Evidence for metabolic turnover of polyphenolics in tropical brown algae

Polyphenolic chemical defenses of plants have traditionally been classified as immobile or quantitative and as such are believed to have low to negligible rates of turnover. This assumption is an important element in many ecological theories of chemical defense that invoke cost versus benefit relationships, because (1) turnover increases the metabolic cost of maintaining an effective level of defense, and (2) changes in the rate of turnover could affect the conclusions of studies that rely upon static concentration (standing crop) measurements, since changes in compound synthesis may not emerge as corresponding changes in compound concentration. By using a stable-isotope labeling technique, we measured rates of synthesis and turnover for the polyphenolic compounds of marine brown algae in laboratory and field experiments. During the laboratory experiment, we observed the relatively rapid turnover of phlorotannins in a population of the tropical brown alga Lobophora variegata. In order to determine if such metabolic turnover in brown algae occurred under natural conditions, we then measured in situ rates of synthesis, polymerization, and turnover for extractable phlorotannins in two species of tropical marine brown algae, Sargassum hystrix var. buxifolium (Fucales) and Dictyopteris justii (Dictyotales), over a 17-day period in the field. We found that phlorotannins in L. variegata and S. hystrix var. buxifolium demonstrated rapid rates of turnover in laboratory culture and in situ field experiments, respectively. The trends for D. justii also support the presence of turnover. Results indicate that (1) the assumption that algal polyphenolics can be grouped with the tannins of vascular plants as "immobile" defenses needs to be reevaluated, (2) estimates of the metabolic cost of algal polyphenolics that presume negligible rates of turnover may significantly underestimate the total cost of defense, and (3) studies designed to test the predictions of ecological theories for the phlorotannin concentrations of tropical brown algae may be affected by changes in the rates of metabolic turnover.