46
H.C. Wainman et al.
and Lean, 1987a; Smith et aI., 1987; Li and Platt, 1982). Over the whole photic
zone, with irradiance ranging from saturating to strongly limiting in most cases,
the influence of light is likely to be far larger than that of any variations in nutrient
supply.
It is nonetheless commonly observed that the changes in relative allocation to
lipids in response to varying incubation irradiance are small compared with those
observed in protein and polysaccharide components. Perhaps, partly for this reason, most attempts to determine the environmental controls on the synthesis of
lipids by natural communities of micro algae have emphasized the patterns observed under saturating incubation irradiance (Wainman and Lean, 1996, 1992;
Cuhel and Lean, 1987a; Smith et aI., 1987; Lancelot and Mathot, 1985a). It is
important to realize, however, that the relative allocation to lipids (typically 1030% of total photosynthate) is generally smaller than that to either protein or
polysaccharide (typically 30-60%). Even modest photosynthetically active radiation (PAR)-dependent changes in the proportion of total photosynthate directed to
lipid therefore translate into large relative changes in lipid synthesis, often comparable with those observed for protein or polysaccharide (Wainman and Lean,
1992; Cuhel and Lean, 1987a; Smith et aI., 1987; Li and Platt, 1982). In addition,
the irradiance required to saturate lipid synthesis in a variety of freshwater phytoplankton communities is systematically higher than that for total carbon incorporation (Wainman and Lean, 1992; Priscu et aI., 1987), indicating that observations made only at saturating light intensities are insufficient for accurate
measurement of integrated lipid synthesis throughout the photic zone.
Although photosynthesis can proceed only during the illuminated phase of the
die I cycle, algae can reallocate photosynthate to support continued synthesis of
essential cellular components in darkness. Dark synthesis is well documented for
protein, which can sometimes be synthesized, overnight, at rates rivaling those
observed during the light phase (Cuhel and Lean, 1987a,b; Lancelot and Mathot,
1985a; but cf. Fernandez et aI., 1992). If an accurate determination of lipid
synthesis and the environmental factors that control it is desired, then the possibility of changes during the dark phase must be considered.
The high degree of light regulation of fatty acid synthesis in higher plants
would suggest that lipid synthesis should be minimal in darkness. Catabolism of
storage lipids (principally triacylglycerols) in higher plants, however, generally
involves a high degree of interorganelle cooperation and transportation, and extensive catabolism is associated mainly with specialized situations such as germination of oil seeds (Gurr and Harwood, 1991) and not with short-term energy
storage on the diel time scale. We may then anticipate relatively little synthesis or
catabolism of lipids overnight, and measurements of photosynthate allocation in
natural microalgae often support this expectation (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a). Lipid synthesis in plants can, however, be supported in
darkness by using reductant generated from the pentose phosphate pathway (Gurr
and Harwood, 1991). Catabolism is difficult to measure in isolation from the other
factors that can effect losses of photosynthate from natural microalgal populations
(e.g., excretion and predation), but there is evidence that overnight lipid catabol-
H.C. Wainman et al.
and Lean, 1987a; Smith et aI., 1987; Li and Platt, 1982). Over the whole photic
zone, with irradiance ranging from saturating to strongly limiting in most cases,
the influence of light is likely to be far larger than that of any variations in nutrient
supply.
It is nonetheless commonly observed that the changes in relative allocation to
lipids in response to varying incubation irradiance are small compared with those
observed in protein and polysaccharide components. Perhaps, partly for this reason, most attempts to determine the environmental controls on the synthesis of
lipids by natural communities of micro algae have emphasized the patterns observed under saturating incubation irradiance (Wainman and Lean, 1996, 1992;
Cuhel and Lean, 1987a; Smith et aI., 1987; Lancelot and Mathot, 1985a). It is
important to realize, however, that the relative allocation to lipids (typically 1030% of total photosynthate) is generally smaller than that to either protein or
polysaccharide (typically 30-60%). Even modest photosynthetically active radiation (PAR)-dependent changes in the proportion of total photosynthate directed to
lipid therefore translate into large relative changes in lipid synthesis, often comparable with those observed for protein or polysaccharide (Wainman and Lean,
1992; Cuhel and Lean, 1987a; Smith et aI., 1987; Li and Platt, 1982). In addition,
the irradiance required to saturate lipid synthesis in a variety of freshwater phytoplankton communities is systematically higher than that for total carbon incorporation (Wainman and Lean, 1992; Priscu et aI., 1987), indicating that observations made only at saturating light intensities are insufficient for accurate
measurement of integrated lipid synthesis throughout the photic zone.
Although photosynthesis can proceed only during the illuminated phase of the
die I cycle, algae can reallocate photosynthate to support continued synthesis of
essential cellular components in darkness. Dark synthesis is well documented for
protein, which can sometimes be synthesized, overnight, at rates rivaling those
observed during the light phase (Cuhel and Lean, 1987a,b; Lancelot and Mathot,
1985a; but cf. Fernandez et aI., 1992). If an accurate determination of lipid
synthesis and the environmental factors that control it is desired, then the possibility of changes during the dark phase must be considered.
The high degree of light regulation of fatty acid synthesis in higher plants
would suggest that lipid synthesis should be minimal in darkness. Catabolism of
storage lipids (principally triacylglycerols) in higher plants, however, generally
involves a high degree of interorganelle cooperation and transportation, and extensive catabolism is associated mainly with specialized situations such as germination of oil seeds (Gurr and Harwood, 1991) and not with short-term energy
storage on the diel time scale. We may then anticipate relatively little synthesis or
catabolism of lipids overnight, and measurements of photosynthate allocation in
natural microalgae often support this expectation (Cuhel and Lean, 1987a; Lancelot and Mathot, 1985a). Lipid synthesis in plants can, however, be supported in
darkness by using reductant generated from the pentose phosphate pathway (Gurr
and Harwood, 1991). Catabolism is difficult to measure in isolation from the other
factors that can effect losses of photosynthate from natural microalgal populations
(e.g., excretion and predation), but there is evidence that overnight lipid catabol-