Photosynthetic characteristics and UV stress tolerance of Antarctic seaweeds along the depth gradient

The photosynthetic characteristics through P-E curves and the effect of UV radiation on photosynthesis (measured as rapid adjustment of photochemistry, F _v/F _m) and DNA damage (as formation of CPDs) were studied in field specimens of green, red and brown algae collected from the eulittoral and sublittoral zone of Fildes Peninsula (King George Island, Antarctic). The content of phenolic compounds (phlorotannins) and the antioxidant activity were also studied in seven brown algae from 0 to 40 m depth. The results indicated that photosynthetic efficiency (α) was high and did not vary between different species and depths, while irradiances for saturation (E _k) averaged 55 μmol m^?2 s^?1 in subtidal and 120 μmol m^?2 s^?1 in eulittoral species. The studied species exhibited notable short-term UV tolerance along the vertical zonation. In intertidal and shallow water species, decreases in F _v/F _m by UV radiation were between 0 and 18 %, while in sublittoral algae, decreases in F _v/F _m varied between 3 and 35 % relative to PAR treatment. In all species, recovery was high averaging 84–100 %. The formation of CPDs increased (15–150 %) under UV exposure, with the highest DNA damage found in some subtidal species. Phlorotannin content varied between 29 mg g^?1 DW in Ascoseira mirabilis from 8 m depth and 156 mg g^?1 DW in Desmarestia menziesii from 17 m depth. In general, phlorotannin concentrations were constitutively high in deeper sublittoral brown algae, which were correlated with higher antioxidant activities of algal extracts and low decreases in photosynthesis. UV radiation caused a strong decrease in phlorotannin content in the deep-water Himantothallus grandifolius, whereas in D. menziesii and Desmarestia anceps, induction of the synthesis of phlorotannins by UV radiation was observed. The antioxidant activity was in general less affected by UV radiation.     

Lack of Physiological Depth Patterns in Conspecifics of Endemic Antarctic Brown Algae: A Trade-Off between UV Stress Tolerance and Shade Adaptation?

A striking characteristic of endemic Antarctic brown algae is their broad vertical distribution. This feature is largely determined by the shade adaptation in order to cope with the seasonal variation in light availability. However, during spring-summer months, when light penetrates deep in the water column these organisms have to withstand high levels of solar radiation, including UV. In the present study we examine the light use characteristics in parallel to a potential for UV tolerance (measured as content of phenolic compounds, antioxidant activity and maximum quantum yield of fluorescence) in conspecific populations of four Antarctic brown algae (Ascoseira mirabilis, Desmarestia menziesii, D. anceps and Himantothallus grandifolius) distributed over a depth gradient between 5 and 30 m. The main results indicated that a) photosynthetic efficiency was uniform along the depth gradient in all the studied species, and b) short-term (6 h) exposure to UV radiation revealed a high tolerance measured as chlorophyll fluorescence, phlorotannin content and antioxidant capacity. Multivariate analysis of similarity indicated that light requirements for photosynthesis, soluble phlorotannins and antioxidant capacity are the variables determining the responses along the depth gradient in all the studied species. The suite of physiological responses of algae with a shallower distribution (A. mirabilis and D. menziesii) differed from those with deeper vertical range (D. anceps and H. grandifolius). These patterns are consistent with the underwater light penetration that defines two zones: 0–15 m, with influence of UV radiation (1% of UV-B and UV-A at 9 m and 15 m respectively) and a zone below 15 m marked by PAR incidence (1% up to 30 m). These results support the prediction that algae show a UV stress tolerance capacity along a broad depth range according to their marked shade adaptation. The high contents of phlorotannins and antioxidant potential appear to be strongly responsible for the lack of clear depth patterns in light demand characteristics and UV tolerance.     

