3. Irradiance and Lipid Production
53
(1985a,b). For diatom-dominated communities in Belgian coastal waters, there
was, on average, a relatively large reallocation from polysaccharide and LMW to
protein, and a small loss from lipid, during the night (Lancelot and Mathot,
1985a). We used the eight sampling dates from Georgian Bay to calculate the
average overnight metabolism and reallocation rates for our freshwater community over a substantial part of the main productive season. Unlike Lancelot and
Mathot (\ 985a,b), we were able to examine the patterns not only for phytoplankton previously exposed to saturating irradiance but also those exposed to
limiting irradiance. We assumed that the loss of labeled carbon from the particulate phase represented primarily respiratory and excretory losses from the phytoplankton, which we termed carbon loss rate. Consistent with Lancelot and
Mathot (1985a,b), we also defined a quantity termed metabolism rate, which is the
sum of the carbon loss term and nighttime protein synthesis.
Both the loss and the metabolism rates increased with increasing previous
incubation irradiance (Fig. 3.3A). The difference between the two rates was
relatively small, reflecting a relatively small conversion of metabolized carbon
into new protein. Even at saturating irradiance, only a small fraction of the
mobilized carbon was reincorporated into protein, suggesting that energy supply
rate alone does not account for the low levels of nighttime protein synthesis.
Nutrient limitation also seems an unlikely explanation, considering that SRSi and
the PDI suggested little or no shortage of Si or P on most sampling dates (Table
3.1 ).
The rate of overnight protein synthesis increased with previous incubation
irradiance (Fig. 3.3B), up to the second-highest irradiance, at which photosynthesis was usually close to light saturation (Furgal and Smith, 1997; and cf. Ik
values in Table 3.1). A similar dependence of overnight protein synthesis on
previous irradiance has been reported for Lake Ontario phytoplankton (Cuhel and
Lean, 1987a,b), although the overnight protein synthesis rates reported for Lake
Ontario were far larger than the current results for Georgian Bay. The major
source of carbon for protein synthesis was polysaccharide, and polysaccharide
loss rates increased with previous incubation irradiance (Fig. 3.3B). On average,
LMW (not shown) and lipids (Fig. 3.3C) were also lost overnight, and the loss rate
increased with irradiance. Polysaccharide contributed 45-75% of the total overnight metabolism, the proportion varying in a nonsystematic way among irradiance levels. LMW contributed 18-23% whereas lipid contributed a rather variable share (7-35%, averaging 19%), with considerable but nonsystematic differences among irradiance levels.
As the confidence intervals in Fig. 3.3 show, there was considerable variation
around the mean seasonal value for overnight rates of gain or loss from each
photosynthate class. Regression analysis showed that for lipid, the variation was
best explained by the rate of lipid synthesis over the preceding light phase. The
percentage explained variation ranged from 72% to 97% among irradiance levels
and the slopes from - 0.345 to - 0.172, with no significant differences among
irradiance levels. Thus, the rate of lipid loss (a negative rate of carbon exchange)
in the dark increased as the rate of lipid synthesis in the light increased. Factors
53
(1985a,b). For diatom-dominated communities in Belgian coastal waters, there
was, on average, a relatively large reallocation from polysaccharide and LMW to
protein, and a small loss from lipid, during the night (Lancelot and Mathot,
1985a). We used the eight sampling dates from Georgian Bay to calculate the
average overnight metabolism and reallocation rates for our freshwater community over a substantial part of the main productive season. Unlike Lancelot and
Mathot (\ 985a,b), we were able to examine the patterns not only for phytoplankton previously exposed to saturating irradiance but also those exposed to
limiting irradiance. We assumed that the loss of labeled carbon from the particulate phase represented primarily respiratory and excretory losses from the phytoplankton, which we termed carbon loss rate. Consistent with Lancelot and
Mathot (1985a,b), we also defined a quantity termed metabolism rate, which is the
sum of the carbon loss term and nighttime protein synthesis.
Both the loss and the metabolism rates increased with increasing previous
incubation irradiance (Fig. 3.3A). The difference between the two rates was
relatively small, reflecting a relatively small conversion of metabolized carbon
into new protein. Even at saturating irradiance, only a small fraction of the
mobilized carbon was reincorporated into protein, suggesting that energy supply
rate alone does not account for the low levels of nighttime protein synthesis.
Nutrient limitation also seems an unlikely explanation, considering that SRSi and
the PDI suggested little or no shortage of Si or P on most sampling dates (Table
3.1 ).
The rate of overnight protein synthesis increased with previous incubation
irradiance (Fig. 3.3B), up to the second-highest irradiance, at which photosynthesis was usually close to light saturation (Furgal and Smith, 1997; and cf. Ik
values in Table 3.1). A similar dependence of overnight protein synthesis on
previous irradiance has been reported for Lake Ontario phytoplankton (Cuhel and
Lean, 1987a,b), although the overnight protein synthesis rates reported for Lake
Ontario were far larger than the current results for Georgian Bay. The major
source of carbon for protein synthesis was polysaccharide, and polysaccharide
loss rates increased with previous incubation irradiance (Fig. 3.3B). On average,
LMW (not shown) and lipids (Fig. 3.3C) were also lost overnight, and the loss rate
increased with irradiance. Polysaccharide contributed 45-75% of the total overnight metabolism, the proportion varying in a nonsystematic way among irradiance levels. LMW contributed 18-23% whereas lipid contributed a rather variable share (7-35%, averaging 19%), with considerable but nonsystematic differences among irradiance levels.
As the confidence intervals in Fig. 3.3 show, there was considerable variation
around the mean seasonal value for overnight rates of gain or loss from each
photosynthate class. Regression analysis showed that for lipid, the variation was
best explained by the rate of lipid synthesis over the preceding light phase. The
percentage explained variation ranged from 72% to 97% among irradiance levels
and the slopes from - 0.345 to - 0.172, with no significant differences among
irradiance levels. Thus, the rate of lipid loss (a negative rate of carbon exchange)
in the dark increased as the rate of lipid synthesis in the light increased. Factors