Plant power

So far conservative approaches have been adopted by commercial algal growers. Open ponds, with shallow raceways or channels in which water is kept in circulation, are generally used. These are cheaper than photobioreactors, which resemble pharma industry bioreactors, and make scale-up easier. The species grown are restricted to those that thrive in selective environments, which keep out predatory protozoa and algal species considered as weeds. In current use are Chlorella, which requires a nutrient-rich medium; Spirulina or Arthrospira, which grow in high pH and bicarbonate conditions; and Dunaliella, which thrives in high salinity.

US-based Sapphire Energy plans to open a 122ha pilot facility in New Mexico, US, in 2014, which will likely make it the largest algae production facility for biofuels, says Rosenberg. The company will use large open ponds to grow Spirulina, which has been grown as a health food product for years. Contamination is of less concern because Spirulina flourishes at a high pH, excluding most competitors. Rosenberg explains the company plans to introduce rotifers to target single-celled algae weeds because the rotifers can’t wrap their mouths around the larger, cork-screw-shaped Spirulina when grazing.

Other companies are looking to use photobioreactors, which provide greater control over algal culture conditions and improve biomass productivity. They are also more secure for growing genetically-modified algae. But they are capital intensive and pricey if used solely for biodiesel production. Nonetheless, there are novel solutions under development that aim to trim costs.

Solix in Colarado, US, has produced a flat-panel bioreactor made from plastics, which resembles a raceway pond, while NASA has a flat-panel bioreactor made of plastic, which is laid down on a water surface, exploiting wave movements for mixing culture and regulating temperature. Algenol, in Florida, US, has made a plastic, horizontal half-cylinder vessel that uses a hydrofoil to mix the contents.

Mostly green algae and diatoms are being evaluated for oil, but Susan Golden, a researcher at University of California, US, among others, is studying cyanobacteria – so-called blue-green algae – which are easier to genetically modify than the more complex microalgae.

‘Talk to people who raise cyanobacteria in ponds and they will tell you that predators like amoebae can cause population crashes,’ says Golden. Her group discovered a cyanobacteria mutant apparently resistant to grazing by amoebae. The researchers revealed that the gene involved affected a surface sugar structure, which the predator seems to home in on (Proc. Nat. Acad. Sci. U.S.A., doi: 10.1073/pnas.1214904109).

‘When this chain of sugars was missing, the amoebae weren’t grazing on the organism. One possibility is that the cell makes a new structure not recognised by the amoebae. We call this our Harry Potter hypothesis; when the chain of sugars is missing, some cloak of invisibility is made in its place,’ Golden explains. However, it is also possible that the sugar chain is needed for recognition, and without it the cyanobacteria is not recognised by the amoeba as food.

Some researchers, such as Wim Varmass in Arizona State University, US, are trying to genetically modify cyanobacteria to produce desirable oils, while others aim to release these oils in response to external stimuli (Proc. Nat. Acad. Sci. U.S.A., 2011, 108, 6899), thus increasing productivity and greatly reducing the cost of taking the oil out of algal cells.

The lab strains being developed, however, are unlikely to be the answer to mass production of algal fuel. The strains currently used in products have proven resistant to the geneticist’s tools. ‘Spirulina, for example, is recognised as safe and it grows well but it is terribly difficult to transform,’ says Golden.

Moreover, algae produce oil only under specific conditions. They tend to become oil rich when deprived of nutrients, which cause them to hoard carbon as oil. Studies of algae have identified genes turned on during this starvation phase; ideally scientists want to genetically engineer algae to ‘trick’ them into their starvation phase while they are still growing.

Goldman and her colleagues recently discovered a high-growth cyanobacteria species (Leptolyngbya sp.) thriving in the wild in California, which is easy to genetically engineer. (PLoS ONE, doi:10.1371/journal.pone.0030901 3 ). ‘It’s as easy as falling off a log,’ she says excitedly. A highly productive species, open to modification, would be an ideal candidate for industrial production. Such discoveries mark the start of the domestication of algae for biodiesel and other products.

But algae do not give fuel for nothing; they require light, water, nutrients and carbon dioxide. ‘Large algae plants would require very significant amounts of fertiliser and this is the Achilles’ heel of algae production. That is why nutrient recycling is critical for the sustainable production of algal-based fuels,’ says Kurt Liffman, fluid dynamics scientist at CSIRO, Australia’s national science agency.

