Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllum nodosum

Plants have different strategies to cope with herbivory, including induction of chemical defences and compensatory growth. The most favourable strategy for an individual plant may depend on the density at which the plants are growing and on the availability of nutrients, but this has not been tested previously for marine plant–herbivore interactions. We investigated the separate and interactive effects of plant density, nutrient availability, and herbivore grazing on the phlorotannin (polyphenolic) production in the brown seaweed Ascophyllum nodosum. Seaweed plants grown at low or high densities were exposed either to nutrient enrichment, herbivorous littorinid gastropods (Littorina obtusata), or a combination of nutrients and herbivores in an outdoor mesocosm experiment for 2 weeks. Seaweeds grown at a low density tended to have higher tissue nitrogen content compared to plants grown at a high density when exposed to elevated nutrient levels, indicating that there was a density dependent competition for nitrogen. Herbivore grazing induced a higher phlorotannin content in plants grown under ambient, but not enriched, nutrient levels, indicting either that phlorotannin plasticity is more costly when nutrients are abundant or that plants responded to herbivory by compensatory growth. However, there were no significant interactive or main effects of plant density on the seaweed phlorotannin content. The results indicate that plants in both high and low densities induce chemical defence, and that eutrophication may have indirect effects on marine plant–herbivore interactions through alterations of plant chemical defence allocation.     

Interactive effects of UV radiation and enhanced temperature on photosynthesis, phlorotannin induction and antioxidant activities of two sub-Antarctic brown algae

Lessonia nigrescens and Durvillaea antarctica, two large sub-Antarctic brown algae from the southern Chilean coast, were exposed to solar UV radiation in an outdoor system during a summer day (for 11 h) as well as to artificial UV radiation under controlled laboratory conditions at two temperatures (15 and 20 °C) for 72 h. Chlorophyll a fluorescence–based photoinhibition of photosynthesis was measured during the outdoor exposure, while electron transport rates, lipid peroxidation, antioxidant activity and content of phlorotannins were determined at different time intervals during the laboratory exposure. Under natural solar irradiances in summer, both species displayed well-developed dynamic photoinhibition: F _v/F _m values decreased by 70 % at noon coinciding with the levels of PAR >1,500 μmol m^?2 s^?1 and UV-B radiation >1 W m^?2 and recovered substantially in the afternoon. In treatments including UV radiation, recovery in D. antarctica started already during the highest irradiances at noon. The results from laboratory exposures revealed that (a) elevated temperature of 20 °C exacerbated the detrimental effects of UV radiation on photochemical parameters (F _v/F _m and ETR); (b) peroxidative damage measured as MDA formation occurred rapidly and was strongly correlated with the decrease in F _v/F _m, especially at elevated temperature of 20 °C; (c) the antioxidant activity and increases in soluble phlorotannins were positively correlated mainly in response to UV radiation; (d) phlorotannins were rapidly induced but strongly impaired at 20 °C. In general, short-term (2–6 h) exposures to enhanced UV radiation and temperature were effective to activate the photochemical and biochemical defenses against oxidative stress, and they continued operative during 72 h, a time span clearly exceeding the tidal or diurnal period. Furthermore, when algae were exposed to dim light and control temperature of 15 °C for 6 h, F _v/F _m increased and lipid peroxidation decreased, indicating consistently that algae retained their ability for recovery. D. antarctica was the most sensitive species to elevated temperature for prolonged periods in the laboratory. Although no conclusive evidence for the effect of the buoyancy of fronds was found, the interspecific discrepancies in thermo-sensitivity in the UV responses found in this study are consistent with various ecological and biogeographical differences described for these species.     

Light quality and concentration of proteins,RNA, DNA and photosynthetic pigments in two species of marine plankton algae

Chlorophyll, protein, DNA and RNA concentrations in cultures ofCyclotella nana (Hustedt) andDunaliella tertiolecta (Butcher) were higher in blue light and lower in green light than in white light of the same intensity. Total carotenoid concentrations were highest in green light. Total pigment concentrations were highest in cells grown in blue light, lowest in those from green light. The relative changes in pigment concentrations associated with differences in spectral composition of the radiation are comparable to those often observed in phytoplankton from various depths in stratified natural water. Light adaptation in algae as a response to light quality is suggested. The chemical composition of plankton algae can be expected to vary considerably with depth.     

