A microencapsulation technique for introducing pure compounds in zooplankton diets

Sodium alginate microcapsules have low permeability, high digestibility, and are specifically suited as artificial food particles for zooplankton. The microcapsules may be used to introduce pure compounds into zooplankton diets and to study the biochemical fate of those compounds. We encapsulated a highly labile molecule, Chl a, and traced it and its principal degradation products in zooplankton grazing experiments. Chlorophyll was not degraded or lost during the encapsulation procedure and remained stable for at least 6 d at 4°C in the dark. The microcapsules were ingestible and digestible by female Calanus pac~jcus, and fecal pellets were formed at normal rates, 40-60 min after ingestion. Two quantitative feeding experiments were conducted with pure chlorophyll. The mean loss ofingestedpigmentwas85.9%(+7.2%)and76.1% (+6.1%). In the second experiment, we analyzed the medium to determine if any chlorophyll may have gone into solution; ~2% of the total “lost” chlorophyll went into solution. We conclude that a significant amount of ingested Chl a is degraded into non-fluorescent compounds during passage through copepod guts. Acknowledgments We thank David Martin-Suarez for help in the quantitative experiments, 0. Holm-Hansen for the use of the fluorometer, and H. Felbeck for laboratory space. The high degree of teamwork among authors made it difficult to determine order of authorship. This study was supported by grant OCE 89-00497 from the National Science Foundation to M. E. Huntley and M. Vemet, and a Research Education for Undergraduates grant from NSF which supported P. J. Perl. Estimates of zooplankton grazing and vertical flux of particulate matter have become increasingly dependent on measurements of chlorophyll a (Chl a) and its pheopigment derivatives, which have been considered to be conservative tracers of phytoplankton biomass (Currie 1962). The principal assumptions are that Chl a is quantitatively converted to pheopigment during gut passage in zooplankton (Shuman and Lorenzen 1975) and that both types of pigments are detected by the fluorometric technique (Holm-Hansen et al. 1965). However, recent studies show that pigment is indeed lost during digestion, to the extent that in several studies pigment recovery was significantly reduced to as low as 66% on a molar basis (Helling and Baars 1985; Conover et al. 1986). Whether the loss of fluorescence is constant (Helling and Baars 1985; Dagg and Walser 1987) or varies (Head 1988; Lopez et al. 1988) is still a matter of controversy. Chl a is commonly introduced to grazers as a component of living cells, mixed with other cellular components. To understand the quantitative degradation of Chl a during gut passage, free of interference from other compounds, requires techniques that allow the quantitative administering of pure Chl a through a process that will not be detrimental to this highly labile molecule. The