Growing Azolla to produce sustainable protein feed: The effect of differing species and CO₂ concentrations on biomass productivity and chemical composition

BACKGROUND: Since available arable land is limited and nitrogen fertilizers pollute the environment, cropping systems ought to be developed that do not rely on them. Here we investigate the rapidly growing, N₂-fixing Azolla/Nostoc symbiosis for its potential productivity and chemical composition to determine its potential as protein feed. RESULTS: In a small production system, cultures of Azolla pinnata and Azolla filiculoides were continuously harvested for over 100days, yielding an average productivity of 90.0-97.2kg dry weight (DW) ha^-1d^-1. Under ambient CO₂ levels, N₂ fixation by the fern's cyanobacterial symbionts accounted for all nitrogen in the biomass. Proteins made up 176-208gkg^-1 DW (4.9 × total nitrogen), depending on species and CO₂ treatment, and contained more essential amino acids than protein from soybean. Elevated atmospheric CO₂ concentrations (800ppm) significantly boosted biomass production by 36-47%, without decreasing protein content. Choice of species and CO₂ concentrations further affected the biomass content of lipids (79-100gkg^-1 DW) and (poly)phenols (21-69gkg^-1 DW). CONCLUSIONS: By continuous harvesting, high protein yields can be obtained from Azolla cultures, without the need for nitrogen fertilization. High levels of (poly)phenols likely contribute to limitations in the inclusion rate of Azolla in animal diets and need further investigation.