Screening for a low-cost Haematococcus pluvialis medium reveals an unexpected impact of a low N:P ratio on vegetative growth

Haematococcus pluvialis is the current better source of natural astaxanthin, a high-value carotenoid.
Traditionally, the production process of astaxanthin by this algae is achieved by a two-stage system:
during the first stage, vegetative “green” cells are produced and then converted, in the second stage, into
cysts that accumulate astaxanthin. In this work, a medium screening strategy based on the mixing of a
3-component hydroponic fertilizer was applied to identify a new formulation optimized for the vegetative
stage. A maximal and high cell density of 2 x 106 cells mL−1 was obtained in a medium containing
a high level of phosphate relative to nitrate, resulting in a N:P ratio much lower than commonly used
media for H. pluvialis. In this medium, cells remained at the vegetative and motile stage during a
prolonged period of time. Both high cell density culture and motile stage persistence was proved to
be related to the N:P feature of this medium. We conclude that the macrozoid stage of H. pluvialis
is favored under high-P and low-N supply and that low-cost hydroponic fertilizers can be successfully
used for achieving high density cultures of vegetative cells of H. pluvialis.