Conversion of wet microalgae to biodiesel with microalgae carbon based magnetic solid acid catalyst

•A microalgae carbon based magnetic solid acid catalyst was successfully prepared.•Effects of three preparation conditions on the performance of MCMSA were studied.•MCMSA showed high specific surface area, high acidity and high magnetism.•MCMSA showed high efficiency in biodiesel production from wet microalgae.•MCMSA is easy to be separated and can maintain catalytic activity in five cycles.