A. Sciandra et al., GROWTH-COMPENSATING PHENOMENA IN CONTINUOUS CULTURES OF DUNALIELLA-TERTIOLECTA LIMITED SIMULTANEOUSLY BY LIGHT AND NITRATE, Limnology and oceanography, 42(6), 1997, pp. 1325-1339
Two nitrogen-limited continuous cultures of Dunaliella tertiolecta wer
e grown on light/dark cycles. One was submitted to limiting photon flu
x density (PFD) and the other to nonlimiting PFD. The growth rate was
identical in the two cultures despite the difference in the PFD condit
ions. Once equilibria were reached in both cultures, the PFD were reve
rsed to simulate a decrease and an increase in light conditions. A lar
ge suite of variables was measured to characterize the response of the
cells, mainly through the interactions of carbon and nitrogen assimil
ation pathways. A decrease in irradiance led to a rapid decrease in al
gal biovolume; the biovolume-based growth rate (mu) descended to level
s lower than those estimated for cells of the light-limited culture. A
n increase in irradiance rapidly led to an increase in mu, which attai
ned values greater than those observed in high-light cells of the othe
r culture before the shift. For the two cultures before and after the
shift, cell carbon and cell volume were strongly correlated, showing t
he same pattern of diel variations. The specific C fixation rates (mu
C) of the two cultures before and after the light shift declined signi
ficantly during the light periods. Before the Light shift, mu C was pa
radoxically higher in the low Light culture and could not be predicted
only from the light levels. This suggests that some compensatory phen
omena may occur during light and nitrogen limitations. Time variations
of cell Chl a due to photoacclimation in both cultures were correlate
d with their N status. Similarly, the regulation of carboxylase activi
ties (Rubisco) by the Light levels was sensitive to the degree of N li
mitation. We found that nitrogen limitation has an overriding effect c
ompared to Light for the regulation of cell volume, C fixation and res
piration rates, Chi a synthesis, electron transport system, and Rubisc
o activities. In cultures subjected to low irradiance, NO3- uptake rat
e decreased several hours into the dark phase, suggesting a time lag b
etween the end of photosynthate production and the exhaustion of compo
unds necessary for dissolved inorganic nitrogen assimilation. The impl
ications of these results concerning phytoplankton growth modeling in
a variable environment are important because there are no existing mod
els that correctly integrate the simultaneous effects of light and nit
rogen on primary production. It is shown that the effects of these lim
itations were not additive in the range of light and N limitations tes
ted in this experiment.