Variation in phlorotannin content within two species of brown macroalgae (Desmarestia anceps and D. menziesii) from the Western Antarctic Peninsula

Phlorotannins are metabolites found only in the Phaeophyceae (brown algae) and have a variety of metabolic roles, including both primary (e.g. cell wall construction) and secondary (e.g. herbivore defence and UV protection). This study aimed to establish the level of variation of phlorotannins in two species of dominant Antarctic macroalgae (Desmarestia anceps and D. menziesii). Thirteen samples were taken from specific locations throughout the thallus of multiple individuals at two depths at three locations near Anvers Island, Antarctica. Overall average concentrations were 0.117±0.003 g g^–1dwt in D. anceps and 0.052±0.002 g g^–1dwt in D. menziesii. Concentrations varied greatly at the 13 sampling locations in each individual, but not in any consistent manner. Significant variation occurred between locations in both species and between depths in D. anceps.     

Induced resistance in a brown alga: phlorotannins, genotypic variation and fitness costs for the crustacean herbivore

In the marine littoral, strong grazing pressure selects for macroalgal defenses such as the constitutive and inductive production of defense metabolites. Induced defenses are expected under spatiotemporally varying grazing pressure and should be triggered by a reliable cue from herbivory, thereby reducing grazing pressure via decreased herbivore preference and/or performance. Although induced resistance has frequently been demonstrated in brown macroalgae, it is yet to be investigated whether induced macroalgal resistance shows genetic variation, a prerequisite for evolutionary responses to selection. In addition, consequences of induced resistance on herbivore performance have rarely been tested while the role of brown algal phlorotannins as inducible defense metabolites remains ambiguous. Using preference bioassays, we tested various cues, e.g., natural grazing, waterborne cues or simulated grazing to induce resistance in the brown alga Fucus vesiculosus. Further, we investigated whether there are induced responses in phlorotannin content, genetic variation in induced resistance or incurred performance costs to the mesoherbivore isopod, Idotea baltica. We found that both direct grazing and waterborne grazing cues decreased the palatability of F. vesiculosus, while increasing the total phlorotannin content. Since the sole presence of the herbivore also increased the total soluble phlorotannins, yet failed to stimulate deterrence, we concluded that phlorotannins alone do not explain increased resistance. Induced resistance varied between algal genotypes and thus showed potential for evolutionary responses to variation in grazing pressure. Induced resistance also incurred performance costs for female I. baltica via reduced egg production. Our results show that the induced resistance of F. vesiculosus decreases grazing pressure by deterring herbivores as well as impairing their performance. Resistance may be induced in advance by waterborne cues and spread effectively throughout the F. vesiculosus belt. Through lowering herbivore performance, induced resistance may also reduce future grazing pressure by decreasing the population growth rate of I. baltica.     

Induction of phlorotannin production in a brown alga: defense or resource dynamics?

Increase of phenolic secondary metabolites, phlorotannins, in brown algae due to gastropod grazing has been interpreted as an anti-herbivore adaptation. Here we tested whether such a response could be due to changes in truly available resources for the alga, not by the grazing activity of snails as such. We allowed two species of snails, Theodoxus fluviatilis and Physa fontinalis to graze on Fucus vesiculosus . These species feed on epibiota and particulate matter on the thallus but do not eat the thallus of F. vesiculosus . We further simulated snail grazing by nutrient enhancement, removal of epibiota and by a combination of the two. Manipulations of nutrient and light availability revealed the crucial role of epibiota in mediating resource availability for F. vesiculosus . Nutrient enhancement alone increased epibiota and decreased phlorotannins. Cleaning the thallus resulted in increased growth, and together with nutrient enhancement also in a trade-off with phlorotannins. Presence of T. fluviatilis on the thallus induced phlorotannin production, a response differing from the simulations of snail grazing. However, we suggest that the increase in phlorotannins may not be an induced defense but rather a consequence of a specific way of resource manipulation by this snail species. T. fluviatilis removes hyaline hairs that facilitate nutrient uptake. P. fontinalis did not remove hyaline hairs and the response of the alga to its grazing was similar to the treatment where we mechanically removed epibiota suggesting that cleaning of the thallus is the major mechanism how this snail species affects F. vesiculosus . Genetic variation in phlorotannin concentrations highly exceeded the induced responses of simulated or real snail grazing. This casts doubt for the efficiency of induced phlorotannin production to act as a defense, but is not contradictory with the interpretation of phlorotannins responding to variation in resource availability.     

