The effect of nitrogen depletion on the cell size, shape, density and gravitational settling of Nannochloropsis salina, Chlorella sp. (marine) and Haematococcus pluvialis

Changes in cell size, density, and morphology of three microalgae were characterized across a batch growth curve in relation to nitrate availability. These results were then evaluated theoretically and experimentally in the context of gravity sedimentation. Nannochloropsis salina, Chlorella sp. (marine), and Haematococcus pluvialis (freshwater) were cultivated autotrophically under self-depletion of nitrate. Comparing results from the growth and senescent stages of batch cultivation, average cell diameters increased by ~30% for all species. However, there was variation within the population, with small-sized cells present even during senescence. Nitrogen deprivation decreased the average density of cells in all three species and broadened the density distributions within the populations. The net effect of nitrogen deprivation was to decrease the settling velocity, with the increase in cell size only partly counteracting the decrease in cell density. Cells generally became more spherical but this did not affect settling. Good agreement was found between experimental batch settling and theoretical settling velocities based on the measured parameters of low-density and small cells within the population. This indicates that approaches that use only median values of the cell density and size are unlikely to be predictive of sedimentation performance.