A recent report for the National Research Council in the US, Sustainable development of algal biofuels in the United States, noted that 10bn gallons of algal oil/year would require about half the total US consumption of nitrogen from ammonia and significant quantities of phosphorus from phosphate rock, itself a finite resource. This could have unintentional impacts on food prices; moreover, ammonia production itself requires significant inputs of energy. ‘If large-scale production of algal oil is to occur in the future, methods will have to be found for recycling fertilisers, as the existing supply is limited,’ Chisti acknowledges.

Wastewaters from municipal, industrial and agricultural activities could be used for cultivating algae feedstocks for biofuels. Indeed, the first open ponds for algae – since copied by many in the algal-growing world – were designed for wastewater treatment, not biomass production. China, for example, has massive potential in generating wastewater streams because of its large populations of people, pigs and other animals; a recent report by APEC (Asia-Pacific Economic Cooperation), Resource potential of algae for sustainable biodiesel production in the APEC economies, estimated that such streams could be used to produce 9G L of algal biodiesel.

There are, however, alternatives to sewage and slurry. Chisti believes that genetically engineering nitrogen fixing capabilities into green algal is achievable and could reduce a need for nitrogen fertiliser. Golden is sceptical though: ‘To engineer nitrogen fixation into an organism would be a hard thing to do.’ But, she points out that some cyanobacteria, for example, Nostoc, naturally fix nitrogen, and this would a more viable option. ‘I don’t understand why everyone is not demanding nitrogen-fixing cyanobacteria. An organism that could fix nitrogen could be good for production,’ she says.

Since carbon dioxide – the source of the carbon – is an essential nutrient for algal growth, some scientists are investigating the use of flue gas from coal-fired power plants as a source of CO₂. The US Department of Energy, for example, is looking at the potential upsides of co-locating algal production facilities with stationary industrial CO₂ sources. Scientists have mapped power plant sources of CO₂ located within 20 miles of municipal wastewater facilities in preferred climate regions. Many think it’s inevitable that algal plants will be cited to tap such nutrient sinks, abundant in the populated cities of Europe, North America and elsewhere.

The last five years have seen several pilot facilities for algae-based biofuels emerging. CSIRO is in the process of designing a nutrient recycling system and has an algal growth facility at its labs just outside Melbourne. AzCATI (Arizona Center for Algae Technology and Innovation) in the US provides test facilities for the algae industry and research community. In Holland, Wageningen University opened the AlgaePARC (Algae Production and Research Centre) to test outdoor photobioreactors. ‘Our goal is to have a design of a commercial plant with operational concepts in four years,’ explains Rouke Bosma, operational manager at this facility. ‘It will compare productivity in four different production facilities, including raceway ponds, tubular reactors and flat panels,’ he adds. 

Bosma argues that producing only biofuels from microalgae is a non-starter, echoing the views of many algal scientists. ‘Nowadays, awareness is increasing that biodiesel production alone is economically not feasible. For that reason, biorefinery initiatives are starting with a focus on several products from the same algal production facility.’ Some lipids could be recovered for biodiesel, others as a feedstock for the chemical industry; omega-3 fatty acids, proteins and carbohydrates could be recovered for food, feed and bulk chemicals, and the oxygen produced could also be recovered.

Will algae biodiesel replace regular diesel any time soon? ‘The decreasing cost of renewable energy and improvements in battery technology means that the electrification of our land and sea transport systems will be the long-term, most effective answer to greenhouse gases,’ predicts Liffman. ‘Air transport, however, is a different case. Algal-based aviation fuels may be a long-term viable option,’ he says.

Chisti believes that we are looking at a time frame of 10 years at least for algal fuels to become competitive with petroleum fuels. ‘But they do look promising and the possibility of their commercialisation at some scale in the future cannot be ruled out,’ he says. ‘I believe most in the scientific community and some companies recognise that commercialisation of algal fuels will take substantial developmental effort and time. Production of liquid transport fuels from algae has been proven repeatedly. No doubt about it. The issue is one of economics.’

In the past, a fall in oil prices pushed algae back into research obscurity, but climate change and sustainability concerns over other biofuels make this unlikely to reoccur. Greener algal biofuels will be part of our future fuel economy.