EFFECTS OF UV RADIATION ON GROWTH AND PHLOROTANNINS IN FUCUS GARDNERI (PHAEOPHYCEAE) JUVENILES AND EMBRYOS1

Diminishing levels of atmospheric ozone are increasing UV stress on intertidal algae. Early developmental stages tend to be more susceptible to environmental stresses; however, little research has examined how these stages are protected from UV radiation (UVR). Many brown algae contain high levels of phlorotannins, which are thought to function in screening UVR. In this study, we tested the effects of ambient levels of UV‐B and UV‐A on growth and phlorotannin production in 1‐ to 2‐cm juvenile and microscopic postsettlement embryos of the intertidal alga Fucus gardneri Silva. Algae were grown in four light treatments: 1) ambient light; 2) under cellulose acetate, which lowered light quantity but did not affect light quality; 3) under Mylar^TM, which filtered UV‐B; and 4) under Plexiglas^TM, which blocked UV‐A and UV‐B. Over a 3‐week period, UV‐B inhibited and UV‐A enhanced the growth of F. gardneri embryos, whereas the growth of juveniles was not affected. Phlorotannin concentrations of both embryos and juveniles did not differ in any of the light treatments. Our results suggest that embryos of F. gardneri are susceptible to UV light but develop a tolerance to it as they mature. This tolerance may result from increases in phlorotannin concentrations that occur during maturation; however, phlorotannin production in embryonic or juvenile stages is either not induced by UV light or takes more than 3 weeks to occur.     

The phylogenetic position of the subantarctic marine macroalga Desmarestia chordalis (Phaeophyceae) inferred from nuclear ribosomal ITS sequences

The phylogenetic position of the subantarctic brown macroalga Desmarestia chordalis was inferred from nuclear ribosomal DNA internal transcribed spacer sequences, which were compared with published homologous sequences of putative relatives. According to the molecular data, D. chordalis is closely related to the Antarctic species D. menziesii, and more distantly related to other taxa of similar morphology, i.e. Antarctic D. anceps and northern hemisphere D. aculeata. Accepted: 2 August 1999     

Seasonal variation in the relationship between growth rate and phlorotannin production in the kelp Ecklonia radiata

Most theories for the evolution of plant chemical defences assume that defences are costly. In order to investigate the potential cost of phlorotannin production in the brown alga (kelp) Ecklonia radiata, I measured the correlation between changes in phlorotannin levels and growth rates for E. radiata in two seasons, spring and autumn. In spring, when both phlorotannin levels and growth rates in E. radiata were highest, there was a significant negative relationship between changes in phlorotannin levels and growth, consistent with a cost of phlorotannin production. No relationship was evident in autumn, indicating that moderate levels of phlorotannins and growth could be maintained simultaneously. These and related results suggest that the cost of defence will vary as a function of (1) endogenous patterns of growth or metabolite production and (2) variation in available resources. Evidence for a cost of phlorotannins in E. radiata adds to the paradox of the maintenance of high levels of phlorotannins in Australasian brown algae.     

Phlorotannins versus other factors affecting epiphyte abundance on the kelp Ecklonia radiata

We examined factors affecting the abundance and distribution of epiphytes (fouling) on the sublittoral kelp Ecklonia radiata. We first assessed the importance of phlorotannins as chemical defences against epiphytes by (a) correlating epiphyte loads on different parts of the thallus with the phlorotannin content of those tissues, and (b) experimentally testing the effects of variation in phlorotannin concentration against the settlement and growth of gametes of Ulva lactuca, a common epiphyte in the system. Tissue phlorotannin content was, at best, only weakly related to epiphyte loads, with r ² values typically <0.10. Inhibition of Ulva gametes only occurred at concentrations >10 mg l⁻¹, which is 5 orders of magnitude greater than levels of phlorotannins in the water column around beds of E. radiata, and 1–3 orders of magnitude greater than estimated levels in the boundary layer at the surface of the plant. We concluded that phlorotannins have a negligible impact on patterns of epiphytism on E. radiata, and proceeded to investigate other factors influencing the distribution and abundance of epiphytes. In our samples the relative age of different parts of the thallus was strongly correlated with epiphyte abundance, with epiphyte densities greatest on the oldest tissue and least on the youngest. Distal parts of the thalli also had greater epiphyte loads than basal parts. Field experiments in which kelp tissue was suspended at two heights in an E. radiata forest for varying lengths of time confirmed the importance of the length of time that the tissue was in the water, and its height in the water column, to the development of an epiphyte community. Comparison of epiphyte loads on tissue from primary (smooth) and secondary (rough) laminae in these experiments indicated that surface rugosity also affected fouling. Macroherbivores were rare on E. radiata, and abundances of mesofauna and epiphytes were positively related, suggesting that grazers were not important determinants of patterns of epiphyte abundance. Although phlorotannins have been previously suggested to play an important role as defences against epiphytes, we suggest that water-soluble compounds such as phlorotannins are less likely to be effective defences against epiphytes than non-polar metabolites, which can adhere to the surface of the producing organism.     

