Digestion and alteration of the biochemical constituents of a diatom (Thalassiosira weisflogii) ingested by an herbivorous zooplankton (Calanus pacificus)

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Abstract
A laboratory experiment was conducted in which 14C‐labeled diatoms (Thalassisora weissflogii) were fed to an herbivorous copepod zooplankton (Calanus pacificus). Digestion efficiencies, as defined by changes in concentration relative to biogenic silica from the diet to fecal pellets, were calculated for 14C, organic carbon, nitrogen, and chlorophyll‐type pigments as well as proteins (as amino acids) and polysaccharides (as aldoses).The majority of organic material ingested (≥65% of all components except pigments, 30%) was digested. Digestion efficiencies were similar to reactivity patterns observed in previous studies of the same organisms (total N > organic C > total chlorophyll‐type pigments). Polysaccharides and particularly amino acids were clearly important nutrients for copepods. However, together with chlorophyll‐type pigments, these biochemicals accounted for only 57% of organic C in the diatom and only 35% in fecal pellets. Although lipids represented some fraction of the uncharacterized materials, an important, relatively stable diatom component remained unidentified and apparently was concentrated in the fecal pellets.Individual amino acids and especially aldoses were digested to differing degrees, apparently due to preferential digestion of intracellular materials with concomitant preservation of cell‐wall polymers in the fecal pellets. Overall, egested materials were dramatically altered in composition relative to the diet. Digestion efficiencies for all components were similar in magnitude and pattern to reactivities observed in temperate coastal sites in which diatoms and copepods are important contributors to plankton populations and sinking particle fluxes.

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