Planet Earth Online

Slime power: bioenergy from the sea

14 December 2012

All plants turn sunlight energy and CO₂ into organic molecules such as sugars and lipids (oils), which we can extract and use to produce biofuels like bioethanol, biobutanol and biodiesel. This bioenergy is likely to be a big help in reducing our dependence on fossil fuels.

But the biomass - the raw materials - have to be both sustainable and economically viable, and that's proving to be a bit of a problem.

The most common biofuel - bioethanol - is made from sugar cane and maize. It currently accounts for 90 per cent of the world's biofuel supply, with biodiesel from plant oil such as rape seed and palm accounting for the rest. But these 'first generation' biofuels are still far from meeting even existing demand for bio-based alternatives to petroleum. And we can't simply increase production, because these energy crops compete with food crops for land and water.

So attention has turned to the other 70 per cent of the Earth's surface - the oceans - and the potential for aquatic plants to provide a sustainable fuel source.

Algae are a diverse group of photosynthetic aquatic organisms which includes seaweed (macroalgae) and microscopic floating plants such as phytoplankton (microalgae). Like other plants, algae also make sugar and oil molecules through photosynthesis. In the right conditions some species of microalgae can accumulate oil in quantities up to half their dry cell weight, and this means they have the potential to provide up to 24 times the amount of oil per acre than palm, the most productive terrestrial crop. Algae are also incredibly flexible: they can grow in a wide range of conditions including marine, brackish or nutrient-rich wastewater. All this means algae could be a highly efficient energy source.

Microalgal biodiesel has other benefits over traditional land-grown biofuels. It has high levels of polyunsaturated fatty acids so it can remain fluid at low temperatures, which improves the performance of diesel engines in cold conditions.

Yet despite these potential benefits to yield and performance, growing, harvesting and processing algae remains expensive. Microalgae are grown in large open ponds or in photobioreactors - artificial environments that provide light, CO₂ and nutrients. But the amount of light and constant temperatures needed for optimal growth come at a price, especially in the UK where sunlight can be scarce. Using artificial light would have significant economic and carbon costs at the scale required for fuel production, and even in warmer regions where light and temperature are more reliable, the costs of harvesting and processing large amounts of algae are still high.

So large-scale production of algal biofuel is in its infancy, but there is considerable global investment to realise its potential. The US Navy and shipping giant Maersk have successfully tested algal biofuels in their ships and are investing in further research and development.

Macroalgae offer different opportunities. Large brown seaweed grows very fast and is common around the UK coast, particularly in Scotland. Because of its structure seaweed can easily be biodegraded to produce methane gas. This could provide a local source of biogas for remote coastal communities, where grid connections are poor and gas supplies expensive. Macroalgae could also be fermented to make ethanol. But, as with microalgae, using macroalgae on a commercial scale presents problems.

There's an estimated ten million tonnes of seaweed around the Scottish coast, but harvesting these wild stocks could spell disaster for the surrounding ecosystem. Seaweed is home to many small fish and invertebrates which are an essential part of the food web for migrating seabirds. Standing stocks of seaweed also provide defence against erosion and flooding, which would be lost if harvested at scale.

Seaweed farms could provide an answer. These are already commonplace in China where nine million tonnes of seaweed are cultivated each year. The plants are grown on long ropes held afloat by plastic buoys and harvested by hand.

While most of China's seaweed is currently used for food, textiles, cosmetics and medicines, attention is now turning to cultivating it for fuel. But Europe's relatively high labour costs mean harvesting by hand wouldn't be economically viable; we would need to develop a machine to harvest and process enough macroalgae for fuel production.

And crucially, we also need to understand the potential impacts of industrial-scale seaweed farms on the marine ecosystem.

In future, micro- and macroalgae could be a sustainable source of energy; there is plenty of evidence to suggest that large-scale biofuels production from algae is possible. But there are still many unknowns. We need to ensure their long-term sustainability - economically, socially and environmentally. The next steps must include assessments of the short- and long-term impacts of growing algae, so we can be confident of the sustainable limits of production and make sure that marine ecosystems and biodiversity are properly looked after.

Keywords: Marine life, Natural resources, Plants, Sustainability,

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