Polar extracts of the brown alga Fucus vesiculosus (L.) reduce assimilation efficiency but do not deter the herbivorous isopod Idotea baltica (Pallas)

Plant secondary metabolites provide resistance against herbivory if they are capable of decreasing herbivore performance or deterring feeding. Here we test the effects of phlorotannin-containing polar extracts from the brown alga Fucus vesiculosus on the assimilation efficiency and feeding preferences of the herbivorous isopod Idotea baltica. We test the effects separately for each sex of the isopod to test the hypothesis that the digestive efficiencies of the sexes may have diverged. Phlorotannin-containing polar extracts were incorporated into an agar-based artificial food in varying amounts. We also varied the quality of the food, using high-quality food containing animal proteins and low-quality food containing only algal material. The total assimilation efficiency as well as the assimilation efficiencies of carbon and nitrogen decreased with an increasing concentration of phlorotannin-containing extracts. The negative effects on carbon and nitrogen assimilation were higher when extracts were incorporated in the high-quality food. Females were consistently more efficient than males in all assimilation measures; there were no interactions between sex and food quality or phlorotannin concentration, suggesting that the digestive efficiency of the sexes had diverged but that the two sexes responded similarly to phlorotannin enriched artificial food. Although phlorotannin-containing extracts were harmful to performance, they did not deter feeding in preference tests. On the contrary: the isopods preferred the food containing more phlorotannins, suggesting that phlorotannins may act as cues in host recognition. Thus phlorotannins were inefficient deterrents, and I. baltica seems to be adapted to utilizing phlorotannin-containing foods despite their detrimental effects on assimilation. We suggest that F. vesiculosus provides a predation refuge and temporal constancy in availability for I. baltica, both of which select for the ability to utilize it as a food. Phlorotannin-containing polar extracts from F. vesiculosus are still harmful digestibility reducers, and may thereby prevent complete specialization and generate selection for utilization of a wider host range.     

Feeding rates of common Antarctic gammarid amphipods on ecologically important sympatric macroalgae

Single species feeding trials employing both fresh algal tissues and alginate food pellets containing dried finely ground algal tissues were conducted to examine the relative palatability of sympatric Antarctic macroalgae (three brown and five red macroalgal species) to three common herbivorous gammarid amphipods (Prostebbingia gracilis Chevreux, Gondogeneia antarctica (Chevreux) Thurston, and Metaleptamphopus pectinatus Chevreux). In fresh algal tissue bioassays, both the amphipods P. gracilis and G. antarctica consumed significantly greater amounts of the red alga Palmaria decipiens over all other seven species of macroalgae. The amphipod M. pectinatus failed to consume measurable quantities of fresh thalli of any macroalgae and therefore is likely to feed on other resources. In food pellet bioassays, the consumption rates of amphipods fed with eight different species of macroalgae were compared with consumption rates on a highly palatable control green alga. Alginate pellets containing finely ground tissues of P. decipiens were consistently the most palatable of any of the macroalgae to P. gracilis and G. antarctica, while pellets containing the brown algae Desmarestia menziesii, D. anceps and the red alga Plocamium cartilagineum were not consumed by any of the three amphipod species. Regression analysis indicated that feeding rates of the amphipods P. gracilis and G. antarctica on alginate food pellets were not significantly correlated with known species-specific parameters of macroalgal nutritional quality (%N, %C, C:N ratio, soluble protein, soluble carbohydrate, and lipid). Therefore, differences in amphipod macroalgal palatability are most likely related to other factors including physical and/or chemical deterrents.     

