The two common methods of measuring biomass in algae cultures
employed in this study, optical density measurements at 750 nm
(OD750) and AFDW, were markedly different. Compared with AFDW
measurements, OD750 significantly over predicted biomass loss rates
in all three organisms tested (Fig. 6). This was most pronounced in
N. salina, with rates determined by OD750 up to 57.4% greater than
rates determined by AFDW. Changes in cell size and number over the
dark period, due to cell division, likely explain the discrepancy in
OD750 and AFDWmeasurements and therefore calculated rates of biomass
loss. Cells grown under a light:dark photoperiod often divide in
a circadian rhythm. Cell counts were performed for N. salina culture before
and after the dark period to investigate this possibility. Cell number
increased after dark incubation, which corresponded with smaller cell
sizes, especially at higher temperatures. At the wavelength of 750 nm,
optical density is mostly a measure of light scattering and therefore, in
addition to measuring changes in cell number, this optical method is
highly susceptible to changes in cell size [46]. We caution the use of
optical methods to measure night biomass loss and, when possible,
gravimetric methods are recommended.