Science - January 11, 2011

Fish farming goes ‘cradle to cradle’

The algae that are fed to shellfish can be bred more sustainably if they are fed on the waste water from fishponds. PhD researcher Michiel Michels is going to optimize this approach to breeding algae and test its economic feasibility.

Feeding algae with waste water makes fish breeding more sustainable
Even in December it is warm in the greenhouse housing the marine laboratory of Zeeland University of Applied Sciences in Vlissingen. Pumps are humming in amongst a tangle of cables and air bubbles rise noisily up a couple of transparent cylinders. But what really steals the show is the brand new algae breeding reactor, with its 20 meter long transparent plexiglass tubes through which a brown substance slowly flows: seawater in which diatoms (a kind of alga) grow. This ingenious device, designed by environmental scientist Michiel Michels, was officially launched last month. Michels, a doctoral researcher in the Bioprocess technology chair group, will be using it over the next few years to perfect his breeding system for algae.
The device has sensors which continually measure the levels of carbon dioxide and oxygen in the algae slurry. The system automatically adjusts the concentrations of these gases to keep them at optimal levels. A murkiness monitor indicates whether the numbers of algae are going up or down. Michiels points to a graph on his computer screen, with a rising line. 'Here you can see that the murkiness level has gone up considerable since this morning', he says. 'That means that the numbers of algae have gone up.' As soon as there is more than one gram of algae per litre of seawater, the system automatically starts harvesting by syphoning off the algae slurry. At the same time, new seawater carrying extra nutrients enters the system. 'It looks amazing; we are working continuously', says the researcher proudly. 'We harvested the first algae yesterday.'

Cradle to cradle
Michels' algae project comes under a foundation for plaice fisheries development called Zeeuwse Tong. This is an umbrella organization under which companies and research institutes, including Wageningen UR, have joined forces to explore the scope for creating a closed cycle in fish farms located behind the dykes. This would make them highly sustainable. 'The waste water from the plaice farms can be a source of nutrients for cultivating diatoms, and these are eaten in turn by farmed shellfish, so you close the cycle', explains Michels. Personally, he thinks it's great to be able to make use of all that waste water. 'Like this, we make the cradle-to-cradle concept very concrete', he says.
When algae became popular about three years ago with both climate and food scientists, Michels started up a few algae breeding projects together with industry. 'I liked it so much that I got in touch with Rene Wijffels, the algae expert at Bioprocess Technology at Wageningen', he explains. 'And that is how the ball got rolling.' The diatom Chaetoceros muelleri was used for the trials because of its high nutritional value, thanks to its omega 3 fatty acids.

Flying start
But it is not easy to breed this species of alga, as it is very delicate and susceptible to damage from what is known as shear stress. 'That is the hydrodynamic force which the algae are subjected to if they are put through a pump, for example', explains the researcher. Michels exposed the algae to various levels of shear stress and identified the threshold above which the plants got damaged. It was particularly the energy-saving, fast pumps that caused damage. 'The solution was an outsized pump that we set to work slowly. This did not damage the algae', he explains.
Michels is now focusing on maximizing the algae growth at different times of year. He thinks that the main factors in their growth are the optimal algae density and the effect of oxygen accumulation. He has already published his first article and has every confidence that the work will be completed in time. 'If you see how much data I'll be gathering in a short time, then I don't think it can be a problem at all.'