The dilute sulfuric acid hydrolysis was employed to optimize saccharification of the green seaweed Valoniopsis pachynema by response surface methodology (RSM) and the retrieved hydrolysates were characterized. The RSM quantitative evaluation of total sugar recovery on acid hydrolysates indicated that 3% dilute sulfuric acid was sufficient to retrieve optimum level of saccharification with 2.15 h incubation time at 60 degrees C. Fourier-transform infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDAX) images of seaweed evidenced that seaweed surface polysaccharides were hydrolyzed by 3% dilute sulfuric acid, further this treatment showed improved polymer dissolution when compared to other concentrations. The acid treated seaweed pellets indicated effective colonization and encouraged growth in adhesion analysis of microalgae C. vulgaris MSU-AGM 14. By combining with algae culture medium and at different concentrations of acid hydrolysate showed higher biomass (0.111 +/- 0.001 g L-1 d(-1)) and lipid productivity (18.18 +/- 0.19 mg L-1 d(-1)) and improved elemental contents in mixotrophic condition. These results evidenced that mixotrophic cultivation enhances microalgae growth and lipid productivity by presence of extracellular polysaccharide and leads to initial colonization and prolonged period of growth. The overall results expressed that dilute 3% sulfuric acid hydrolysis retrieved high amount of total sugar content (38.5 +/- 0.2 mg g(-1)) from green seaweed V. pachynema and promotes microalgae biomass and lipid productivity significantly. (c) 2020 Elsevier Ltd. All rights reserved.