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

Resource-allocation models predict trade-offs between growth and chemical defense. 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 marine systems, field and laboratory tests of the CNBH with phlorotannin-producing algae have generally supported the predictions of the model. However, these tests have all measured phlorotannin concentrations in adult algae rather than juveniles, which are susceptible to higher grazing pressures. We experimentally tested some of the predictions of the CNBH in early post-settlement stages of a common intertidal macroalga, Fucus gardneri, by growing F. gardneri embryos in media enriched with 3 nutrients: nitrogen, phosphorus, and iron. Phlorotannin concentrations across all treatments were correlated with embryo size but not with growth rates. As predicted by the model, nitrogen enrichment significantly enhanced embryo growth rates and decreased phlorotannin concentrations. Iron enrichment alone had no effect on phlorotannin concentrations, but did affect growth. The effects on growth were primarily in altering morphology rather than changing the overall size of the embryos. Phosphorus enrichment had no effect on growth, but did significantly lower phlorotannin concentrations. Surprisingly, there was a significant iron-phosphorus interaction effect on both growth and phlorotannin concentrations. Enrichment with a combination of iron and phosphorus had a stronger negative effect on growth and phlorotannin concentrations than would have been predicted based on the individual effects of these 2 nutrients. The combination of iron and phosphorus enrichment may have physiologically stressed the embryos, resulting in decreased phlorotannin production. Our results suggest that ontogeny plays a strong role in determining secondary metabolite levels and that nutrient addition can affect secondary metabolite production in embryos by (1) altering resource allocation patterns, or (2) providing a physiological stress that results in reduced secondary metabolite production.