Henrik Pavia

New trends in marine chemical ecology

This essay is the outcome of a colloquium convened in November 2005 at the Benthos Laboratory of the Stazione Zoologica Anton Dohrn in Ischia, Italy, on chemical ecology and the role of secondary metabolites in the structuring and functioning of marine biodiversity. The participants of the workshop are part of the European Network of Excellence MarBEF (Marine Biodiversity and Ecosystem Function), a consortium of 56 European marine institutes to integrate and disseminate knowledge and expertise on marine biodiversity. Here we review some of the new trends and emerging topics in marine chemical ecology. The first section deals with microbial chemical interactions. Microbes communicate with each other using diffusible molecules such as N-acylhomoserine lactones (AHL). These are regulators in cell-density-dependent gene regulation (quorum sensing) controlling microbial processes. In chemical interactions with higher organisms, microbes can act either as harmful pathogens that are repelled by the host’s chemical defense or as beneficial symbionts. These symbionts are sometimes the true producers of the host’s secondary metabolites that have defensive and protective functions for their hosts. We also describe how allelochemicals can shape phytoplankton communities by regulating competition for available resources, and also interactions among individuals of the same species. Compounds such as the diatom-derived unsaturated aldehydes have been demonstrated to act as info chemicals, and they possibly function as a diffusible bloom-termination signal that triggers an active cell death and bloom termination at sea. The same molecules have also been shown to interfere with the reproductive capacity of grazing animals deterring future generations of potential predators. Such compounds differ from those that act as feeding deterrents since they do not target the predator but its offspring. Many of the neurotoxins produced by dinoflagellates act as feeding deterrents, and laboratory experiments have shown that ingestion of these algae by some microzooplankton and macrozooplankton can cause acute, responses such as death, incapacitation, altered swimming behavior, and reduced fecundity and egg-hatching success. These effects may rarely occur in nature because of low individual grazing rates on dinoflagellate cells and grazing on other food sources such as microflagellates and diatoms. We also consider the nutritional component of marine plant-herbivore interactions, especially in the plankton, and the information available on the effects of growing conditions of algae on the production of toxic metabolites. Species producing saxitoxins seem to consistently produce the highest amounts of toxins (on a per cell basis) in the exponential phase of growth, and there is a decrease in their production under nitrogen, but not under phosphorus stress, where the production actually increases. We try to explain the circumstances under which organisms defend themselves chemically and argue that the most likely explanatory model for the production of secondary metabolites used for defense in planktonic organisms is the carbon nutrient balance hypothesis, which predicts that most algae produce their toxins mainly under conditions where carbon is in excess and nitrogen (or other nutrients) is limiting. We also discuss chemically mediated macroalgal-herbivore interactions in the benthos and the large variation in concentration of seaweed defense metabolites at different spatial and temporal scales. Seaweeds have been shown to produce a large variety of secondary metabolites with highly variable chemical structures such as terpenoids, acetogenins, amino acid derivates, and polyphenols. Many of these compounds probably have multiple simultaneous functions for the seaweeds and can act as allelopathic, antimicrobial, and antifouling or ultraviolet-screening agents, as well as herbivore deterrents. We also provide examples of interactions between marine benthic invertebrates, especially sponges, molluscs, and cnidarians, that are mediated by specific secondary metabolites and discuss the role of these in shaping benthic communities.

