Importance of a micro-diet for scleractinian corals

This study investigated the ability of 3 coral species - zooxanthellate (Stylophora pistillata and Galaxea fascicularis) and azooxanthellate (Tubastrea aurea)-to feed on pico- and nano-plankton (particles < 100 mum). Coral nubbins were incubated for 6 h in flow chambers containing the planktonic particles (experimental chambers). Control chambers were also set up to follow the natural changes in the planktonic community. Changes in the concentrations of dissolved organic carbon (DOC), bacteria, cyanobacteria and flagellates were monitored during the incubation. Results showed that ingestion rates were proportional to prey concentrations. In terms of number of prey ingested per polyp, bacteria were the first group ingested. When converted into carbon and nitrogen, nanoflagellates represented the most important contribution, amounting to 84-94 % of the total carbon and 52-85% of the total nitrogen ingested. Bacteria, cyanobacteria and picoflagellates accounted only for 1-7 % of the ingested carbon. At the end of the incubation, DOC concentrations increased in the chambers containing T. aurea and G. fascicularis. However, in all chambers containing S. pistillata, DOC concentrations decreased (from 101.69 +/- 13.53 to 93.59 +/- 4.67 mumol DOC1(-1)) equal to a rate of 0.251 +/- 0.217 nmol DOC polyp(-1) h(-1). In symbiotic species, pico- and nanoplankton accounted for only 6.6-7.8 % of the carbon supplied by photosynthesis but were a major supply of nitrogen. In S. pistillata, the amount of nitrogen supplied by pico- and nanoplankton ingestion (1.2 ng N polyp(-1) h(-1)) was as high as the amount supplied by the dissolved nitrogen uptake (1.55 ng N polyp(-1) h(-1)). Pico- and nanoplankton may, therefore, constitute an important food source for these corals.