Toxic and non-toxic cyanobacterial biomass as a resource for sustainable agriculture: A lettuce cultivation experiment.

Cyanobacteria represent a promising resource for sustainable agriculture, as they have demonstrated the ability to restore 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 cyanobacterial biomass (cyanotoxin-containing or not) as a sustainable resource for soil amendment, biostimulants or fertilizers for lettuce cultivation, 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-producing strain, (3) a CYN-producing strain, and (4) the same CYN-producing 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. Nevertheless, 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 amendment, biostimulant or fertilizer for lettuce cultivation, even with trace amounts of CYN (1-40 µg/g), may enhance plant yield without leading to cyanotoxin accumulation in edible tissues above the WHO-recommended tolerable daily intake.

Impacts of cyanobacterial biomass and nitrate nitrogen on methanogens in eutrophic lakes.

Methanogenesis is a key process in carbon cycling in lacustrine ecosystems. Knowledge of the methanogenic pathway is important for creating mechanistic models as well as predicting methane emissions. Due to low concentrations of methyl substrates in freshwater lakes, the proportion of methylotrophic methanogenesis is believed to be negligible in such environments. However, the high abundance of methylotrophic methanogens previously detected in Dianchi Lake suggests that methylotrophic methanogenesis may be underestimated in eutrophic lakes, whereas their influencing factors and mechanisms are not yet clear. In this study, the effects of cyanobacteria biomass (CB) or/and nitrate nitrogen on methanogenesis, especially methylotrophic pathway, in eutrophic lakes were investigated using microcosm simulation experiments combined with chemical analysis and high-throughput sequencing techniques. The results showed that either CB or nitrate nitrogen had significant effects on methane flux, the archaeal diversity and community structure of methanogens. Functional prediction, together with the result of chemical analysis, revealed that CB could promote methylotrophic methanogenesis by providing methyl organic substrates, while nitrate nitrogen increased the relative abundance of obligate methylotrophic methanogens by competitively inhibiting the other two methanogenic pathways. In eutrophic lake where both CB and nitrate present at a high concentration, methylotrophic methanogenesis could play a much more important role than previously believed.

Formation and inhibition of polycyclic aromatic hydrocarbons from the gasification of cyanobacterial biomass in supercritical water.

Polycyclic aromatic hydrocarbons (PAHs) formation and inhibition from supercritical water gasification (SCWG) of cyanobacterial biomass were investigated. High reaction temperature, long residence time, and low feedstock concentration favoured higher molecular weight (HMW) PAH formation. The total PAH yield reached 34.80 µg g-1 at 500 °C, 22.5 MPa, and 10 min. The main PAHs formed in the liquid phase and the solid residue were 3-ring and 4-ring PAHs, which were generated from the cycloaddition reaction of lower molecular weight (LMW) PAHs. In addition, 2-ring PAHs were produced from the Diels-Alder reaction of phenols and unsaturated hydrocarbons. The possible control methods for PAH formation during the SCWG of cyanobacterial biomass were proposed. H2O2 addition effectively inhibited the reaction pathways underlying PAH formation, and the addition at more than 1.0% concentration suppressed H2 production. The work revealed that the inhibition of PAHs was achieved in terms of improving the oxidisation condition during the SCWG process for converting wet biomass or organic wastes to energy sources.

Volatile fatty acid recovery by anaerobic fermentation from blue-green algae: Effect of pretreatment.

The aims of this study were to quantify how pretreatment affects production of volatile fatty acids (VFAs) from cyanobacterial biomass and production of subsequent microbial lipid by an oleaginous microorganism that uses the VFAs as carbon sources. The highest biomass solubilization was obtained using thermal-alkaline (th-alkaline) pretreatment (33.1%), followed by alkaline pretreatment (29.1%), and thermal pretreatment (7.2%), but the highest VFA yield was obtained using alkaline pretreatment (0.54±0.02g/gVS), followed by the untreated condition (0.47±0.03g/gVS), and th-alkaline pretreatment (0.44±0.02g/gVS). Although VFA yield was higher using alkaline pretreatment condition than in the untreated condition, the difference was not great. However, lipid productivity by Cryptococcus curvatus after the alkaline pretreatment condition was 2.0-fold higher than that under the untreated condition. This study confirmed the feasibility of using biologically produced VFAs from cyanobacterial biomass for microbial lipid production by the oleaginous microorganism.

Modification of cyanobacterial bloom-derived biomass using potassium permanganate enhanced the removal of microcystins and adsorption capacity toward cadmium (II).

Cyanobacterial biomass shows high adsorption capacity toward heavy metal ions. However, the cyanotoxins in the cyanobacterial biomass inhibit its application in heavy metals removal. In order to safely and effectively remove Cd(II) from water using cyanobacterial bloom-derived biomass (CBDB), KMnO4 was used to modify CBDB. The results indicated that the microcystins in the CBDB were successfully removed by KMnO4. Potassium permanganate oxidation caused the transformation of hydroxyl to carboxyl on the CBDB, and formed manganese dioxide on the surface of CBDB. The oxidized CBDB showed higher adsorption capacity toward Cd(II) than that of unoxidized treatment. The optimal KMnO4 concentration for increasing the adsorption capacity of CBDB toward Cd(II) was 0.2g/L. The adsorption isotherm of Cd(II) by oxidized- or unoxidized-CBDB was well fitted by Langmuir model, indicating that the adsorption of Cd(II) by CBDB was monolayer adsorption. The desorption ratio of Cd(II) from oxidized CBDB was higher than that from unoxidized CBDB in the desorption process using NH4NO3 and EDTA as desorbent. The results presented in this study suggest that KMnO4 modified CBDB may be used as a safe and high efficient adsorbent in Cd(II) removal from water.