OPTIMAL DEFENSE THEORY: ELASTICITY ANALYSIS AS A TOOL TO PREDICT INTRAPLANT VARIATION IN DEFENSES

Optimal defense theory (ODT) predicts that plants should have the highest defense levels in parts that have the highest value in terms of fitness. Assumptions about differences in fitness value among plant parts have previously been based on general reasoning, e.g., that reproductive tissue is more valuable than vegetative tissue since fitness ultimately depends on reproduction. Here we present a first example of how demographic elasticity (proportional sensitivity) analysis can be used as a tool to generate more objective and specific estimates of the fitness value of different plant parts, based on the entire life cycle. These estimates were further used to generate testable predictions about intraplant variation in defenses of the brown seaweed Ascophyllum nodosum (knotted wrack). The predictions were clearly supported by the results from both analyses of chemical defense levels (phlorotannins) and bioassays (feeding preference experiments), which showed that reproductive tissues had the lowest defense level of the three examined tissue types, in contrast to previous assumptions. We suggest that a stronger focus on variation in life history among species and populations, with demographic perturbation analysis as one important tool, could lead to a better understanding of intraplant variation in defense levels, in seaweeds as well as vascular plants and other modular organisms.

Maximum species richness at intermediate frequencies of disturbance : Consistency among levels of productivity

Development of a mechanistic understanding and predictions of patterns of biodiversity is a central theme in ecology. One of the most influential theories, the intermediate disturbance hypothesis (IDH), predicts maximum diversity at intermediate levels of disturbance frequency. The dynamic equilibrium model (DEM), an extension of the IDH, predicts that the level of productivity determines at what frequency of disturbance maximum diversity occurs. To test, and contrast, the predictions of these two models, a field experiment on marine hard-substratum assemblages was conducted with seven levels of disturbance frequency and three levels of nutrient availability. Consistent with the IDH, maximum diversity, measured as species richness, was observed at an intermediate frequency of disturbance. Despite documented effects on productivity, the relationship between disturbance and diversity was not altered by the nutrient treatments. Thus, in this system the DEM did not improve the understanding of patterns of diversity compared to the IDH. Furthermore, it is suggested that careful consideration of measurements and practical definitions of productivity in natural assemblages is necessary for a rigorous test of the DEM.

Increased chemical resistance explains low herbivore colonization of introduced seaweed

The success of introduced species is often attributed to release from co-evolved enemies in the new range and a subsequent decreased allocation to defense (EICA), but these hypotheses have rarely been evaluated for systems with low host-specificity of enemies. Here, we compare herbivore utilization of the brown seaweed, Fucusevanescens, and its coexisting competitors both in its native and new ranges, to test certain predictions derived from these hypotheses in a system dominated by generalist herbivores. While F. evanescens was shown to be a preferred host in its native range, invading populations supported a less diverse herbivore fauna and it was less preferred in laboratory choice experiments with important herbivores, when compared to co-occurring seaweeds. These results are consistent with the enemy release hypothesis, despite the fact that the herbivore communities in both regions were mainly composed of generalist species. However, in contrast to the prediction of EICA, analysis of anti-grazing compounds indicated a higher allocation to defense in introduced compared to native F.evanescens. The results suggest that the invader is subjected to less intense enemy control in the new range, but that this is due to an increased allocation to defense rather than release from specialized herbivores. This indicates that increased resistance to herbivory might be an important strategy for invasion success in systems dominated by generalist herbivores.

INDUCIBLE AND CONSTITUTIVE DEFENSES OF VALUABLE SEAWEED TISSUES: CONSEQUENCES FOR HERBIVORE FITNESS

Optimal Defense Theory predicts that plants exposed to herbivory should allocate more resources to produce costly secondary metabolites in tissues with higher fitness values. To increase plant resistance, the secondary metabolites must have a negative impact on the preference and/or performance of herbivores. We tested the hypotheses that induction of secondary metabolites (phlorotannins) in a brown seaweed in response to grazing by herbivorous gastropods will differ between seaweed tissues with different fitness values (basal stipes and annual shoots), and that the subsequent change in food value will affect the fitness (growth and fecundity) of the gastropods. Induction of phlorotannins was significant in both tissue types but was more pronounced in basal stipes, which have a higher fitness value. Basal tissues also had significantly higher constitutive defense levels than did apical tissues. No effects of algal tissue type or grazing history on the growth rate of the gastropods were detected. However, the number of viable eggs was significantly lower for gastropods feeding on basal shoots, and there was a significantly lower proportion of viable eggs produced by gastropods that were offered previously grazed seaweed tissues. The results show that induced resistance, and its variation among different plant parts, can have significant negative effects on herbivore performance that may reduce future herbivore pressure and thus enhance plant fitness.

