Phytoplankton: a significant trophic source for soft corals?

Abstract.

Histological autoradiographs and biochemical analyses show that ¹⁴C-labelled microalgae (diatoms, chlorophytes and dinoflagellates) are used by the soft coral Dendronephthya sp. Digestion of the algae took place at the point of exit of the pharynx into the coelenteron. Ingestion and assimilation of the labelled algae depended on incubation time, cell density, and to a lesser extent on species-specificity. ¹⁴C incorporation into polysaccharides, proteins, lipids and compounds of low molecular weight was analysed. The ¹⁴C-labelling patterns of the four classes of substances varied depending on incubation time and cell density. ¹⁴C incorporation was highest into lipids and proteins. Dissolved labelled algal metabolites, released during incubation into the medium, contributed between 4% and 25% to the total ¹⁴C activity incorporated. The incorporated microalgae contributed a maximum of 26% (average of the four species studied) to the daily organic carbon demand, as calculated from assimilation rates at natural eucaryotic phytoplankton densities and a 1 h incubation period. The calculated contribution to the daily organic carbon demand decreased after prolonged incubation periods to about 5% after 3 h and to 1–3% after 9 h. Thus the main energetic demand of Dendronephthya sp. has to be complemented by other components of the seston.