Paper Reviewed
Brouwer, P., Schluepmann, H., Nierop, K.G.J., Elderson, J., Bijl, P.K., van der Meer, I., de Visser, W., Reichart, G.-J., Smeekens, S. and van der Werf, A. 2018. Growing Azolla to produce sustainable protein feed: the effect of differing species and CO2 concentrations on biomass productivity and chemical composition. Journal of the Science of Food and Agriculture 98: 4759-4768.

The rising population of the planet is expected to generate an increasing demand for meat and dairy products, which will require -- in the words of Brouwer et al. (2018) -- a "3.2- to 40-fold input of plant protein to produce, hence further increasing the pressure on global food supply." It should therefore come as no surprise that scientists are searching for ways to increase feed crop protein above current values or find suitable replacements that produce more protein per unit area than that of current feed crops.

Taking the latter of these two approaches, Brouwer et al. set out to study two cultivars of the aquatic fern Azolla (Azolla pinnata and Azolla filiculoides) to see how the protein produced by these two plants would compare with current feed crops when exposed to both ambient and elevated atmospheric CO2 concentration. This was accomplished by cultivating the two fern species in 30-liter containers in controlled-environments under either ambient (~400 ppm) or elevated (~800 ppm) CO2 air. Then, twice each week over the next 100 days, the authors harvested 33% of the surface area of the containers and performed a series of measurements and analyses to determine the Azolla's growth and protein characteristics.

Results indicated that a doubling of CO2 from 400 to 800 ppm increased plant productivity by 36% for A. filiculoides and 47% for A. pinnata. What is more, the authors note that, surprisingly, the increasing growth rates did not lead to a decrease in nitrogen or protein content, which finding likely occurred due to a CO2-induced increase in nitrogen fixation by cyanobacteria located within the ferns' leaf cavities.

In comparing their findings on Azolla with soybean, Brouwer et al. report that soybeans "can yield 2 tons of protein ha^-1 per year in subtropical regions, but only fixes 50% of the organic nitrogen exported from the field as seed." This contrasts with a 6.8-7.9 tons of protein ha^-1 harvest per year for Azolla under ambient CO2 and an expected 10.3 to 11.8 tons of protein ha^-1 harvest per year under elevated CO2. The major advantage for Azolla, however, is that its protein requires no nitrogen input and no arable land to produce such high yields. Consequently, and in light of their findings, Brouwer et al. conclude that "the high levels of protein and favorable amino acid content compared to soybean suit application of Azolla biomass as feed."