Photosynthetic carbon allocation: Effects of planktivorous fish and nutrient enrichment

Abstract

The proportions of lipid, polysaccharide and protein profoundly influence the nutritional value of food for planktonic organisms. Here, we investigated the effects of nutrients (N and P) and planktivorous fish (Phoxinus sp.) on the partitioning of photosynthetically fixed 14C-labelled bicarbonate into macromolecules (lipids, polysaccharides and proteins) and low molecular weight (LMW) metabolites of epilimnetic phytoplankton living in large (8 m diameter) enclosures from July to mid-October 1991. The enclosures were situated in an oligotrophic lake (north of Montreal, Canada) in both shallow (3.5 m) and deep (11 m) locations. The fractions were calculated using the sequential extraction of 14C-labeled phytoplankton samples incubated for short duration (∼2 hours) at maximum photosynthetic rates. Labelling patterns of all end products exhibited great temporal variations. Overall, the LMW, polysaccharide, lipid and protein fractions accounted for 32.1 ± 10.5%, 31.0 ± 8.8%, 19.2 ± 5.3% and 17.8 ± 6.1% of the total 14C fixed respectively. Percent 14C in the LMW fraction was, in general, lower in the nutrient-enriched enclosure and was usually lower in the deep than the shallow enclosures. The effect of fish on the labelling pattern of LMW varied throughout the sampling period but, overall, its impact was non-significant. The proportions of 14C into lipids and polysaccharides did not show the expected increase under nutrient deficiency. In contrast, the polysaccharide fraction increased, for the most part, with nutrient enrichment and was higher in the deep than the shallow enclosures with little changes in the presence of planktivorous fish. Incorporation of 14C into lipids remained relatively uniform throughout the study and averaged less than 20% of total fixed carbon. Nutrient addition increased the lipid fraction in early July but did not have a significant effect thereafter. In general, the presence of planktivorous fish decreased the lipid fraction especially in the deep N + F enclosures. With nutrient enrichment, the proportion of 14C incorporated into protein decreased in the shallow enclosures while it increased in the deep enclosures. During most of the study, there was no significant effect of fish on the distribution pattern of protein. Percent carbon protein averaged 20% of total carbon fixed during the summer and decreased to 10% in October with lower water temperatures. Any fish effects encountered throughout this study were probably mediated indirectly through changes in the species and cell size compositions of phytoplankton. An experiment where samples were incubated with depth (0.5 to 7 m) showed that, for all treatments, the protein fraction increased with depth at the expense of lipid and LMW metabolites syntheses. The polysaccharide fraction demonstrated no apparent relationships with depth. These changes with depth most likely reflect light attenuation rather than a decrease in temperature. Overall, our findings would suggest that the distribution patterns of photosynthetically incorporated carbon into LMW compounds and macromolecules in natural phytoplankton communities are controlled by a combination of both abiotic (e.g., nutrients, light and temperature) and biotic (species and cell size) environmental factors and the interactions between these factors will in essence influence the direction of carbon flow and the food quality of the phytoplankton. These observations are placed in context with a review of the literature on carbon allocation by natural populations of phytoplankton.