48
B.C. Wainman et al.
(1993) found that the overnight change in the share of new photosynthate in lipid
was small for North Atlantic phytoplankton but that the concentration of new
(labeled) lipid at dusk versus dawn indicated overnight changes ranging from a
50% loss to a 25% gain. There is as yet little indication of what controls the size
and direction of overnight changes in lipid, although light, nutrients, temperature,
and taxonomic affiliation of the dominant phytoplankton all appear to influence
daytime lipid synthesis (Fernandez et aI., 1994; Smith and D'Souza, 1993;
Madariaga, 1992; Wainman and Lean, 1992; Rivkin, 1989).
The first systematic examination of the overnight reallocation of lipids in fresh
waters was recently carried out in one of the Laurentian Great Lakes. This study
provides an opportunity to evaluate the variability of overnight reallocation involving lipids following incubation under irradiance ranging from saturating to
strongly limiting. Besides total lipid synthesis, the synthesis of three major lipid
classes was also determined. The neutral lipids are primarily associated with
storage and are composed largely of triacylglycerol, the glycolipids comprise
mainly the pigments and sugar-containing lipids associated with the plastid, and
the phospholipids comprise mainly the lipids associated with nonplastid membranes (Gurr and Harwood, 1991; Parrish 1987). Together, the three classes normally comprise the great majority of algal lipids (Parrish, 1987). Given the
differences in function and biosynthetic pathways that exist among the three
classes, they may be expected to respond differently to environmental factors such
as light and nutrients (Smith and D'Souza, 1993; Parrish and Wangersky, 1987)
and may help explain the variability of total lipid, which is actually a collection of
functionally diverse macromolecules. Information on lipid class synthesis and
composition for natural microalgae, especially in fresh water, is very scarce to
date.
Oligotrophic Georgian Bay in Lake Huron (Munawar, 1988) has been the
subject of studies to determine the influence of varying light and nutrient status on
productivity and biosynthetic patterns of freshwater phytoplankton (Furgal and
Smith, 1997; Furgal, 1995; Smith and Maly, 1993; Maly, 1992). The results
published to date have concerned only events during the light phase of incubations, but incubations were routinely performed throughout a light-dark cycle,
and here we report on the diel patterns observed.
Full details of sampling and incubation protocols can be found in Furgal (1995)
and Furgal and Smith (1997), and only the essential details are given here. Whole
water samples were taken before dawn from a depth of 5 m at approximately
3-week intervals from June 10 to November 4, 1993. The samples were subsequently incubated under cool-white fluorescent light for 6 h, followed by a 12-h
dark period. This rather aberrant diel cycle was dictated by logistics but nonetheless provided a long enough light period to permit extensive labeling of the algae
and thus the chance to track overnight reallocation. PAR was supplied at 30, 60,
150, and 600 /-Lmol photons . m - 2 • S - I and at in situ temperatures. Total photosynthesis and photosynthate allocation were determined by 14C incorporation. The
extraction protocol for photosynthate allocation was the same variation
of Li and Platt's (1982) method used by Smith and D'Souza (1993). In brief, the
B.C. Wainman et al.
(1993) found that the overnight change in the share of new photosynthate in lipid
was small for North Atlantic phytoplankton but that the concentration of new
(labeled) lipid at dusk versus dawn indicated overnight changes ranging from a
50% loss to a 25% gain. There is as yet little indication of what controls the size
and direction of overnight changes in lipid, although light, nutrients, temperature,
and taxonomic affiliation of the dominant phytoplankton all appear to influence
daytime lipid synthesis (Fernandez et aI., 1994; Smith and D'Souza, 1993;
Madariaga, 1992; Wainman and Lean, 1992; Rivkin, 1989).
The first systematic examination of the overnight reallocation of lipids in fresh
waters was recently carried out in one of the Laurentian Great Lakes. This study
provides an opportunity to evaluate the variability of overnight reallocation involving lipids following incubation under irradiance ranging from saturating to
strongly limiting. Besides total lipid synthesis, the synthesis of three major lipid
classes was also determined. The neutral lipids are primarily associated with
storage and are composed largely of triacylglycerol, the glycolipids comprise
mainly the pigments and sugar-containing lipids associated with the plastid, and
the phospholipids comprise mainly the lipids associated with nonplastid membranes (Gurr and Harwood, 1991; Parrish 1987). Together, the three classes normally comprise the great majority of algal lipids (Parrish, 1987). Given the
differences in function and biosynthetic pathways that exist among the three
classes, they may be expected to respond differently to environmental factors such
as light and nutrients (Smith and D'Souza, 1993; Parrish and Wangersky, 1987)
and may help explain the variability of total lipid, which is actually a collection of
functionally diverse macromolecules. Information on lipid class synthesis and
composition for natural microalgae, especially in fresh water, is very scarce to
date.
Oligotrophic Georgian Bay in Lake Huron (Munawar, 1988) has been the
subject of studies to determine the influence of varying light and nutrient status on
productivity and biosynthetic patterns of freshwater phytoplankton (Furgal and
Smith, 1997; Furgal, 1995; Smith and Maly, 1993; Maly, 1992). The results
published to date have concerned only events during the light phase of incubations, but incubations were routinely performed throughout a light-dark cycle,
and here we report on the diel patterns observed.
Full details of sampling and incubation protocols can be found in Furgal (1995)
and Furgal and Smith (1997), and only the essential details are given here. Whole
water samples were taken before dawn from a depth of 5 m at approximately
3-week intervals from June 10 to November 4, 1993. The samples were subsequently incubated under cool-white fluorescent light for 6 h, followed by a 12-h
dark period. This rather aberrant diel cycle was dictated by logistics but nonetheless provided a long enough light period to permit extensive labeling of the algae
and thus the chance to track overnight reallocation. PAR was supplied at 30, 60,
150, and 600 /-Lmol photons . m - 2 • S - I and at in situ temperatures. Total photosynthesis and photosynthate allocation were determined by 14C incorporation. The
extraction protocol for photosynthate allocation was the same variation
of Li and Platt's (1982) method used by Smith and D'Souza (1993). In brief, the