3. Irradiance and Lipid Production
47
ism can be extensive in some populations and may at times be the principal source
of carbon for protein synthesis in the dark (Priscu et aI., 1987). This seemingly
inconsistent physiological behavior in different algal communities may reflect
control by environmental factors that are currently only poorly understood.
Our purpose here is to improve our understanding of lipid synthesis in natural
populations of microalgae by explicitly examining the implications of the varying
irradiance environment in the photic zone and the overnight metabolic and reallocation activities of the algae. We deal specifically with data for temperate
freshwater phytoplankton, which have been much less studied in regard to lipid
synthesis and photosynthate allocation than their marine counterparts. We try to
show that lipid synthesis when integrated over the photic zone and the diel cycle is
not the same as that observed only during the light phase under saturating irradiance but that it is usefully predictable from light-phase measurements alone.
3.2. Metabolism and Reallocation
3.2.1. Lipids in Relation to Other Macromolecular Classes
The majority of studies reporting measurements of lipid synthesis by microalgae
have concentrated on daytime rates, but a considerable number have also reported
on the metabolism and reallocation of recently fixed lipid carbon as revealed by
overnight changes in labeling patterns (Marafi6n et aI., 1995; Fernandez et aI.,
1994, 1992; Smith and D'Souza, 1993; Hawes, 1990; Glover and Smith, 1988;
Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a; McConville et aI., 1985). A
widely accepted view, seemingly consistent with the biochemical fundamentals as
we know them, is that overnight changes in phytoplankton mainly involve a
continuation of protein synthesis at the expense of polysaccharides (including
low-molecular-weight metabolites [LMW]) , with comparatively minor changes
in total lipids (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a,b). Study of
overnight changes has focused largely on the patterns observed when the preceding incubation light intensity was sufficient to saturate total photosynthesis, although several authors have noted that energy supply during the light period can
influence subsequent metabolism and reallocation significantly in the context of
nighttime protein synthesis (Fernandez et aI., 1992; Cuhel and Lean, 1 987a,b).
A closer examination of the literature gives cause to question whether reallocation involving lipid is always small in extent or consistent in direction. Of the
reallocation studies published to date, few show more than one example of a diel
pattern and rarely is it obvious that diel patterns were systematically and regularly
measured. Where more comprehensive results are reported, they can reveal considerable variations. For example, Glover and Smith (1988) found that the share
of total labeled photosynthate in marine Synechococcus sp. at the end of the dark
period decreased by 10% at some stations but increased by >50% at others,
compared with the share at the end of the preceding light period. The estimated
concentration of newly synthesized lipid varied similarly. Smith and D'Souza
47
ism can be extensive in some populations and may at times be the principal source
of carbon for protein synthesis in the dark (Priscu et aI., 1987). This seemingly
inconsistent physiological behavior in different algal communities may reflect
control by environmental factors that are currently only poorly understood.
Our purpose here is to improve our understanding of lipid synthesis in natural
populations of microalgae by explicitly examining the implications of the varying
irradiance environment in the photic zone and the overnight metabolic and reallocation activities of the algae. We deal specifically with data for temperate
freshwater phytoplankton, which have been much less studied in regard to lipid
synthesis and photosynthate allocation than their marine counterparts. We try to
show that lipid synthesis when integrated over the photic zone and the diel cycle is
not the same as that observed only during the light phase under saturating irradiance but that it is usefully predictable from light-phase measurements alone.
3.2. Metabolism and Reallocation
3.2.1. Lipids in Relation to Other Macromolecular Classes
The majority of studies reporting measurements of lipid synthesis by microalgae
have concentrated on daytime rates, but a considerable number have also reported
on the metabolism and reallocation of recently fixed lipid carbon as revealed by
overnight changes in labeling patterns (Marafi6n et aI., 1995; Fernandez et aI.,
1994, 1992; Smith and D'Souza, 1993; Hawes, 1990; Glover and Smith, 1988;
Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a; McConville et aI., 1985). A
widely accepted view, seemingly consistent with the biochemical fundamentals as
we know them, is that overnight changes in phytoplankton mainly involve a
continuation of protein synthesis at the expense of polysaccharides (including
low-molecular-weight metabolites [LMW]) , with comparatively minor changes
in total lipids (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a,b). Study of
overnight changes has focused largely on the patterns observed when the preceding incubation light intensity was sufficient to saturate total photosynthesis, although several authors have noted that energy supply during the light period can
influence subsequent metabolism and reallocation significantly in the context of
nighttime protein synthesis (Fernandez et aI., 1992; Cuhel and Lean, 1 987a,b).
A closer examination of the literature gives cause to question whether reallocation involving lipid is always small in extent or consistent in direction. Of the
reallocation studies published to date, few show more than one example of a diel
pattern and rarely is it obvious that diel patterns were systematically and regularly
measured. Where more comprehensive results are reported, they can reveal considerable variations. For example, Glover and Smith (1988) found that the share
of total labeled photosynthate in marine Synechococcus sp. at the end of the dark
period decreased by 10% at some stations but increased by >50% at others,
compared with the share at the end of the preceding light period. The estimated
concentration of newly synthesized lipid varied similarly. Smith and D'Souza