Light quality in relation to growth,photosynthetic rates and carbon metabolism in two species of marine plankton algae

The effect of light quality on growth, photosynthesis and carbon metabolism in two species of marine algae,Cyclotella nana (Hustedt) andDunaliella tertiolecta (Butcher), was examined. Relative growth constants forC. nana were 0.37, 0.29 and 0.25 in blue, white and green light, respectively. Corresponding constants were 0.41, 0.31 and 0.29 forD. tertiolecta. Photosynthetic rates in both species were higher in blue light and lower in green light compared with white light of the same intensity. More than 60% of¹⁴C assimilated byC. nana orD. tertiolecta grown in blue or green light was incorporated into the ethanol-insoluble fraction, compared with 10 to 30% in this fraction in white light. The relative importance of the various components within this fraction was independent of light quality. Although less¹⁴C was assimilated into the ethanol-soluble fraction in blue or green light, there was a relative increase in some amino acids and organic acids in this fraction and a decrease in sugars and sugar phosphates relative to white light of the same intensity. These differences were independent of light intensity, photosynthetic rate and cell density in the cultures.     

Relationship between larval and juvenile growth rates in two marine gastropods,Crepidula plana and C. fornicata

Previous studies on various marine mollusc species have shown that both larval and juvenile growth rates are substantially heritable, but few workers have examined the extent to which larval and juvenile growth rates covary. We examined the relationship between larval and juvenile growth rates in seven laboratory experiments conducted between 1986 and 1993, using the prosobranch gastropods Crepidula plana Say and C. fornicata (L.). In most experiments larvae were reared individually, measured twice nondestructively to determine larval grwoth rate, allowed or stimulated (daily 5-h exposure to 20 mM excess K⁺ in seawater) to metamophose, and then measured at least twice after metamorphosis to determine juvenile growth rates. Generally, there was no significant (p >0.10) relationship between larval and juvenile growth rates, suggesting that in these two species selection can act independently on the two stages of development. A positive correlation (p=0.007) between larval and juvenile growth rates was observed for C. fornicata in one experiment, but only for offspring from females maturing the most rapidly in laboratory culture. Even for these larvae, however, variation in larval growth rate explained<2% of the variation in juvenile growth rate, so that larval and juvenile growth rates are at most only weakly associated in this species.     

Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.     

Interactions between nutritional status and long-term responses to ultraviolet-B radiation stress in a marine diatom

Influences of nutritional status on the photoinhibitory effects of ultraviolet-B radiation (UVBR: 290 to 320 nm) on the specific growth rates (_obs) and biomass of Phaeodactylum tricornutum were determined using nutrient-replete batch cultures and nutrient-limited continuous cultures. P. tricornutum cultures were exposed to UVBR doses representative of current mid-latitude and ozone-depletion intensities. Specific growth rates and biomass were inhibited from 2 to 16% by UVBR during nutrient-replete growth. However, no effect of UVBR on _obs or biomass was detectable when nutrient limitation exceeded the potential for limitation by UVBR. Thus, a competitive interaction appears to occur between macronutrient stress and UVBR stress, such that _obs and biomass will be determined by the most limiting factor. Our results suggest that measurable decreases in phytoplankton _obs and biomass from UVBR are most likely in nutrient-rich areas of the ocean, while these parameters may not be appropriate for measuring UVBR stress in regions of nutrient limitation.     

Effects of nitrogen deficiency on nitrate reductase,nitrate assimilation and photosynthesis in unicellular marine algae

Nitrate reductase (NR) activity appeared in ammoniumgrown cultures of 5 species of marine algae, representing 4 classes, after a short period of nitrogen starvation. In nitrogen-limited chemostat cultures of Nannochloropsis oculata and Chlorella stigmatophora there was an inhibition of photosynthetic carbon fixation during nitrate assimilation. In these organisms, nitrate assimilation was light-dependent and inhibited by 3-(3′,4′-dichloro-)-1-1-dimethyl urea (DCMU). In N. oculata, an obligate autotroph, nitrite assimilation was dependent on light absolutely. Physiological changes that occur in these organisms during nitrogen deficiency enable them to assimilate nitrogen rapidly when it becomes available.     

Relationship between the position preference and nutritional state of individuals in schools of juvenile roach (Rutilus rutilus)

Summary Position preferences of well-fed and food-deprived juvenile roach were investigated in schools of 2 and 4 fish in the laboratory. Food-deprived fish appeared significantly more often in the front position than their well-fed conspecifics. For fish at the same hunger level, individuals at the front of the school had the highest feeding rate. These results represent the first evidence for a relationship between the nutritional state of individual fish and their positions in a school and suggest a functional advantage of the preference. Offprint requests to: J. Krause at the present address     

Pattern of distribution of selected trace elements in the marine brown alga,Sargassum filipendula Ag. from Sri Lanka

