3. Irradiance and Lipid Production
67
under rather severe stress such as strong nutrient limitation (Parrish and Wangersky, 1987). There are times when lipid synthesis is much higher (30-40% of
total production), and this may well be luxury storage, perhaps in response to
nutrient limitation.
The reason for the relative stability in the lipid percent of areal synthesis across
seasons is that the increase in a-lipid is concurrent with a decrease in Ik-lipid so
overall synthesis of lipid is maintained. The variability that was observed in lipid
production was related to changes in the photosynthetic parameters, particularly
a. When a-lipid and a are similar, then the lipid percent of areal production is
high. Because this parity between a's occurs most often in the late fall and winter
and they are farthest apart in summer (Fig. 3.8), one would expect that areal lipid
production would be highest in winter and lowest in summer; but that is not so
(Fig. 3.11) because these are times when Pm and Ik are low. The real highs in areal
lipid production appeared when the a-ratio and the light saturation ratio are high,
which is a rare situation because they are usually inversely related. The seasonality in the lipid percent of Pm is not, unfortunately, a good predictor of the areal
production of lipid relative to total production (Fig. 3.13). Predicted areal production from Pm has a strong bias related to season. Perhaps it should not be surprising
that Pm' which reflects only part of the photoadaptive strategy of the algae, is not
indicative of areal production. Nonetheless, most work to date on the environmental control of lipid synthesis by phytoplankton has emphasized observations at Pm
and may need to be re-evaluated in the light of the present findings.
3.4. Conclusions
A plausible interpretation of the present observations is that phytoplankton adapt
the photosynthetic parameters governing lipid production to maintain the columnintegrated proportion of biosynthesis directed to lipids within a fairly narrow
range and, thus, a balanced cell composition. This would be consistent with the
idea that the total lipid pool is, primarily, a constitutive component and that
mixing is, on average, strong enough that the plants respond mainly to the average
effective light climate. Focusing on behavior in selected portions of the vertical
light gradient, such as the light-saturated zone, gives only a partial and biased
view of phytoplankton activity. Future research could profitably focus on the
average light climate as the single variable most likely to give good predictions of
column-integrated lipid production.
3.5. Research Directions
A number of studies could be undertaken to further understand the control of algal
lipid production.
1. The lake light climate as well as the hypsographical data could be measured so
that algal lipid budgets for whole lakes could be calculated.
67
under rather severe stress such as strong nutrient limitation (Parrish and Wangersky, 1987). There are times when lipid synthesis is much higher (30-40% of
total production), and this may well be luxury storage, perhaps in response to
nutrient limitation.
The reason for the relative stability in the lipid percent of areal synthesis across
seasons is that the increase in a-lipid is concurrent with a decrease in Ik-lipid so
overall synthesis of lipid is maintained. The variability that was observed in lipid
production was related to changes in the photosynthetic parameters, particularly
a. When a-lipid and a are similar, then the lipid percent of areal production is
high. Because this parity between a's occurs most often in the late fall and winter
and they are farthest apart in summer (Fig. 3.8), one would expect that areal lipid
production would be highest in winter and lowest in summer; but that is not so
(Fig. 3.11) because these are times when Pm and Ik are low. The real highs in areal
lipid production appeared when the a-ratio and the light saturation ratio are high,
which is a rare situation because they are usually inversely related. The seasonality in the lipid percent of Pm is not, unfortunately, a good predictor of the areal
production of lipid relative to total production (Fig. 3.13). Predicted areal production from Pm has a strong bias related to season. Perhaps it should not be surprising
that Pm' which reflects only part of the photoadaptive strategy of the algae, is not
indicative of areal production. Nonetheless, most work to date on the environmental control of lipid synthesis by phytoplankton has emphasized observations at Pm
and may need to be re-evaluated in the light of the present findings.
3.4. Conclusions
A plausible interpretation of the present observations is that phytoplankton adapt
the photosynthetic parameters governing lipid production to maintain the columnintegrated proportion of biosynthesis directed to lipids within a fairly narrow
range and, thus, a balanced cell composition. This would be consistent with the
idea that the total lipid pool is, primarily, a constitutive component and that
mixing is, on average, strong enough that the plants respond mainly to the average
effective light climate. Focusing on behavior in selected portions of the vertical
light gradient, such as the light-saturated zone, gives only a partial and biased
view of phytoplankton activity. Future research could profitably focus on the
average light climate as the single variable most likely to give good predictions of
column-integrated lipid production.
3.5. Research Directions
A number of studies could be undertaken to further understand the control of algal
lipid production.
1. The lake light climate as well as the hypsographical data could be measured so
that algal lipid budgets for whole lakes could be calculated.