3. Irradiance and Lipid Production
55
that promote lipid synthesis during the day therefore appear to stimulate greater
lipid consumption at night. Night synthesis rates for protein and loss rates for
polysaccharide and LMW also tended to be significantly and positively correlated
with day rates, with r2 values of 0.83-0.97. Only protein and polysaccharide at the
highest irradiance levels gave poor correlations (r 2 of 0.21 and 0.54, respectively).
No single environmental factor (e.g., PDI or I k ) gave better prediction of overnight
synthesis or loss rates than did the day rate of synthesis for any of the photosynthate classes.
We used multiple regression to ask whether physicochemical variables, together with day rates, might give better explanations of the night rates. For
example, we found that daylength and day rate together could provide reasonable
predictions of overnight protein synthesis at the highest irradiance level (r 2 = 0.78;
P = .02). In general, however, any gains in predictive power were minimal. It was
particularly surprising that nutrients, temperature, and day length had little apparent influence on the night rates because all are considered important influences on
the physiology of microalgae generally and on photosynthate allocation specifically. Taxonomic succession may have confounded the situation (Fernandez et aI.,
1992; Feuillade et aI., 1992; Madariaga, 1992; Rivkin, 1989), but we lack species
counts with which to test this idea.
In summary, the die I reallocation patterns observed in Georgian Bay largely
confirmed generalizations that have been made on the basis of apparently more
limited data sets (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a). In particular, the results showed that lipids are subject to a net loss overnight but to a small
extent compared with polysaccharides. The loss is predictable from the day rates
so that production over the 24-hour cycle can, on average, be estimated as 87%
(average T2/Tl slope among irradiance levels, Table 3.2) of that over the light
phase only. This pattern holds true for Georgian Bay phytoplankton, ranging
widely in nutrient status and photo adaptive state and after exposure to irradiance
ranging from strongly limiting to saturating.
3.2.4. Reallocation Among Lipid Classes
All three major lipid classes (neutral, glyco-, and phospholipids) contain fatty
acids that are synthesized primarily in the plastid but then modified by reactions
outside the plastid. In the case of glycolipids of higher plants, about half of the
fatty acids never leave the plastid during the synthetic process. The other fatty
acids of glycoJipids, and all those contained in neutral and phospholipids, are
thought to be exported to the cytoplasm as C 16 or C 18 molecules and then
FIGURE 3.3. Relationship of PAR during the light period (fLmol photons· m - 2 • S -1) to
(A) overnight respiration (0) and metabolism (D) rates, (B) carbon exchange rates (positive denotes net increase or synthesis; negative denotes net loss or catabolism) for protein
(0) and polysaccharide (D). and (C) carbon exchange rates for total lipids. Respiration.
metabolism. and exchange rates in C . ChI a-I. h - I, showing means ± 2 SE bars.
55
that promote lipid synthesis during the day therefore appear to stimulate greater
lipid consumption at night. Night synthesis rates for protein and loss rates for
polysaccharide and LMW also tended to be significantly and positively correlated
with day rates, with r2 values of 0.83-0.97. Only protein and polysaccharide at the
highest irradiance levels gave poor correlations (r 2 of 0.21 and 0.54, respectively).
No single environmental factor (e.g., PDI or I k ) gave better prediction of overnight
synthesis or loss rates than did the day rate of synthesis for any of the photosynthate classes.
We used multiple regression to ask whether physicochemical variables, together with day rates, might give better explanations of the night rates. For
example, we found that daylength and day rate together could provide reasonable
predictions of overnight protein synthesis at the highest irradiance level (r 2 = 0.78;
P = .02). In general, however, any gains in predictive power were minimal. It was
particularly surprising that nutrients, temperature, and day length had little apparent influence on the night rates because all are considered important influences on
the physiology of microalgae generally and on photosynthate allocation specifically. Taxonomic succession may have confounded the situation (Fernandez et aI.,
1992; Feuillade et aI., 1992; Madariaga, 1992; Rivkin, 1989), but we lack species
counts with which to test this idea.
In summary, the die I reallocation patterns observed in Georgian Bay largely
confirmed generalizations that have been made on the basis of apparently more
limited data sets (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a). In particular, the results showed that lipids are subject to a net loss overnight but to a small
extent compared with polysaccharides. The loss is predictable from the day rates
so that production over the 24-hour cycle can, on average, be estimated as 87%
(average T2/Tl slope among irradiance levels, Table 3.2) of that over the light
phase only. This pattern holds true for Georgian Bay phytoplankton, ranging
widely in nutrient status and photo adaptive state and after exposure to irradiance
ranging from strongly limiting to saturating.
3.2.4. Reallocation Among Lipid Classes
All three major lipid classes (neutral, glyco-, and phospholipids) contain fatty
acids that are synthesized primarily in the plastid but then modified by reactions
outside the plastid. In the case of glycolipids of higher plants, about half of the
fatty acids never leave the plastid during the synthetic process. The other fatty
acids of glycoJipids, and all those contained in neutral and phospholipids, are
thought to be exported to the cytoplasm as C 16 or C 18 molecules and then
FIGURE 3.3. Relationship of PAR during the light period (fLmol photons· m - 2 • S -1) to
(A) overnight respiration (0) and metabolism (D) rates, (B) carbon exchange rates (positive denotes net increase or synthesis; negative denotes net loss or catabolism) for protein
(0) and polysaccharide (D). and (C) carbon exchange rates for total lipids. Respiration.
metabolism. and exchange rates in C . ChI a-I. h - I, showing means ± 2 SE bars.