Distribution of macroalgae and macroalgal communities in gradients of physical conditions in Potter Cove, King George Island, Antarctica

The vegetation of a small fjord and its adjacent open shore was documented by subaquatic video. The distribution of individual species of macroalgae and the composition of assemblages were compared with gradients of light availability, hydrography, slope inclination, substratum, and exposition to turbulence and ice. The sublittoral fringe is usually abraded by winterly ice floes and devoid of large, perennial algae. Below this zone, the upper sublittoral is dominated by Desmarestia menziesii on steep rock faces, where water movements become irregular, or by Ascoseira mirabilis and Palmaria decipiens on weakly inclined slopes with steady rolling water movements. In the central sublittoral above 15 m, where turbulence is still active, Desmarestia anceps is outcompeting all other species on solid substratum, However, the species is not able to persist on loose material under these conditions. Instead, Himantothallus grandifolius may occur. Deeper, where turbulence usually is negligible, Desmarestia anceps also covers loose material. The change of dominance to Himantothallus grandifolius in the deep sublittoral cannot completely be explained at present. Himantothallus grandifolius also prevails in a mixed assemblage under the influence of grounding icebergs. Most of the smaller algae are opportunists with different degrees of tolerance for turbulence, but some apparently need more stable microhabitats and thus are dependent from continuing suppression of competitive large phaeophytes.The present work is part of the Argentinian-German cooperation program RASCALS (Research on Antarctic shallow coastal and littoral systems).     

Resistance of the brown alga Fucus vesiculosus to herbivory

Herbivory is typically intense in marine littoral environments; thus, macrophytes are expected to evolve defenses against grazing. Although putative defenses of macrophytes are widely studied, there is lack of studies demonstrating the main premises of defense adaptations: the consequences of herbivory to macrophytes, genetic variation of defense traits and the costs and benefits of defenses in natural environment. We conducted a factorial experiment, where we manipulated amount of herbivory, growing depth and nutrient availability, and measured resistance to herbivory as well as genetic variation and costs of phlorotannins, putative defensive secondary metabolites, in the brown alga Fucus vesiculosus . Herbivory on algae varied with depth: grazing did not cause losses close to the surface, but, most of the algal production was consumed at the deeper end of the algal belt. The higher the genotypic phlorotannin content the less damage the genotype received implying that phlorotannins acted as a resistance trait. Production of phlorotannins was associated with costs for growth. Consistent with the prediction that the cost of defense will be greatest when resources are limiting, the cost appeared only in the deep end of the algal belt where growth was slowed down. Phlorotannins displayed phenotypic plasticity; the three factors influenced phlorotannins interactively, with the main tendencies of nutrient enrichment decreasing and herbivory and increasing depth increasing phlorotannins. Despite this plasticity, variation of phlorotannins was mainly due to the genotype of algae. These results emphasize the role of herbivory as a selective agent for algal defenses and the importance of genetic variation in the constitutive level of phlorotannins in interactions with natural enemies. The cost of phlorotannins may constrain the evolution of resistance in environments where growth is limited by light availability.     

Acclimation to UV radiation and antioxidative defence in the endemic Antarctic brown macroalga Desmarestia anceps along a depth gradient

The endemic Antarctic brown macroalga Desmarestia anceps colonizes the subtidal between 5 and 30 m in Potter Cove on King George Island (South Shetland Islands, Antarctica). Experiments were conducted to study photosynthetic activities, antioxidative enzymes and UV tolerance of field-grown individuals with respect to the light histories along different subtidal positions. Individuals collected from the upper (5.5 m) and mid-subtidal (9.0 m) are characterized by high maximum electron transport rates (ETRmax) measured by PAM-fluorometry and high activities of superoxide dismutase (SOD) supported by considerable activities of glutathione reductase. Individuals of this species from the upper subtidal are able to tolerate high irradiances of UV-B radiation because its photosynthetic apparatus is putatively well protected by phlorotannins. In contrast, individuals from lower subtidal positions (13.5 and 15.5 m) showed an opposite trend: lower ETRmax and SOD activities as well as a lower UV tolerance of photosynthesis. Moreover, a non-denaturing polyacrylamide gel electrophoresis (native PAGE) of a partially purified crude extract reveals that D. anceps has probably six isoforms of SOD. These intra-specific patterns imply a high phenotypical plasticity of D. anceps with respect to its photosynthesis and photoprotective mechanisms. Overall, photosynthesis, UV tolerance and antioxidative potential are highly regulated in D. anceps corresponding to the respective light regimes along its natural growth sites.     

