Inducible Chemical Resistance to Herbivory in the Brown Seaweed Ascophyllum nodosum

Abstract

Physical damage caused by herbivores or artificial clipping is known to induce responses in secondary chemistry as well as increased resistance to further grazing in a large number of terrestrial vascular plants, but this has only rarely been described for marine algae. In the present study, it was found that a few weeks of grazing by the gastropod Littorina obtusata can induce substantially increased concentrations of phlorotannins in the intertidal brown seaweed Ascophyllum nodosum. In contrast, grazing by the isopod Idotea granulosa and simulations of herbivory through momentary and continuous clipping caused no significant changes in phlorotannin levels. This indicates a high degree of specificity in the elicitation of chemical responses to physical damage, something which previously has been shown for terrestrial vascular plants but not for marine algae. Ascophyllum plants that had been grazed by L. obtusata were also less susceptible than undamaged plants to further grazing by gastropods, but no such induced resistance was found in the experiments with I. granulosa. Feeding experiments with undamaged Ascophyllum plants and artificial food containing different levels of phlorotannins provided further support for the sensitivity of the feeding behavior of L. obtusata, and the insensitivity of I. granulosa, to intraspecific variation in the phlorotannin content of Ascophyllum. Since L. obtusata is specialized to live and feed on a few fucoid species, including A. nodosum, the results imply that phlorotannins have an important mediating role in the interactions between these macroalgae and L. obtusata. The experimentally induced increase of phlorotannins was consistent with results from phlorotannin analyses of Ascophyllum individuals from natural populations, where plants that had been heavily grazed by L. obtusata contained significantly higher levels of phlorotannins compared to undamaged plants. A field survey of the distribution and abundance of L. obtusata revealed that the density of the gastropod is highly variable at the same spatial scale as the phlorotannin content of Ascophyllum in the study area. These results suggest that grazing by L. obtusata can be an important factor in explaining natural phenotypic variation in the phlorotannin content of Ascophyllum.Together with the results from a few other studies on the interactions between mesoherbivores and marine algae, the results of this study support the previously proposed hypothesis that it is feeding by relatively small, less mobile herbivores that is most likely to cue for induced production of defense chemicals in seaweeds. More studies on such interactions may reveal that the apparent rarity of inducible chemical defenses in seaweeds is misleading.

Desorption electrospray ionization mass spectrometry reveals surface-mediated antifungal chemical defense of a tropical seaweed

“…S7); however, if bromophycolides are indeed effective antimicrobial chemical defenses in nature, one might expect low microbial abundances in bromophycolide-rich areas. Macroalgae have been reported to up-regulate chemical defenses in response to bites from small grazers (40)(41)(42), although it is unclear whether in those studies defenses were induced throughout the alga or exclusively at sites of challenge. Another possibility is that patches are associated with sites of localized algal damage from which bromophycolides have leaked, providing antifungal defenses where tissues are vulnerable to waterborne microbes.…”

No sex-related dispersal limitation in a dioecious, oceanic long-distance traveller: the bull kelp Durvillaea antarctica

“…However in this case, phlorotannin levels were higher in Totoralillo Sur. Increases in phlorotannin concentrations have been related to changes in PAR and UVR levels (Swanson andDruehl 2002, Cruces et al 2013) and herbivore pressure (Van Alstyne 1988, Pavia andToth 2000). Thus, site-specific environmental conditions cause variation in pigments and phlorotannin concentrations in D. antarctica and appear to be more important than sex-specific differences.…”

CONTENTS OF SOLUBLE, CELL-WALL-BOUND AND EXUDED PHLOROTANNINS IN THE BROWN ALGA Fucus vesiculosus, WITH IMPLICATIONS ON THEIR ECOLOGICAL FUNCTIONS

“…They are suggested to have multiple ecological roles: some phlorotannins act as chemical defenses against herbivory (e.g., Steinberg, 1988;Targett and Arnold, 1998;Arnold and Targett, 2000;Pavia and Toth, 2000a) and as antifouling substances (Sieburth and Conover, 1965;Wikström and Pavia, 2004), although the evidence for this function is equivocal (Jennings and Steinberg, 1997). Concentrations of phlorotannins show phenotypic plasticity in response to environmental parameters, such as salinity, nutrient and light availability, ultraviolet irradiation, and intensity of herbivory (Yates and Peckol, 1993;Peckol et al, 1996;Pavia et al, 1997;Pavia and Toth, 2000b;Honkanen et al, 2002;Swanson and Druehl, 2002).…”