Wetenschap - 7 juli 2011

Sustainable farming at sea

Seaweed cultivation could solve the looming global food crisis and prevent the last rainforests being turned over to farmland, says a group of Wageningen researchers. Seaweed is chockfull of high quality protein and grows amazingly fast. Wageningen's first testing facility for algae cultivation is officially launched on July 7.


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'Last week there were porpoises swimming around here, but with waves like these you can't see whether they are here today as well.' Seaweed researcher Willem Brandenburg gazes across a wild and choppy Oosterschelde estuary. A couple of hundred metres off the coast, four connected floating frames, each about four by four metres in size, bob up and down on the waves. The seaweed grows on ropes hanging from slats inside the frames. This is Europe's first experimental farm for growing seaweed. The algae floats are themselves anchored to a larger work float. 'It is now at least wind force 4, which is borderline safe to work in', says Brandenburg.
Brandenburg and his team are doing research on environmentally friendly seaweed cultivation. A few weeks ago, they started the first field trials. In order to establish what the local effect of the farming system was on the surrounding plants and animals, a team of divers regularly inventories the sea life around the floats. Which was rich, according to the baseline measurement. The sea and the seabed are inhabited by shrimps, anemones, seaweed pipefish and a large North Sea crab that has been christened Charlie.
Elephants and pandas
Brandenburg is a man with a mission. He sees the development of marine farming as a possible solution to the problem of how to feed a growing world population without sacrificing nature. 'Conventional agriculture is not exactly sustainable', he claims. 'We could double food production on land, so that we can still feed the world in 2050, but then there will be a big problem for the elephants and pandas that we would also like to keep alive.' The solution, thinks Brandenburg, lies in marine farms in which seaweed is cultivated on a large scale. Seaweed contains a lot of high quality protein, amounting to one quarter of the dry material, and if grown on a large scale could supply the protein requirements of the burgeoning world population. This would require an area of sea twice the size of Portugal, Brandenburg has calculated. And he believes that can be done sustainably, as long as you choose the right locations.
At the moment, Asia leads the field in algae cultivation, with 90 percent of the total production of roughly seven million tons. But the way it is done in Asia is very damaging to the environment. The algae are grown in coastal areas on ropes or nets with loamy clay pots full of fertilizer hanging between them. The nutrients from these are released slowly, but most of them are dispersed into the sea. 'This massive overload of nutrients damages the coastal ecosystem', says Brandenburg.

Purifying effect
The potential for growing algae in the Netherlands is excellent. The sea contains enough nutrients and the Dutch species of algae grow well. The Wageningen research focuses on the cultivation of three indigenous species of seaweed: sea lettuce, oarweed and seabelt. Fundamental questions that arise include: at which depth do the Dutch seaweed species thrive best? Researchers also want to know how the seaweed responds to environmental factors such as temperature, light and nutrients. Different species of seaweed grow at a range of different depths. Algae growers can make use of this to grow algae in layers. 'Chlorophytes mainly absorb red light, and you can grow them high up the rope where there is enough red light', explains Brandenburg. 'Brown and red algae absorb the green and blue light from the spectrum and that is exactly the light that penetrates deeper into the water, so these species do better further down the rope.'
The researchers aim to grow the seaweed with a minimum of nutrients. At strategic places, for example at river mouths, no fertilizer is needed at all. The rivers transport large amounts of nutrients, especially phosphates, to the sea: about 30 million tons worldwide, and you can grow an awful lot of seaweed on that. Marine farming at such locations is even more attractive because the shortage of phosphates for conventional agriculture is growing more and more acute. Algae also thrive on carbon dioxide, increasing concentrations of which are causing acidification of the oceans. So with large-scale seaweed cultivation you can reduce the pressure on land and clean up the oceans. Due to this purifying effect, Brandenburg also sees scope for using algae as biofilters in fish farming. The sea vegetables efficiently absorb the nutrients excrete d by the fish.
Sturdy slabs
Our small boat soon drops us off at the work float. Spray from the waves and the howling wind makes this a wet workplace today. Researcher Julia Wald steps onto one of the rocking nursery floats. The first seaweed rope she pulls up turns out to be almost empty. 'Two weeks ago there were great slabs of sea lettuce on here, at least 300 centimetres long', she says. 'No idea why they have now disappeared; perhaps the current has torn them off.' The empty ropes are taken back to the lab for research. The sturdier looking seabelt has done considerably better. The researchers had attached a few tiny pieces to the rope four weeks ago, and now they have grown to substantial slabs of more than a metre in length.
A day later, Julia Wald is back in Wageningen working on her aquarium experiments, which complement the field trials in important ways. In a controlled setting, the researchers collect information about the effect of fertilizer, light, temperature and seaweed density on the growth rate. In ten aquaria and several large 1000-litre tanks, fronds of seaweed float gracefully about. Wald picks a handful of sea lettuce from a tank and examines the pale green, lettuce-like substance critically. 'These algae get too little phosphate', she concludes. 'Look, holes are developing in them and the colour is far too pale as well.' Measurements confirm her observations: the plants contain roughly half as much chlorophyll as healthy algae.

Economic aspects
A lot of water will flow through the Oosterschelde before Dutch seaweed farming can get going on a large scale. The team first has to get the finer details of algae cultivation under its belt. The next hurdle is how to efficiently harvest the algae and partially process them at sea. Then there is the challenge of upscaling the cultivation without harming marine life. Brandenburg and Wald think that large-scale cultivation should mainly be done in the open sea, where there is more space than along the coast. Algae cultivation that makes use of drilling platforms and wind turbine parks would be ideal. 'You can also deliberately locate algae farms on places along the coast where you want to improve the water quality, for example at river mouths', says Wald. The researchers are also looking into the economic aspects of algae cultivation. You can process the algae biomass as a whole, but with the help of some technology you can also harvest separate components. And this raises the value of a kilo of algae. But to extract the various components from cultivated algae, you need new techniques from the field of bio-refinery. But sustainability is at the top of the list of requirements, emphasizes Julia Wald. That will safeguard a future among the marine farms of residents such as Charlie the North Sea crab.

Indispensible algae
Algae already plays a bigger role in our daily lives than many of us realize. And not just in the well-known sushi: binding agents based on seaweed find their way into countless everyday products such as sauces, soups, ice cream and toothpaste. Besides these uses, the industry also harvests vitamins, minerals and antioxidants from these primitive plants. In future they could also supply protein for food and animal feeds.