These results corresponded to the species' known bicarbonate utilisation abilities and validated the use of the assay as a screening tool.
Highlights
The global impact and consequences of climate change, due to greenhouse gas emissions, are increasingly recognised as a concern for policy makers
Dunaliella salina (WCSA, Upington) was grown in natural seawater collected from Seapoint, Cape Town which had been supplemented with trace metals and vitamins as specified in f/2 medium (Anderson 2005) as well as (g l-1): NaNO3 (0.25), NaH2PO4 (0.0004), FeC6H5O7 (0.0029) and Na2EDTA.2H2O (0.0087)
The Wilbur and Anderson assay was performed using a solution of bovine carbonic anhydrase (CA) of known concentration in order to evaluate the method statistically
Summary
The global impact and consequences of climate change, due to greenhouse gas emissions, are increasingly recognised as a concern for policy makers. Reductions in carbon dioxide (CO2) emissions, in particular, would require significant lifestyle adjustments, which are difficult to achieve. An attractive addition to emissions reductions and other sequestration methods is the utilisation of CO2 through biological means. The use of microalgae (including cyanobacteria) to photosynthetically fix CO2 is a potentially favourable method, given their high productivities. Microalgal systems may be more environmentally benign than other alternatives (geological storage, mineral carbonation) which are energy intensive (Binaghi et al 2003; Maeda et al 1995). CO2 is fixed from the atmosphere or directly injected into the algal solution as a gas. CO2 uptake efficiencies are low, due to limitations in mass transfer and photosynthetic efficiency (Stewart and Hessami 2005; Ugwu et al 2008, Langley et al 2012)
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