Haematococcus pluvialis microzooid and palmella cells display distinct metabolic responses to excess manganese

Current understanding of the metabolism and adaptation of the commercially important microalga Haematococcus pluvialis in response to stress is limited, in part due to its complex life cycle. Manganese (Mn) is an essential micronutrient but a toxic pollutant when present in excess. This study coupled high-resolution elemental imaging, spectroscopy, and microscopy, alongside metabolic techniques to characterise responses to a high, sub-lethal concentration of Mn in motile microzooids and non-motile palmella cells within the same culture. Some microzooids in response to Mn showed pronounced vacuolisation, loss of cell wall integrity, and significant Mn internalisation within vacuoles, while some cells showed no structural changes and had Mn localisation in the cell wall. Palmella showed less vacuolisation with Mn localised around starch and astaxanthin granules. Microzooids sequestered Mn using intracellular sulphate moieties, probably on sulphated polysaccharides, and carboxyl groups in the cell wall. In response to Mn abundance, microzooids upregulated protein synthesis, depleted lipid energy reserves, with increased membrane fluidity and lipid peroxidation. Palmella also synthesised polysaccharides but otherwise showed no drastic metabolic changes, reflecting higher tolerance to Mn stress. Under normal conditions, microzooids and palmella showed distinctive carbohydrate and protein composition. This knowledge enhances our understanding of the biochemical mechanisms that H. pluvialis uses to manage excess concentrations of this essential micronutrient.