Chemical inducers regulate ROS signalling to stimulate astaxanthin production in Haematococcus pluvialis under environmental stresses: A review

•Chemical inducers combined with abiotic stress are a useful strategy for astaxanthin enrichment in H. pluvialis.•Stress-induced ROS are indispensable for the successful use of chemical inducers in astaxanthin production.•Chemical inducers can regulate ROS signalling or provide precursors to stimulate astaxanthin accumulation.•Suitable chemical inducers matching abiotic stress is feasible for commercial astaxanthin by H. pluvialis.