Effects of C/N ratio on the growth and protein accumulation of heterotrophic Chlorella in broken rice hydrolysate.

BACKGROUND: Microalgae protein is considered as a sustainable alternative to animal protein in the future. Using waste for microalgal culture can upgrade low-value raw materials into high-value products, helping to offset the cost of microalgal protein production. In this study we explored the feasibility of using microalgae heterotrophic fermentation to convert broken rice hydrolysate (BRH) into protein. RESULTS: The results showed that the increase of BRH supplemental ratio was beneficial to the increase of biomass production but not beneficial to the increase of intracellular protein content. To further improve protein production, the effect of C/N ratio on intracellular protein accumulation was studied. It was found that low C/N ratio was beneficial to the synthesis of glutamate in microalgae cells, which in turn promoted the anabolism of other amino acids and further the protein. When the C/N ratio was 12:1, the biomass productivity and protein content could reach a higher level, which were 0.90 g/L/day and 61.56%, respectively. The obtained Chlorella vulgaris biomass was rich in essential amino acids (41.80%), the essential amino acid index was as high as 89.07, and the lysine content could reach up to 4.05 g/100 g. CONCLUSIONS: This study provides a theoretical basis and guidance for using Chlorella vulgaris as an industrial fermentation platform to convert broken rice into products with high nutritional value.

Evaluation of cadmium (Cd) transfer from paddy soil to rice (Oryza sativa L.) using DGT in comparison with conventional chemical methods: derivation of models to predict Cd accumulation in rice grains.

The consumption of rice contaminated with soil cadmium (Cd) threatens human health. It is essential to ensure the production of rice that meets food quality standards. Therefore, a large-scale field survey was conducted in Zhejiang province, southeastern China, to investigate the relationship between Cd accumulation in rice grains and Cd bioavailability in soil, and thus to establish a model to predict Cd contents in rice grains based on soil properties. For this purpose, a total of 156 paired rice and soil samples were collected. Pearson's correlation analysis revealed that Cd measurements obtained by diffusive gradient in thin films (DGT) had a higher correlation (r = 0.818, p < 0.001) with the Cd in rice grains as compared to the Cd measured by the DTPA, CaCl2, EDTA, and HCl extraction methods, which indicated that the DGT technique was a reliable method for the assessment of Cd bioavailability in soils. In addition, among the four extraction methods, the DTPA-extractable Cd showed the highest correlation with the Cd contents in rice grains. Therefore, we developed two predictive models (modelDGT and modelDTPA) to predict Cd levels in rice grains via Cubist multivariate mixed linear regression, using "soil DGT-measured Cd, pH, and oxide contents of Ca, Si, and Fe" or "soil DTPA-extractable Cd, pH, OM, and oxide contents of Ca and Fe" as explanatory variables, respectively. The overall modelDGT and modelDTPA had R2 values of 0.95 and 0.93, respectively, and relative error values of 0.30 and 0.33, respectively. Simple correlation analysis showed direct and close relationships between the measured Cd in rice grains and the Cd concentrations predicted by the Cubist modelDGT and modelDTPA, with R2 values of 0.979 and 0.922, respectively. Therefore, Cd levels in rice grains could be predicted very well based on the two prediction models, and thus, the two models derived in this study are effective in identifying soils in which the Cd in rice grains will exceed food safety standards, thereby helping to ensure safe rice production.

Alteration of bioaccumulation mechanisms of Cu by microalgae in the presence of natural fulvic acids.

The impact of natural fulvic acids (FAs) on the toxicity and bioaccumulation of Cu by Chlorella pyrenoidosa was studied. FAs extracted from Taihu Lake were separated into three fractions using dialysis bags: F1 (<500 Da), F2 (500-1000 Da) and F3 (>1000 Da). The results showed that the F3 fraction with a larger molecular weight contained less acidic groups and unsaturated aliphatic structures than F1 and F2, and it showed stronger alleviation of the toxicity of Cu to algae. In the presence of F1∼F3, the bioaccumulation curve of Cu in algae intersected with the straight line in the binary system of Cu-algae at approximately 5.3 × 10-3－6.0 × 10-3 mM of Cu equilibrium concentration, showing an inhibition of bioaccumulation of Cu in lower concentrations but an enhancement in higher Cu concentrations. The ratio of {Cu}ads/{Cu}int was used to clarify the transformation mechanism on adsorption; the transition interval occurred at a ratio of 3.5-4.4. This ratio indicated a shift from a mechanism of slow trending to equilibrium to a mechanism with rapid increase, which may be due to the bridging action of Cu to form a ternary complex of FA-Cu-algae and the occurrence of multilayer adsorption. The promotion order of F1> F3> F2 was consistent with percentages of the carboxyl group in total acidic functional groups in the FAs. This research is helpful for improving the accuracy of present models for the prediction of heavy metal risks in aqueous environments.

Dissolved organic matter affects the bioaccumulation of copper and lead in Chlorella pyrenoidosa: A case of long-term exposure.

This study evaluated the impact of dissolved organic matter (DOM) of varying molecule weights (MWs) on long-term exposure to Cu and Pb in Chlorella pyrenoidosa. Citric acid, fulvic acid, and humic acid, in the order of increasing MWs, were selected to represent DOM. The results showed that DOM with larger MWs had stronger inhibitory effects on the bioavailability of Cu to algae. However, the biosorption isotherm of Pb in the presence of DOM was different: as Pb equilibrium concentration increased, the biosorption capacity increased sharply to a maximum, then decreased. The maximum values ranged between 0.186 and 0.398 mmol g-1, as the solution DOM concentration and MW changed, exhibiting a stoichiometric relationship between DOM, Pb and algae. The ternary complex of Pb-DOM-alga formed in a limited Pb concentration range, and increased the percentage of internalized Pb. This research helps to understand the role of DOM in metal uptake in phytoplankton.