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 neglig ible rates of turnover. This assumption is an important element in many eco logical theories of chemical defense that invoke cost versus benefit relati onships, because (1) turnover increases the metabolic cost of maintaining a n effective level of defense, and (2) changes in the rate of turnover could affect the conclusions of studies that rely upon static concentration (sta nding crop) measurements, since changes in compound synthesis may not emerg e as corresponding changes in compound concentration. By using a stable-iso tope labeling technique, we measured rates of synthesis and turnover for th e polyphenolic compounds of marine brown algae in laboratory and field expe riments. During the laboratory experiment, we observed the relatively rapid turnover of phlorotannins in a population of the tropical brown alga Lobop hora variegata. In order to determine if such metabolic turnover in brown a lgae occurred under natural conditions, we then measured in situ rates of s ynthesis, 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 th e field. We found that phlorotannins in L. variegata and S. hystrix var. bu xifolium 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 al gal polyphenolics can be grouped with the tannins of vascular plants as "im mobile" defenses needs to be reevaluated, (2) estimates of the metabolic co st of algal polyphenolics that presume negligible rates of turnover may sig nificantly underestimate the total cost of defense, and (3) studies designe d to test the predictions of ecological theories for the phlorotannin conce ntrations of tropical brown algae may be affected by changes in the rates o f metabolic turnover.