Removal of Dissolved Brown Algal Phlorotannins Using Insoluble Polyvinylpolypyrrolidone (PVPP)

Tannins, a large and diverse group of phenolic secondary metabolites, are common in terrestrial plants and marine brown algae. It is sometimes desirable to remove the tannins from plant or algal extracts, e.g., when isolating enzymes and nucleic acids, when using certain colorimetric methods to quantify the tannin content, or to create reliable controls when using tannins in experimental studies. Insoluble polyvinylpolypyrrolidone (PVPP) can be used to specifically remove tannins from solution. In the present study, we evaluated the effect of different factors (amount of PVPP, number of PVPP treatments, type of solvent, pH, and incubation time) on the PVPP removal of dissolved brown algal phlorotannins. Our results imply that there is a limited amount of phlorotannins that can bind to a given amount of PVPP, and that it is preferable to use low quantities of PVPP repeatedly, compared to using fewer treatments with a high amount of PVPP. Furthermore, we found no consistent effect on the removal of phlorotannins due to solvent type (acetone, methanol, distilled water or filtered seawater). There was a slight decrease in the amount of phlorotannins removed from extracts with increasing pH when repeatedly treated with PVPP. All phlorotannins were removed from extracts with pH ≤6.2, and 89% of the initial phlorotannin content was removed at pH 9.7. These results are compared with previous methodological studies on tannin removal with PVPP. Furthermore, the implications of phlorotannin removal in analytical and ecological investigations are discussed.

Induction of toxin production in dinoflagellates: the grazer makes a difference

The dinoflagellate Alexandrium minutum has previously been shown to produce paralytic shellfish toxins (PST) in response to waterborne cues from the copepod Acartia tonsa. In order to investigate if grazer-induced toxin production is a general or grazer-specific response of A. minutum to calanoid copepods, we exposed two strains of A. minutum to waterborne cues from three other species of calanoid copepods, Acartia clausi, Centropages typicus and Pseudocalanus sp. Both A. minutum strains responded to waterborne cues from Centropages and Acartia with significantly increased cell-specific toxicity. Waterborne cues from Centropages caused the strongest response in the A. minutum cells, with 5 to >20 times higher toxin concentrations compared to controls. In contrast, neither of the A. minutum strains responded with significantly increased toxicity to waterborne cues from Pseudocalanus. The absolute increase in PST content was proportional to the intrinsic toxicity of the different A. minutum strains that were used. The results show that grazer-induced PST production is a grazer-specific response in A. minutum, and its potential ecological importance will thus depend on the composition of the zooplankton community, as well as the intrinsic toxin-producing properties of the A. minutum population.

Novel chemical weapon of an exotic macroalga inhibits recruitment of native competitors in the invaded range

