Introduction
The intensification of aquaculture production has raised increasing concern regarding the environmental impacts of nutrient-rich effluents, particularly those containing high concentrations of nitrogen and phosphorus compounds. A sustainable alternative for mitigating these impacts involves the use of microalgae, such as Desmodesmus armatus, which are capable of growing in wastewater while simultaneously assimilating excess nutrients. This study evaluated the growth performance of D. armatus in synthetic aquaculture effluents formulated with varying concentrations of carbon (C), nitrogen (N), and phosphorus (P), aiming to assess its efficacy as a bioremediation agent. Additionally, the approach aligns with circular economy principles by integrating nutrient recovery with the generation of high-value algal biomass, with potential application as aquafeed.
Material and methods
The experimental design followed a two-factor factorial arrangement, investigating the effects of carbon (C) and macronutrient (nitrogen and phosphorus) concentrations on the growth of Desmodesmus armatus and its nutrient bioremediation performance. Four levels of C-glucose (4.2, 2.8, 1.4, and 0 mM) were combined with four dilution levels of nitrogen (N) and phosphorus (P) (100%, 67%, 33%, and 10%), with the highest concentrations corresponding to 8.6 mM N and 0.64 mM P. The synthetic effluent medium was based on the formulation proposed by Visvanathan et al. (2008), with modifications to C, N, and P concentrations and the inclusion of ammonia and nitrite as nitrogen sources. Bold’s Basal Medium (BBM) was used as the control, resulting in a total of 17 treatments, each with two replicates. Cultures were conducted in 250 mL Erlenmeyer flasks containing 100 mL of working volume, maintained under orbital shaking (100 rpm), at 24 °C, with a 12:12 h light: dark photoperiod and an irradiance of 300 µE m⁻² s⁻¹. The cultivation protocol consisted of an initial inoculation phase followed by three consecutive subcultures, each lasting 12 days. Daily monitoring included measurements of optical density at 720 nm and cell concentration determined by flow cytometry. On days 0, 6, and 12, additional analyses were performed to assess photosynthetic efficiency (Fv/Fm), pH, total ammoniacal nitrogen, nitrite-nitrogen (NO₂⁻), nitrate-nitrogen (NO₃⁻), phosphate-phosphorus (PO₄³⁻), and total organic carbon (TOC) (2017). Statistical analysis included two-way ANOVA followed by Tukey’s post hoc test. Principal Component Analysis (PCA) was employed to identify treatment effects and multivariate trends.
Results and Discussion
Desmodesmus armatus exhibited growth under all tested conditions, with significant variation in response to the availability of organic carbon. Higher C-glucose concentrations (4.2 and 2.8 mM) consistently promoted algal growth that was greater than the control, resulting in increased biomass productivity regardless of nitrogen and phosphorus dilution levels. The principal component analysis (fig. 1) revealed a clear separation among treatments with different C-glucose concentrations (0, 1.4, 2.8 and 4.2 mM), indicating a strong influence of carbon availability on system performance. Dimension 1 (Dim1), which accounts for 44.7% of the total variance, distinctly separates the carbon-free treatments from those with higher concentrations (particularly 4.2 mM), while Dimension 2 (Dim2), explaining 36.5%, further discriminates intermediate groups. Variables associated with bioremediation, such as the consumption of ammonium (ConsNH4), nitrite (ConsNO2), and nitrate (ConsNO3), along with the initial nitrogen concentration (N0), were more closely related to treatments with low or no carbon supplementation. In contrast, variables linked to cultivation performance, including final productivity (Pr_final), final relative productivity (Pr_rel_final), and carbon consumption (ConsC), showed a positive association with treatments receiving higher glucose concentrations. These findings suggest that increased carbon availability enhances the growth of Desmodesmus armatus and is associated with changes in the consumption patterns of nutrients derived from aquaculture effluents.
Conclusion
The results demonstrate that higher C-glucose concentrations significantly enhance the growth and biomass productivity of Desmodesmus armatus. Carbon supplementation also influenced nutrient consumption patterns, highlighting the key role of this microalga not only in optimizing high-value biomass production for potential use in aquafeed, but also in bioremediation through the efficient removal of nutrients from aquaculture effluents.
Reference
APHA. (2017). Standard Methods for examination of water and wastewater. In American Public Health Association (APHA).
Visvanathan C., Hung, N.Q., Jegatheesan, V. (2008). Hydrogenotrophic denitrification of synthetic aquaculture wastewater using membrane bioreactor. Process Biochemistry, 43(6), 673-682.