Patterns of gammaridean amphipod abundance and species composition associated with dominant subtidal macroalgae from the western Antarctic Peninsula

The communities of gammaridean amphipods associated with eight dominant macroalgal species were examined near Palmer Station, Western Antarctic Peninsula. A total of 78,415 individuals belonging to 32 amphipod taxa were identified with mean densities ranging up to 20 individuals/g algal wet wt. The most abundant amphipod taxon, Metaleptamphopus pectinatus, was found to associate predominately with the brown alga Desmarestia menziesii, while the second most common taxon, Jassa spp. occurred primarily on the red alga Gigartina skottsbergii. Non-metric multidimensional scaling analysis demonstrated that the population densities of each amphipod species and amphipod species composition were similar on the same algal species but dissimilar on different species of algae. Comparisons of amphipod communities associated with a given algal species but from different sampling sites indicated that although the structure of species-specific macroalgal-associated amphipod communities can vary across spatial scales of 3 km, 50% of the macroalgal species examined showed no significant inter-site differences in associated amphipod community structure. Spearman rank correlation analyses showed that higher abundances of amphipods occurred on the macroalgae with the highest number of branches. As many Antarctic amphipods are known consumers of macroalgae, their remarkable abundances are likely to play a significant role in mediating energy and nutrient transfer in nearshore Antarctic Peninsular macroalgal communities.     

Galactolipids rather than phlorotannins as herbivore deterrents in the brown seaweed<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis>

The first investigation of seaweed chemical defense against herbivores involved the brown seaweed Fucus vesiculosus and suggested defense via phlorotannins. The first demonstration of seaweed induction of secondary metabolites in response to herbivory also involved the genus Fucus and assumed a defensive function for phlorotannins. Many other investigations correlate herbivore feeding preference with changing levels of phlorotannins in this genus and others, but few directly test the effects of phlorotannins. No studies have assessed Fucus chemical defenses using bioassay-guided separation to investigate the complete complement of compounds deterring herbivores. We investigated the deterrence of F. vesiculosus chemical extracts using herbivore bioassays to guide our chemical investigations. Although crude extracts from F. vesiculosus strongly deterred feeding by the sea urchin Arbacia punctulata, phlorotannins from this extract did not deter feeding at 2x or 4x natural concentration by dry mass. Feeding deterrence was due to: (1) a polar galactolipid in the ethyl acetate-soluble extract, and (2) a non-phenolic compound, or compounds, in the water-soluble extract. Although this is the first evidence of galactolipids deterring herbivores, such defenses could be geographically and taxonomically widespread. The galactolipid we discovered in Fucus occurs in marine dinoflagellates, and a related metabolite that deters herbivory has recently been discovered in a tropical green seaweed. We were unable to identify the second deterrent compound, but deterrence occurred in a fraction containing carbohydrates, including sulfated sugars, but no phlorotannins. Given the polarity of these chemical deterrents, they could co-occur with and confound bioassays of phlorotannins if investigators test phlorotannin-containing algal extracts without further purification.     

