Chlorella vulgaris is used for food and feed applications due to its nutraceutical, antioxidant and anticancer properties. An airlift photobioreactor comprising transparent draft tube was used for C. vulgaris cultivation. The effect of reactor parameters like hydrodynamics (0.3-1.5 vvm), light intensity (85-400 lmol m -2 s -1 ), photoperiod (12-24 h) and gas-phase carbon dioxide (CO 2 ) concentration (5-15% v/v) were evaluated on microalgae and associated bacterial growth, biochemical profile; with special emphasis on x-3, x-6 fatty acids, and vitamin B 12 . The optimal growth of C. vulgaris without CO 2 supplementation was observed at 1.2 vvm, which was associated with higher algal productivity, chlorophyll, vitamin B 12 content, and bacterial load along with 72% of nitrate removal. The higher light intensity (400 lmol m -2 s -1 ) and photoperiod (24:0) increased biomass productivity and x-3 fatty acid content (in lipid) up to 2-3 fold. The elevated levels of gas-phase CO 2 concentration (15% v/v) enhanced EPA content up to 7% and biomass productivity up to 171 mg L -1 day -1 . However, the increase in CO 2 concentration lowered vitamin B 12 content (up to 30%) and bacterial load (2-3 log). Also, all the cultivation conditions favoured desirable x-6/x-3 ratio(in the range of 1-2).

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Spirulina evaluated as a source of vitamin B12 through the modulation of vitamin B12 deficiency mediated physiological and biochemical changes in experimental animals. The B12 deficient male weanling Wistar rats were fed with Spirulina‐supplemented diet for 10 weeks. An increase in urinary methylmalonic acid (22.70 ± 4.08 µmol/moles of creatinine) and plasma homocysteine (16.55 ± 0.48 µmol/L) levels in the B12 deficient group was observed, while these were equal to control in the Spirulina fed group (8.71 ± 0.48 µmol/mol of creatinine and 6.88 ± 1.18 µmol/L, respectively). The vitamin B12 levels in serum (874.27 ± 89.69), plasma (615.53 ± 26.5 pg/ml), kidney (10.19 ± 1.066 ng/g), and liver tissues (6.37 ± 0.62 ng/g) in the Spirulina fed group were similar to control. Severe atrophic changes in the testes and altered tissue architecture in lung and spleen as seen in the B12 deficient group were normalized in the Spirulina fed group. The study validates that Spirulina can improve the vitamin B12 status. Practical applications The present study showed that the supplementation of Spirulina in the diet of vitamin B12 deficient rats leads to the normalization of vitamin B12 deficiency‐induced circulatory and functional biomarkers along with biochemical and histological changes. Vegetarian sources for vitamin B12 are limited and the results presented here provide scientific validation for the use of Spirulina as a potential vegetarian source of bioavailable vitamin B12.