Effects of nutrient enrichment on growth and phlorotannin production in Fucus gardneri embryos

Resource-allocation models predict trade-offs between growth and chemical d efense. The carbon/nutrient balance hypothesis (CNBH) predicts that plants will allocate carbon to growth when nutrients are abundant and allocate it to carbon-based antiherbivore defenses when nutrients are Limiting. In mari ne systems, field and laboratory tests of the CNBH with phlorotannin-produc ing algae have generally supported the predictions of the model. However, t hese tests have all measured phlorotannin concentrations in adult algae rat her than juveniles, which are susceptible to higher grazing pressures. We e xperimentally tested some of the predictions of the CNBH in early post-sett lement stages of a common intertidal macroalga, Fucus gardneri, by growing F. gardneri embryos in media enriched with 3 nutrients: nitrogen, phosphoru s, and iron. Phlorotannin concentrations across all treatments were correla ted with embryo size but not with growth rates. As predicted by the model, nitrogen enrichment significantly enhanced embryo growth rates and decrease d phlorotannin concentrations. Iron enrichment alone had no. effect on phlo rotannin concentrations, but did affect growth. The effects on growth were primarily in altering morphology rather than changing the overall size of t he embryos. Phosphorus enrichment had no effect on growth, but did signific antly lower phlorotannin concentrations. Surprisingly, there was a signific ant iron-phosphorus interaction effect on both growth and phlorotannin conc entrations. Enrichment with a combination of iron and phosphorus had a stro nger negative effect on growth and phlorotannin concentrations than would h ave been predicted based on the individual effects of these 2 nutrients. Th e combination of iron and phosphorus enrichment may have physiologically st ressed the embryos, resulting in decreased phlorotannin production. Our res ults suggest that ontogeny plays a strong role in determining secondary met abolite levels and that nutrient addition can affect secondary metabolite p roduction in embryos by (1) altering resource allocation patterns, or (2) p roviding a physiological stress that results in reduced secondary metabolit e production.