Summary 1. Allelopathy is an important non-resource interaction in terrestrial plant communities that may affect invasions by non-indigenous plants. The ‘novel weapons hypothesis’ (NWH) predicts that non-indigenous plants will become invasive if they have allelopathic compounds that assemblages in the new range are not adapted to. Recently, the non-indigenous, chemically rich macroalga Bonnemaisonia hamifera (Hariot) has become one of the most abundant filamentous red algae in Scandinavian waters. 2. We used B. hamifera to specifically test the aspect of the NWH that concerns invasion success based on novel allelochemicals in the invaded range. Allelopathic interactions were tested through effects on the growth rate of adult native macroalgae in co-cultures with B. hamifera and through the settlement success of native macroalgal propagules and microalgae on surfaces coated with 1,1,3,3-tetrabromo-2-heptanone. We also investigated whether 1,1,3,3-tetrabromo-2-heptanone can be transferred from B. hamifera to its native host algae, as a means of pre-emptive competition. 3. The settlement of native macroalgal propagules and microalgae was strongly inhibited on surfaces coated with 1,1,3,3-tetrabromo-2-heptanone at ecologically relevant concentrations, but there were no effects of adult B. hamifera on growth rates of adults of the six native naturally co-occurring species. The compound was shown to be transferred from B. hamifera to the surface of its native host algae at inhibitory concentrations in both laboratory and field experiments. 4. By inhibiting the settlement of propagules on its thallus and on surrounding surfaces, B. hamifera achieves a competitive advantage over native macroalgae, a finding that parallels previous reports on soil- and litter-mediated allelopathic interactions among vascular plants. Because competition for available substrata in marine benthic systems is intense, the ability to reserve space may be vital for B. hamifera’s successful invasion. This is the first example of an allelopathic compound that can be transferred by direct contact from an exotic to a native species, with an active and unaltered function. 5. Synthesis. Our results clearly show that the main secondary metabolite of the invasive red alga B. hamifera has strong allelopathic effects towards native competitors, suggesting that its novel chemical weapon is important for the highly successful invasion of new ranges.

Intraplant habitat and feeding preference of two gastropod herbivores inhabiting the kelp Laminaria hyperborea

The habitat and feeding preference of two gastropod mesoherbivore species, Ansates ( Helcion ) pellucida and Lacuna vincta , inhabiting the kelp Laminaria hyperborea were investigated by studying the distribution, habitat choice and food preference of the herbivores between new and old kelp fronds. Nitrogen content and chemical defence (phlorotannin) level of the kelp fronds were measured in order to determine whether they were correlated with the feeding preference of the herbivores. The habitat choice of Lacuna vincta was correlated with the food preference of this species, while the habitat choice of A. pellucida probably was due to some other factor, e.g. competition or mortality. Ansates pellucida preferred to reside on the old fronds of Laminaria hyperborea , but consumed an equal amount of both tissue types. Lacuna vincta , however, preferred the new Laminaria hyperborea fronds both as habitat and food. There was a large variation in the phlorotannin content among the tissue samples of L. hyperborea . However, there were no overall statistically significant differences in tissue nitrogen or phlorotannin content between the new and the old L. hyperborea fronds.

Physical and biological disturbances interact differently with productivity: effects on floral and faunal richness.

Physical and biological disturbances are ecological processes affecting patterns in biodiversity at a range of scales in a variety of terrestrial and aquatic systems. Theoretical and empirical evidence suggest that effects of disturbance on diversity differ qualitatively and quantitatively, depending on levels of productivity (e.g., the dynamic equilibrium model). In this study we contrasted the interactive effects between physical disturbance and productivity to those between biological disturbance and productivity. Furthermore, to evaluate how these effects varied among different components of marine hard-substratum assemblages, analyses were done separately on algal and invertebrate richness, as well as richness of the whole assemblage. Physical disturbance (wave action) was simulated at five distinct frequencies, while biological disturbance (grazing periwinkles) was manipulated as present or absent, and productivity was manipulated as high or ambient. Uni- and multivariate analyses both showed significant effects of physical disturbance and interactive effects between biological disturbance and productivity on the composition of assemblages and total species richness. Algal richness was significantly affected by productivity and biological disturbance, whereas invertebrate richness was affected by physical disturbance only. Thus, we show, for the first time, that biological disturbance and physical disturbance interact differently with productivity, because these two types of disturbances affect different components of assemblages. These patterns might be explained by differences in the distribution (i.e., press vs. pulse) and degree of selectivity between disturbances. Because different types of disturbance can affect different components of assemblages, general ecological models will benefit from using natural diverse communities, and studies concerned with particular subsets of assemblages may be misleading. In conclusion, this study shows that the outcome of experiments on effects of disturbance and productivity on diversity is greatly influenced by the composition of the assemblage under study, as well as on the type of disturbance that is used as an experimental treatment.