Nancy M. Targett

MINIREVIEW—PREDICTING THE EFFECTS OF BROWN ALGAL PHLOROTANNINS ON MARINE HERBIVORES IN TROPICAL AND TEMPERATE OCEANS

Phlorotannins are polyphenolic brown algal secondary metabolites that can function as herbivore deterrents, digestive inhibitors, and antibacterial agents; thus, they are putative defensive agents against the natural enemies of these algae (see Rosenthal and Janzen 1979, Norris and Fenical 1982, Ragan and Glombitza 1986, Hay and Fenical 1988, Bernays et al. 1989, Steinberg 1992 for reviews, but see Targett et al. 1995). Phlorotannins also contribute to absorption in the UV-B range (280–320 nm) (Pavia et al. 1997) and may function in chelation of metal ions (Ragan and Glombitza 1986). They are found in almost all brown algal orders including Ectocarpales, Dictyotales, Laminariales, Fucales, and Ascoseirales. Phlorotannin concentrations have been determined for a wide range of brown algae and have been found to occur regularly at levels .2% dry mass (DM) throughout the temperate and tropical Atlantic and temperate Pacific (Ragan and Glombitza 1986, Steinberg 1992, Targett et al. 1992, 1995 for reviews). They are generally ,2% DM in brown algae in the tropical Indo-Pacific. The role polyphenolics play in plant antiherbivore defense was traditionally considered to be generalized and nonspecific and their activity dose-dependent or quantitative. They were thought to deter herbivory due to their astringency and to act as digestibilityreducing compounds (Feeny 1976, Rhoades and Cates 1976, Hagerman and Butler 1991, Appel 1993) or as toxins (Bernays et al. 1989, Appel 1993). In marine systems, they were thought to function as a chemical defense against herbivores primarily in temperate and polar areas. However, studies on their efficacy in this role have produced widely conflicting results (Steinberg and Paul 1990, Van Alstyne and Paul 1990), defying our ability to predict their effects accurately. Focusing on phlorotannin chemical characteristics alone is not sufficient to explain their activity over a broad geographical range. In particular, it appears that: 1. polyphenolics act with more specificity of activity than initially suggested (see Zucker 1983, Hagerman and Butler 1991 for reviews, Asquith and Butler 1985, Mole and Waterman 1987, Robbins et al. 1987, Ayres et al. 1997); 2. the reactivity of polyphenolics is affected by the chemical environment of a herbivore’s gut (Martin and Mar-

Epibiosis of Marine Algae and Benthic Invertebrates: Natural Products Chemistry and Other Mechanisms Inhibiting Settlement and Overgrowth

This article reviews chemical and physical mechanisms by which non-motile marine invertebrates and marine algae inhibit the fouling of their surfaces. Inhibition of primary film formation (bacterial, diatoms and protozoans); and macrophyte and epizooite settlement are considered individually followed by discussion of their interactions. Emphasis is placed on ecological and evolutionary considerations, including the advantages and disadvantages for an organism in being fouled, variation in the defenses utilized by an organism and the likelihood of misinterpreting interactions among organisms. The study of natural antifouling defenses is an emerging field offering important theoretical and economic spin-offs.

Chemical ecology of marine organisms: An overview

An overview of marine chemical ecology is presented. Emphasis is placed on antipredation, invertebrate-toxic host relationships, antifouling, competition for space, species dominance, and the chemistry of ecological interactions.

Potent cytotoxins produced by a microbial symbiont protect host larvae from predation

Larvae of the sessile marine invertebrate Bugula neritina (Bryozoa) are protected by an effective chemical defense. From the larvae, we isolated three bryostatin-class macrocyclic polyketides, including the novel bryostatin 20, that deterred feeding by a common planktivorous fish that co-occurs with B. neritina. A unique bacterial symbiont of B. neritina, Endobugula sertula, was hypothesized as the putative source of the bryostatins. We show that: (1) bryostatins are concentrated in B. neritina larvae and protect them against predation by fish; (2) the adults are not defended by bryostatins; and (3) E. sertula produces bryostatins. This study represents the first example from the marine environment of a microbial symbiont producing an anti-predator defense for its host and, in this case, specifically for the host’s larval stage, which is exceptionally vulnerable to predators.

Role of Polyphenolic Molecular Size in Reduction of Assimilation Efficiency in Xiphister Mucosus

Feeding deterrent effects of polyphenolic compounds from both terrestrial and marine environments have been well studied. However, the effects of this compound class on herbivore digestion and assimilation are less well known, particularly with respect to marine herbivores. This study found that the reduction of assimilation efficiency in Xiphister mucosus, a marine herbivorous fish, fed algae treated with marine polyphenolics (phlorotannins) was affected by phlorotannin molecular size (mass). In general, phlorotannins > 16.60 x 10—21 g(= > 10 kDa) significantly decreased the assimilation of X. mucosus; those 16.60 x 10—21 size range. Thus, the size range of phlorotannins that significantly reduced assimilation was also the size range in which they were most concentrated. This work supports the idea that polyphenolics as a chemical class do not all have the same bioactivity, but differ in their activity in a size—dependent manner. It also addresses some of the discrepancies seen in feeding—preference studies, providing an alternative explanation for results.

