Effect of pH and Neem Extract on Metabolite Content and Bacterial Contaminant of Arthrospira platensis Cultures

Arthrospira platensis is a type of microalgae known as a superfood because it has many benefits in the health and renewable
energy sectors. Cultivation of A. platensis as a renewable energy source is carried out in an open pond system. Bacteria, fungi,
grazers and other microalgae frequently contaminated the mass culture of A. platensis. Bacteria were the most dominant
contaminant in microalgae culture, reaching up to 60%. Botanical pesticides are known to reduce contaminants without
damaging microalgae cells. This study presents the combined effect of pH and neem (Azadirachta indica) extract on the growth,
biomass, pigment, secondary metabolite and bacterial contaminants diversity of A. platensis. The results showed that the optimal
conditions for growth and metabolite production in A. platensis were in the pH range of 9–10 with the addition of neem extract.
pH 9 + neem extract produced the highest SGR (specific growth rate), density and cell biomass, namely 2.02 × 106 cells/mL,
0.8 g L-1 d-1 and 6.22 g L-1, respectively. The pH range of 9–10 is optimal for the production of chlorophyll and metabolites
(carbohydrates, proteins and lipids). On the other hand, pH 13 + neem extract produced the highest phycocyanin (0.007 mg L-1)
and reduced the number of bacteria by up to 98%. The next-generation sequencing (NGS) analysis involved three samples
specifically control treatment on day 0 (A), control on day 7 (B) and pH 13 + neem extract (C) because it had the lowest CFU.
The NGS results showed that the bacteria were dominated by the phylum proteobacteria, firmicutes and bacteroidota. Each
sample had a different abundance of OTUs (operational taxonomic units). The bacterial community in sample B (227 OTUs)
was similar to sample C (202 OTUs) but not similar to sample A (518 OTUs). However, the bacteria in A. platensis culture were
not completely harmful because they did not significantly suppress the growth and production of metabolites in A. platensis.
More research is required to investigate the bacteria's relationship pattern with A. platensis