Growing seaweed can solve acidification

Large-scale cultivation of sea lettuce can help reduce acidification of the oceans. And help solve the global food supply problem to boot.

This idea, presented by Wageningen biologist Ronald Osinga, came as a surprise to delegates at the international coral symposium held in Wageningen last week. The symposium was an initiative by the International Society for Reef Studies (ISRS) and focused on the effects of climate change on coral reefs. Acidification of the oceans is one of the problems, and corals are highly sensitive to it. They become bleached and the calcium they contain dissolves.

Marine horticulture

But this does not have to happen, says marine biologist Osinga. On the closing day of the symposium he proposed a solution: sea lettuce (ulva lactuca). As it grows, this marine plant lowers the acidity of water. What is more, it is edible. Osinga and his colleagues have calculated that a ‘marine garden’ of 180,000 square kilometres could provide enough protein for the entire world population. A sea lettuce bed of such gigantic proportions would raise the pH (acidity level) of the Mediterranean Sea by one tenth. That may not seem much, but according to Osinga, it would be enough to compensate for the rise in acidity that started with the industrial revolution.

Fish farming

Linking the cultivation of sea lettuce with fish farming would create a closed food cycle, says Osinga. The waste products of the fish would nourish the sea lettuce. Osinga: ‘Offshore fish-farming is a massive polluter. It’s much better if you can recycle these nutrients. There is a lot of interest nowadays in this sort of integrated concept.’


Osinga and his University of Amsterdam colleague Jaap Kaandorp brought the symposium to Wageningen in order to draw attention to Dutch coral research. Wageningen UR plays a modest role in this research, but that may be about to change through the accession of the ‘BES’ islands (Bonaire, St. Eustatius and Saba) in the Caribbean to the Netherlands. But that is a separate issue, says Osinga, and not the reason for the symposium. ‘It’s a coincidence. But a useful one, with all the attention to the coral reefs around the BES islands.’ Three hundred scientists from all over the world took part in the symposium.

Roelof Kleis, Wageningen University, 22 December 2010. Web site.

1 Response to “Growing seaweed can solve acidification”

  1. 1 Conny Maier 24 December 2010 at 18:43

    This idea seems based on a too simple calculation with the principally logic idea, that photosynthetic organisms scavenge CO2 (and thus theoretically increase pH), but then it seems not further thought or calculated through to the end: How and where could one grow 180,000 km2 of Ulva, which is a very shallow and benthic organism and thus confined to a very narrow stretch of coast line. This amount would apparently be able to provide the world with enough protein – but who wants to have a diet of 100% Ulva? But, apparently such a large amount, which is unrealistic to culture, would be necessary to lower the pH of the Mediterranean by 0.1 pH units and this would be the Mediterranean only, so what about the world oceans? Of course, I have no clue how this was calculated and how viable this statement thus is. But it further seems, that it does not take into account any air-sea gas exchange. Ulva is a very shallow, benthic green alga, thus at shallow there is a high air-sea gas exchange, and the CO2 scavenged by Ulva will fastly be replenished by air CO2, specifically in the wave exposed tidal setting of Ulva. This already minimises the CO2 lowering effect by photosynthesis of Ulva. A 2nd problem is recognised: the inorganic nutrient limitation to allow for intensified seaweed growth – and this would be “solved” by fish farming providing the necessary fertilizer. Only, that fish respire CO2 and thus counterbalance any CO2 scavenging by Ulva! I guess, this is not considered when making the statement that “seaweed growth solves acidification”. The “calculation/idea” does not seem to be based on any realistic scales as the amount of seaweed that has to be grown is unrealistic, nor does it seem to consider mechanisms such as air-sea exchange, tidal waves, currents and other ecological consequenses . . .
    The idea of combining fish farming with seaweed to minimise some of the negative effects with respect to nutrient and CO2 release is ok and makes sense, specifically where fish farming is already an issue, but it should not be linked to OA, just because it is a “hot topic” and thus might make the issue of fish/seaweed farming more attractive to funding agencies and policy makers. With respect to the cause of OA, it is rather dangerous to propagate, that the farming of Ulva (in combination with fish farming) might mitigate effects of ocean acidification as this is something that will be grabbed voluntarily by politicians (and funding agencies) as it appears an economic and ecologic attractive solution, where in fact it does still cause more problems than it solves. It certainly does not solve acidification, but will give some wrong potential cure where other solutions should actually be sought.

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