Suitability of an algal biofuel species, Scenedesmus acutus, as a fertilizer for growth of conventional and genetically modified lettuce

Nitrogen (N) and phosphorus (P) are important elements for global food production, but these nutrients cause pollution in water bodies without proper management. Furthermore, P is a finite resource with geologic reserves that are geographically restricted. Thus, sustainable use of P in agriculture has been the subject of much research over the past decade. This study jointly examines two examples of potential sustainability measures to address nutrient challenges: improved crop varieties and use of recycled fertilizers. We tested the effectiveness of dried biomass of a freshwater alga (Scenedesmus acutus) as a fertilizer with conventional lettuce [Lactuca sativa cv. Conquistador; WT (wild-type)] and genetically modified lettuce of the same variety that was transformed for improved nutrient-use efficiency [(type I Arabidopsis Vacuolar Pyrophosphatase (AVP1)]. In greenhouse conditions, we measured yield, soil nutrient content and losses, and soil P fractions after application of dried S. acutus biomass at different rates, with and without supplemental additions of conventional fertilizer. Yield was higher with commercial fertilizer compared with algal fertilizer, and AVP1 lettuce consistently produced better yield than the conventional lettuce with both fertilizer types, although the effect was stronger with the algal treatments. Soil P was mostly sequestered in the pools of NaHCO3-extractable organic P, NaOH-extractable organic P, and HCl-extractable P, which are poorly available. Although the algal fertilizer was ineffective in supporting short-term growth, the P was retained in the soils, which may improve soil fertility in the long term.