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Chemistry and physiology of feeding deterrent production by the marine diatom Phaeodactylum tricornutum Shaw, Barbara Ann


It has been proposed that some marine phytoplankton use chemical feeding deterrents to reduce or inhibit zooplankton grazing. While a number of studies have shown that certain phytoplankton reduce zooplankton feeding, few studies have dealt with both the chemistry (isolation and identification) and the physiology of production of feeding deterrents. A bioassay to detect feeding deterrents was developed. This bioassay measured the rate of fecal pellet production of the harpacticoid copepod Tigriopus cattfornicus when fed a diet of the diatom Thalassiosira pseudonana in the presence of dissolved feeding deterrents. Using this bioassay, the cellular extracts of several species of phytoplankton were screened. The diatom Phaeodactylum tricornutum and the dinoflagellate Gonyaulax grindleyi were found to deter feeding. Bioassay-guided chemical fractionation was used to isolate the compounds responsible for the feeding deterrent activity of Phaeodactylum tricornutum. Spectroscopic techniques identified four of these compounds as apo-lO'-fucoxanthinal, apo-12'-fucoxanthinal, apo-12-fucoxanthinal, and apo-13'-fucoxanthinone. The IC₅₀ (concentration of compound at which feeding was inhibited by 50%) values ranged from 1.8 to 20 ppm, while the LC50 (concentration of compound at which there was a 50% mortality rate in the test population) ranged between 37 and 340 ppm. Preliminary studies on the physiology of production of these feeding deterrents were performed. In order to carry out these studies, an analytical HPLC method was developed to measure the apo-fucoxanthinoid concentrations in crude cell extracts. Detailed analysis of HPLC data collected on Phaeodactylum tricornutum cell extracts identified another potential feeding deterrent compound, apo-10-fucoxanthinal. Production of apo-fucoxanthinoids during the growth cycle of Phaeodactylum tricornutum and Thalassiosira pseudonana was tracked using HPLC and quantitative bioassays. Although apo-fucoxanthinoids were produced enzymatically from fucoxanthin by both diatoms, P. tricomutum produced much greater amounts of these compounds than T. pseudonana. Thus the degree of production appears to be species-specific. The production of total intracellular apo-fucoxanthinoids increased with the culture age and the degree of phosphate limitation experienced by the culture. The concentration of total intracellular apo-fucoxanthinoids in P. tricomutum was calculated to be 1000 times higher than the amount required to produce a 50% inhibition of fecal pellet production in the copepod Tigriopus califomicus.

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