66
H.C. Wainman et al.
Ik-lipid were of comparable magnitude, whereas bias was positive when Ik was
much larger than Ik-lipid.
3.3.5. Implications of the Irradiance Response
This section of the chapter has been chiefly concerned with the photosynthetic
parameters a, I k , their lipid counterparts a-lipid and Ik-lipid, and the relationship
of these parameters to the percentage of photosynthate allocated to lipids at either
light saturation or on an areal basis. The two main seasonal changes in the lipid
production versus light intensity curve were that (I) the light saturation of lipid
production (/rlipid) occurred at higher levels than overall production in summer
but less than overall production in winter (Fig. 3.4) and (2) the efficiency of lipid
production (a-lipid) decreased relative to the overall efficiency of carbon fixation
(a) in summer (Fig. 3.8).
The seasonal change in a/a-lipid (a-ratio) and I/Ik-lipid (light saturation ratio)
were, not surprisingly, related to the light environment. The light saturation ratio
was lowest when days were longest (Fig. 3.6) and waters were warmest (Fig. 3.5),
whereas the a-ratio was greatest during those conditions (Figs. 3.9 and 3.10). Low
values for a and high values for Ik are indicators of high-light adaptation (Platt et
aI., 1982), so lipid synthesis in midsummer was more high-light adapted than was
the overall biosynthesis. The winter situation has the opposite profile of photosynthetic parameters, indicating that the lipid synthesis is shade-adapted relative
to total biosynthesis. The consequence is that the share of total biosynthesis
directed to lipid is maximal in the upper photic zone during summer and lower in
the photic zone during winter.
Seasonal changes in lipid production compared with overall production at light
saturation are best illustrated by the lipid percent of Pm (Fig. 3.12). In summer,
when the lipid synthesizing apparatus is more light adapted than the overall
photosynthetic apparatus, the lipid percent of production is about 15-30% of Pm'
In winter, the lipid production is 5-20% of Pm' If lipid production is related to
lipid content and it is where it has been measured (Taguchi et aI., 1987), then the
lipid content of epilimnetic algae in summer may be relatively high when lipid
synthesis is light saturated (300-800 !-Lmol photons' m - 2 • S - I in the summer)
but decreases with depth. However, this inference depends also on the rates of
vertical motion associated with mixing, and situations in which the phytoplankton
are static in the vertical light gradient may be unusual. If mixing strength is high,
then no vertical differentiation of lipid content would be expected.
The lipid percent of areal production presents a somewhat different view of
algal lipid production. There is no clear seasonal trend in the lipid percent of areal
production (Fig. 3.11), and it is remarkably stable over the year, with a mean of
16.8 ± 6.4%, compared to protein or carbohydrate fraction of carbon fixation
(Cuhel and Lean, 1987a, Table 4). This seasonal invariability suggests that, usually, lipid is constitutive and not susceptible to "luxury storage." This would be
consistent with measurements that suggest that the lipid pool of phytoplankton is
dominated by lipids associated with structural functions unless the cells are placed
H.C. Wainman et al.
Ik-lipid were of comparable magnitude, whereas bias was positive when Ik was
much larger than Ik-lipid.
3.3.5. Implications of the Irradiance Response
This section of the chapter has been chiefly concerned with the photosynthetic
parameters a, I k , their lipid counterparts a-lipid and Ik-lipid, and the relationship
of these parameters to the percentage of photosynthate allocated to lipids at either
light saturation or on an areal basis. The two main seasonal changes in the lipid
production versus light intensity curve were that (I) the light saturation of lipid
production (/rlipid) occurred at higher levels than overall production in summer
but less than overall production in winter (Fig. 3.4) and (2) the efficiency of lipid
production (a-lipid) decreased relative to the overall efficiency of carbon fixation
(a) in summer (Fig. 3.8).
The seasonal change in a/a-lipid (a-ratio) and I/Ik-lipid (light saturation ratio)
were, not surprisingly, related to the light environment. The light saturation ratio
was lowest when days were longest (Fig. 3.6) and waters were warmest (Fig. 3.5),
whereas the a-ratio was greatest during those conditions (Figs. 3.9 and 3.10). Low
values for a and high values for Ik are indicators of high-light adaptation (Platt et
aI., 1982), so lipid synthesis in midsummer was more high-light adapted than was
the overall biosynthesis. The winter situation has the opposite profile of photosynthetic parameters, indicating that the lipid synthesis is shade-adapted relative
to total biosynthesis. The consequence is that the share of total biosynthesis
directed to lipid is maximal in the upper photic zone during summer and lower in
the photic zone during winter.
Seasonal changes in lipid production compared with overall production at light
saturation are best illustrated by the lipid percent of Pm (Fig. 3.12). In summer,
when the lipid synthesizing apparatus is more light adapted than the overall
photosynthetic apparatus, the lipid percent of production is about 15-30% of Pm'
In winter, the lipid production is 5-20% of Pm' If lipid production is related to
lipid content and it is where it has been measured (Taguchi et aI., 1987), then the
lipid content of epilimnetic algae in summer may be relatively high when lipid
synthesis is light saturated (300-800 !-Lmol photons' m - 2 • S - I in the summer)
but decreases with depth. However, this inference depends also on the rates of
vertical motion associated with mixing, and situations in which the phytoplankton
are static in the vertical light gradient may be unusual. If mixing strength is high,
then no vertical differentiation of lipid content would be expected.
The lipid percent of areal production presents a somewhat different view of
algal lipid production. There is no clear seasonal trend in the lipid percent of areal
production (Fig. 3.11), and it is remarkably stable over the year, with a mean of
16.8 ± 6.4%, compared to protein or carbohydrate fraction of carbon fixation
(Cuhel and Lean, 1987a, Table 4). This seasonal invariability suggests that, usually, lipid is constitutive and not susceptible to "luxury storage." This would be
consistent with measurements that suggest that the lipid pool of phytoplankton is
dominated by lipids associated with structural functions unless the cells are placed