Quantifying in situ rates of phlorotannin synthesis and polymerization in marine brown algae

Using three species of marine brown algae, we describe a stable isotope labeling technique to quantify: (1) in situ rates of phlorotannin synthesis, (2) phlorotannin polymerization or aging, and (3) the related allocation of carbon resources to secondary metabolism. In our field and laboratory assays, Lobophora variegata (Bahamas), Sargassum pteropleuron (Bahamas), and Fucus distichus (California, USA) assimilated various quantities of C-13, but all allocated less than or equal to 1% of the assimilated carbon to the production of phlorotannins. We quantified rates of phlorotannin synthesis both as micrograms of compound produced per gram of tissue per unit of time and as micrograms of compound produced per gram of C assimilated per unit of time. Rates of synthesis, normalized to account for differences in potential photosynthetic rates, are comparable to previously reported rates of phlorotannin accumulation. The aging of phlorotannins from low- (<30 kDa) to high- (>30 kDa) molecular-size polymers was observed in S. pteropleuron within a 28-hr period. Our results indicate that, using this labeling technique, it is possible to make precise measurements of allelochemical metabolism and resource allocation, which are useful both in critically evaluating the assumptions made by ecological models of plant chemistry and in estimating the partial metabolic cost of specific secondary metabolites.