Influence of light and nitrogen on the phlorotannin content of the brown seaweeds Ascophyllum nodosum and Fucus vesiculosus

Phlorotannins, C-based defence compounds in brown seaweeds, show a high degree of spatial and temporal variation within seaweed species. One important model explaining this variation is the Carbon Nutrient Balance Model (CNBM), which states that the relative supply of carbon and limiting nutrients will determine the level of defence compounds in plants. Nitrogen is often considered to be the limiting nutrient for marine macroalgal growth and the CNBM thus predicts that when the carbon:nitrogen ratio is high, photosynthetically fixed carbon will be allocated to production of phlorotannins. In the present study, we evaluated the effects of light (i.e. carbon) and nitrogen on the phlorotannin content of two intertidal brown seaweeds, Ascophyllum nodosum and Fucus vesiculosus. This was done in an observational field study, as well as in a manipulative experiment where plants from habitats with different light regimes were subjected to different nitrogen and light treatments, and their phlorotannin content was measured after 14 days. The results showed that there was a negative relationship between tissue nitrogen and phlorotannin content in natural populations of F. vesiculosus, but not in A. nodosum. In the short term, the phlorotannin content in both algal species was not affected by changes in nitrogen availability. Exposure to sunlight had a positive effect on the phlorotannin content in natural populations of both algal species but, in the manipulative experiment, only F. vesiculosus showed a rapid response to changes in light intensities. Plants subjected to sunlight contained higher phlorotannin content than shaded plants. In conclusion, the results imply that nitrogen availability explains some of the natural variation in the phlorotannin content of F. vesiculosus, but the light environment has greater importance than nitrogen availability in predicting the phlorotannin content of each species.     

Effects of nutrient enrichment and herbivory on morphology,reproduction and chemical content of Turbinaria conoides (Phaeophyceae)

The effects of nutrient enrichment and herbivory on resource allocation patterns among morphology, reproduction, and chemical content of the brown alga, Turbinaria conoides (J. Agardh) Kützing were tested in the shallow subtidal zone of the Gulf of Thailand. The field experimental design comprised 36 plots (50 × 50 cm²) with and without herbivores, and two nutrient levels. Cages (uncaged and fully caged plots) were used to exclude herbivorous fishes and two nutrients levels were achieved by experimental enrichment above ambient nutrient concentrations. For morphology of Turbinaria, the maximum length (holdfast to frond apex), the width of the base of the stipe, number of blades, holdfast and stipe diameter were measured. Biomass, reproductive output, and tissue nutrient (carbon: nitrogen: phosphorus) content of T. conoides were examined. Phlorotannin concentrations were examined using a modified Folin‐Ciocalteu method. The results showed that herbivory had no effect on morphology, reproduction, or phlorotannin concentrations. This could be due to the structural and morphological deterrents of alga, which might minimize grazing effects from herbivorous fishes. Nutrient enrichment had no effect on morphology and reproduction of T. conoides, possibly due to low nutrient demand in Turbinaria. However, nutrient enrichment did affect phlorotannin concentrations, as phlorotannins in the enriched plots were lower than the ambient plots. These results support, in part, the carbon–nutrient balance hypothesis that states algae will allocate fewer resources to production of anti‐herbivore chemical compounds when enriched with potential growth‐limiting nutrients.     

Geographical divergence in host use ability of a marine herbivore in alga–grazer interaction

When the selective environment differs geographically, local herbivore populations may diverge in their host use ability and adapt locally to exploit the sympatric host population. We tested whether populations of the marine generalist herbivore Idotea baltica have diverged in host us ability and whether they locally adapted to exploit the sympatric population of their main host species, the bladderwrack Fucus vesiculosus. We fed isopods from three local populations reciprocally with the sympatric and two allopatric populations of the host. The bladderwrack populations varied in their quality as food for isopods suggesting variation in the selective environment. The ability to exploit the main host showed considerable divergence among the isopod populations. There was no significant interaction between host and isopod origin, indicating that the patterns observed in the reciprocal feeding experiment could be explained by differences in overall suitability of the hosts and differences in overall performance of the isopod populations. Isopod population that was sympatric to a bladderwrack population with low phlorotannin content showed high performance on the algae from the sympatric but low performance on the algae from the two allopatric populations. Performance of isopods, especially in this population, decreased quickly with the increasing phlorotannin content of food algae. We therefore hypothesize that the isopods adapted to a low phlorotannin content were unable to utilize high-phlorotannin algae efficiently. Isopod populations sympatric to the high-phlorotannin bladderwrack populations may be generally better adapted to deal with phlorotannins, being thereby able to utilize a range of bladderwrack populations.