  Some microalgae store large amounts of neutral lipids inside lipid droplets.
Since these lipids can be used to produce biodiesel in a sustainable way,
research is developing on fast non-destructive methods to quantify and monitor
the amount of lipids in microalgal cultures. In this paper, we have developed
with digital holographic microscopy a fast quantitative method to assess the
evolution of the lipid content inside the diatom Phaeodactylum tricornutum
living cells. The method uses a specific processing of recorded hologram
sequences based on the refocusing capability of digital holographic microscopy.
Each lipid droplet volume is evaluated inside the cells on representative
samples of the culture. We have validated the method thanks to correlative
quantitative phase contrast-fluorescence imaging and extrapolated it to larger
calibrated spherical refractive particles, to demonstrate the flexibility of
the method.
