Asian Journal of Biology

The present study evaluated the effects of auxins (indole-3-acetic acid [IAA] and indole-3-butyric acid [IBA]) and cytokinins (kinetin and 6-benzylaminopurine [BAP]) on the growth, photosynthetic pigment synthesis, and biochemical composition of the marine microalga Chaetoceros calcitrans. A laboratory experiment using a completely randomised design was conducted to evaluate the effects of four plant growth regulators at different concentrations (0.1, 0.5, 1.0, and 1.5 mg/L), with untreated cultures serving as controls. The study was conducted at the Department of Botany, Maharaja's College, Ernakulam, Kerala, India, over a period of six months. Batch cultures of C. calcitrans were supplemented with individual concentrations of IAA, IBA, kinetin, and BAP under controlled temperature and illumination. Growth dynamics were monitored using cell-density counts. Photosynthetic pigments (chlorophyll a and carotenoids) were quantified spectrophotometrically. Total carbohydrates, proteins, and lipids were extracted and estimated using standard colorimetric and gravimetric assays. Lower auxin concentrations (0.1 mg/L IAA and 0.1 mg/L IBA) produced the maximum cell density and significantly enhanced chlorophyll a and carotenoid accumulation. Responses to cytokinins varied: BAP maximised cell yield at 0.5 mg/L, whereas kinetin stimulated growth at both 0.1 and 1.5 mg/L. Biochemical accumulation depended strongly on hormone type and dose. Auxin treatments favoured higher accumulation at moderate-to-high concentrations, with 1.0–1.5 mg/L IAA and IBA significantly promoting protein, carbohydrate, and lipid yields. Among the cytokinins, BAP at 0.5 mg/L acted as a broad-spectrum enhancer, inducing peak accumulation of proteins, carbohydrates, and lipids simultaneously, whereas kinetin exhibited dose-dependent target specificity (0.5 mg/L for protein, 1.0 mg/L for carbohydrate, and 1.5 mg/L for lipid). Exogenous plant growth regulators effectively modulated biomass yield, pigment profile, and macromolecular accumulation in C. calcitrans. Low auxin doses (0.1 mg/L) were optimal for biomass and pigment production, whereas 0.5 mg/L BAP served as the most effective single-treatment strategy for boosting overall biochemical productivity for industrial or aquaculture applications.