Influence of marine microalgal metabolites on the feeding behavior of the blue mussel Mytilus edulis

Effects of microalgal ectocrines on the feeding behavior of the blue mussel Mytilus edulis L., collected from Delaware Bay, Lewes, USA, were studied. Unialgal cultures of six species of marine microalgae were grown to the end of their exponential growth phase. Cells were removed by gentle filtration, leaving dissolved microalgal ectocrines in culture filtrates. Filtrates were then tested in filtration rate and particle selection bioassays. In filtration rate bioassays, dissolved ectocrines were delivered to mussels in a flow-through apparatus and removal of beads (6 μm) used to determine rates. Filtration rates were significantly reduced by dissolved ectocrines from two of the microalgal species tested (Olisthodiscus luteus Carter and Dunaliella tertiolecta Butcher). None of the microalgal filtrates stimulated filtration rates of mussels. In particle selection bioassays, ectocrines were adsorbed onto either reverse phase (lipophilic) or normal phase (hydrophilic) microparticles (10 μm) and delivered to mussels with an equal concentration of control treated particles. Mussels showed significant selection, either rejection or preferential ingestion, for ectocrine treated spheres depending upon microalgal species and sphere type used. This research shows that pre-ingestive chemical cues from microalgae can influence mussel feeding behavior. We suggest that epicellular ectocrines are more important in mediating this behavior than dissolved ectocrines.

Marine Tannins: The Importance of a Mechanistic Framework for Predicting Ecological Roles

Since chemical ecology emerged as a field of marine science, it has been strongly influenced by studies of chemically mediated interactions in land-based systems. Marine chemical ecologists, like their terrestrial counterparts, initially focused on identifying natural products and evaluating the potential ecological roles of these products as defenses, attractants, or other cues. Now, like our land-based colleagues, we must increase our focus on the physiological and biochemical mechanisms that underlie the chemical interactions, paying particular attention to regulation of biosynthetic pathways, within-plant and between-plant signaling cues, and comparative and functional genomics. Here, we review the current state of knowledge regarding a heterogenous group of macrophyte natural products, the marine tannins and simple phenolics, to illustrate how such information is critical to future attempts to predict their ecological roles.

Tropical marine herbivore assimilation of phenolic-rich plants

Phenolics in marine brown algae have been thought to follow a latitudinal gradient with high phenolic species in high latitudes and low phenolic species in low latitudes. However, tropical brown algae from the western Caribbean have been shown to be high in phlorotannin concentration, indicating that latitude alone is not a reasonable predictor of marine plant phenolic concentrations. This study shows that the range of high phenolic phaeophytes is not limited to the western Caribbean but encompasses the western tropical Atlantic, including Bermuda and the Caribbean, where algal phlorotannin concentrations can be as high as 25% dry weight (DW). Assimilation efficiencies (AEs) of phenolic-rich and phenolic-poor plants were examined in three tropical marine herbivores (the parrotfish, Sparisoma radians, and the brachyuran crab, Mithrax sculptus, from Belize and the parrotfish, Sparisoma chrysopterum, from Bermuda). AEs of phenolic-rich food by each of the three herbivore species were uniformly high, suggesting that high plant phenolic concentrations did not affect AEs in these species. This is in contrast to some temperate marine herbivores where phenolic concentrations of 10% DW have been shown to drastically reduce AE. The apparent contradiction is discussed in light of the effects of specific herbivore gut characteristics on successful herbivory of high phenolic brown algae.

Biogeographic comparisons of marine algal polyphenolics evidence against a latitudinal trend

SummaryMarine allelochemicals generally are present in greater quantity and diversity in tropical than in temperate regions. Marine algal polyphenolics have been reported as an apparent exception to this biogeographic trend, with literature values for phenolic concentrations significantly higher in temperate than in tropical brown algae. In contrast, our results, the first reported for Caribbean brown algae (orders Dictyotales and Fucales), show that many species have high phenolic levels. In addition, both our study and previous studies with north temperate and tropical species demonstrate that there is marked variation in algal phenolic levels within species from different locations. We conclude that high phenolic concentrations occur in species from both temperate and tropical regions, indicating that latitude alone is not a reasonable predictor of plant phenolic concentrations.

Effect of macrophyte secondary metabolites on feeding preferences of the herbivorous parrotfish Sparisoma radians

Experiments were conducted to test the role of secondary metabolites in determining the natural feeding preference hierarchy of the bucktooth parrotfish Sparisoma radians. The two least preferred food genera of S. radians, Halimeda and Penicillus, both contain 1,4-diacetoxy-1,3-butadiene terpenes, while the most preferred species, Thalassia testudinum, does not. Experiments with agar cylinders containing macrophyte homogenates showed that macrophyte biteability was not a factor. Instead preference could be altered by the application of the diacetoxybutadiene containing terpenes 4,9-diacetoxyudoteal and caulerpenyne or fractions or extracts containing them at naturally occurring concentrations. Concentration of the active terpenes affected the intensity of the fishs preference for the control in pairwise comparisons. Extracts and fractions which did not contain 4,9-diacetoxyudoteal or caulerpenyne did not affect fish feeding preferences at naturally occurring concentrations. Experiments in which S. radians were given no plant choice showed that coating T. testudinum with H. incrassata organic crude extract reduced the number of bites consumed and the biomass consumed to a level equivalent to that obtained for H. incrassata plants.