Effects of UV-B radiation and simulated herbivory on phlorotannins in the brown alga Ascophyllum nodosum

Abstract

Models and experiments seelung to explain intraspec~f~c variation in brown algal phlorotannins (polyphenolics) have mainly focused on the effect of 2 factors, herbivory and resource availability (carbon/nutrients). The possible importance of other biotic and abiotic factors, e.g. pathogenlc micro-organisms, heavy metals and UV radiation, has often been suggested but only rarely experimentally tested. In the present study the effects of increased UV-B irradiance and simulated grazing (clipping) on phlorotannin production in the brown alga Ascophyllum nodosum were investigated. The 2 treatments were applied simultaneously in a crossed factorial experiment in order to test for interactive, as well as separate, effects. Carbon and nitrogen content of the algae was also determined for each treatment. The effect of the experimental treatments on the feeding selectivity of a natural herbivore was tested in a subsequent feeding preference experiment with the crustacea Idotea granulosa. An increase (-50%) in UV-B radiation during a 2 wk period resulted in a signif~cant Increase (-30%) in mean phlorotannin concentration, while no significant changes in phlorotannin levels following simulated grazing were observed. The additional UV light caused a slight increase in the nitrogen content of the algae, indicating that the response in phlorotannin production was not caused by nutrient deficit. Absorption spectra of A. nodosum extracts, before and after removal of phlorotannins with polyvinylpolypyrrolidone, revealed that phlorotannins can contribute to absorption in the UV-B range (280 to 320 nm). The results imply that phlorotannins can function as inducible screens against harmful UV radiation. The grazer I. granulosa showed a clear preference for algae that had been exposed to an addition of UV-B radiation, in spite of their increased phlorotannin levels, supporting the notion that small marine herbivores in general are tolerant to chemical defenses of algae.

Chemical mediation of interactions among marine organisms

“…117 These results contrast with those from a study exposing Ascophyllum nodosum to UV radiation in an experiment designed to determine the effects of UV-B radiation and simulated herbivory on phlorotannin production. 50 Phlorotannin concentrations increased in algae exposed to UV radiation. 50 ' 112 The authors suggested that phlorotannins function as inducible screens against harmful UV radiation because phlorotannins absorb UV-B radiation.…”

“…50 Phlorotannin concentrations increased in algae exposed to UV radiation. 50 ' 112 The authors suggested that phlorotannins function as inducible screens against harmful UV radiation because phlorotannins absorb UV-B radiation. UV-exposed algae were still more susceptible to grazing by the isopod Idotea granulosa, despite their increased phlorotannin levels, than the controls.…”

“…7 ' 46 " 47 These and other UV-absorbing molecules such as scytonemin, 48 " 49 and possibly even brown algal phlorotannins 50 illustrate the variety of roles that natural products play in marine organisms.…”

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). This implies that the pools of soluble phlorotannins are not stable but rather in a state of flux, and their concentration may be determined by the balance between rates of synthesis and turnover (Arnold and Targett, 2000).…”

Species-specific defense strategies of vegetative versus reproductive blades of the Pacific kelps Lessonia nigrescens and Macrocystis integrifolia

“…For example, diterpenes and phlorotannins, the most investigated groups of metabolites isolated from brown algae (Maschek and Baker 2008) were identiWed to act both as anti-herbivory and anti-fouling substances (summarized in Amsler and Fairhead 2006). Phlorotannins have also been invoked to serve or contribute to other vital functions such as protection against UV-radiation, cell-wall formation, and cytokinesis (Pavia et al 1997;Schoenwaelder and Clayton 1999;Schoenwaelder 2002). On the other hand, herbivores are considered to be mainly N-limited, and therefore selective in foraging for N-rich algal tissue (Mattson 1980;DuVy and Paul 1992;Cruz-Rivera and Hay 2000).…”