Toxic and Non-Toxic Cyanobacterial Biomass as a Sustainable Soil Fertilizer: Lettuce Cultivation Experiment

Cyanobacteria represent a promising resource for sustainable agriculture, restoring soil fertility even after death and decay. However, several cyanobacteria can also release secondary metabolites, such as cyanotoxins, which may compromise the quality of agricultural products and pose a potential risk to human health. Depending on the concentration of exposure, few studies reported deleterious effects on plant species when irrigated with cylindrospermopsin (CYN) contaminated water, impairing plant growth and leading to food product contamination, while other studies show promoting effects on plant yield. To evaluate the potential of cyanobacteria biomass (cyanotoxin-containing or not) as a soil-fertilizing product, a study was carried out that consisted of the culture of lettuce plants under controlled conditions, in soil (1) with no extra nutrient addition (control) and supplemented with 0.6 g of freeze-dried Raphidiopsis raciborskii biomass of (2) a non-CYN producer strain, (3) a CYN producer strain, and (4) the same CYN producer strain pasteurized. Results showed no significant differences in photosystem II efficiency with the amendment addition. On the contrary, shoot fresh weight significantly increased in lettuce plants grown with the cyanobacterial biomass addition, especially in condition (3). In addition, there were significant differences in mineral concentrations in lettuce leaves after the cyanobacterial biomass addition, such as K, Na, Ca, P, Mg, Mn, Zn, Cu, Mo, and Co. CYN accumulation was detected under conditions (3) and (4), with concentrations observed in descending order from roots > soil > shoot, although without significant differences. Furthermore, the CYN concentration in edible tissues did not exceed the WHO-proposed tolerable daily intake of 0.03 µg/kg/day. These findings suggest that incorporating cyanobacterial biomass as a soil fertilizer for lettuce cultivation, even with trace amounts of CYN, may enhance plant yield without compromising food safety.