Baseline concentrations together with biological variations of 29 trace elements (Ag, As, Au, Ba, Br, Cd, Ce, Co, Cr, Cs, Eu, Fe, Hf, I, Mn, Mo, Ni, Pb, Rb, Sb, Sc, Se, Sm, Sr, Tb, Th, Yb, Zn and Zr) were investigated in the brown alga, Sargassum filipendula collected from the western coast of Sri Lanka. Several elements (Co, Cr, Fe, Hf, Ni, Sc, Se, Th, Zr and the rare earth elements) were found to be enriched in S. filipendula compared to NIES No. 9 Sargasso reference material. Concentration of strontium in S. filipendula was highest at all sites. Chemical abundance of the rare earth elements decreased approximately linearly with increasing atomic numbers. The pattern of elemental distribution appears to be due to the fact that S. filipendula seems capable of concentrating high levels of trace elements under conditions of their very low availability in sea water. Concentration factors for elements in S. filipendula lie in a higher range compared with those reported in the literaure for brown algae.     

The accumulation levels of heavy metals (Ni,Cr, Sr, & Ag) in marine algae from southwest coast of India

In the context of use of marine algae as biological indicators of heavy metal pollution in coastal waters, six species of marine algae collected from the southwest coast of India were analysed for the levels of heavy metals (Ni, Cr, Sr, and Ag). Interspecies and interclass variations were determined on a spatial and temporal scale. The metal contents varied in the ranges, Ni: 0.20–21.06?µg?g^?1 (mean?=?10.13?µg?g^?1), Cr: non-detectable level (ND)–37.18?µg?g^?1 (mean?=?13.86?µg?g^?1), Sr: 2.19–103.90?µg?g^?1 (mean =?29.40?µg?g^?1), and Ag: ND–6.39?µg?g^?1 (mean?=?1.80?µg?g^?1). Ni and Cr contents were of similar magnitude to those reported for algae from polluted areas.     

Copepod nutritional condition and pelagic production during autumn in Kosterfjorden,western Sweden

The in situ grazing rate and nutritional condition of copepods were studied during October/November 1985, by analyzing gut fluorescence (feeding), body size and lipid composition (nutritional state), and electron transport system (ETS) activity (respiration rate) of copepods from surface-and deep-water in Kosterfjorden on the Swedish west coast. These parameters were related to the physical and biological environment, as defined by light, hydrography, autotrophic and bacterial production and seston in the water column. The results show a gradual build-up of the autumn phytoplankton bloom in the uppermost meters, with a peak in total autotrophic production in mid October of ca 550 mg C m^–2 d^–1, and a bacterial net production corresponding to 15% of this. Phytoplankton exudates made up, on average, 47% of the primary production and more than 50% of this was utilized by the bacteria. Copepods occurring in the surface-water exhibited grazing rates corresponding to between 11 and 18% of their body C d^–1 and potential growth rates of 0 to 9% d^–1. Copepod populations in the surface water were composed of individuals with higher average body-weight and lower lipid-proportion than those from the deep-water.Calanus finmarchicus in the deep-water showed characters indicating diapause condition, while this was not observed forAcartia clausi. Differences in lipid content and composition indicate thatC. finmarchicus, Pseudocalanus sp. andA. clausi represent three successive points on a scale of tolerance for fluctuations in the food environment. Adult femaleMetridia longa was the only one among seven species/stages of copepods in the deep-water ( 50 m depth) that contained phytoplankton pigments.Study performed through Tjärnö Marine Biological Laboratory, University of Göteborg, S-452 00 Strömstad, Sweden     

Feeding preferences and the relationships between food choice and assimilation efficiency in the herbivorous marine snail Lithopoma undosum (Turbinidae)

Preference rankings for 13 macrophytes were established for the subtidal herbivorous snail Lithopoma undosum using two-choice laboratory experiments and consumption rates. L. undosum did not discriminate among three kelp foods (Egregia menziesii, Eisenia arborea and Macrocystis pyrifera) but ate kelp preferentially and more rapidly over all but Ulva spp. among tested macrophytes. Secondary preferences were established for the red alga Pterocladiella capillacea, followed by the coralline Lithothrix aspergillum, whereas the brown seaweeds Zonaria farlowii and Halidrys dioica and the seagrass Phyllospadix torreyi were the least preferred macrophytes. Fastest consumption rates (1.91 g day⁻¹) were measured in trials consisting only of kelp foods. These results indicate that L. undosum exhibits clear feeding preferences even when given less-preferred, non-kelp macrophytes. Using an ash-marker technique, we determined total organic, carbon, and nitrogen assimilation efficiencies (AE%) for six macroalgae used in preference trials. Tested macrophytes were assimilated at different efficiencies but a pattern was not detected between AE (%) and a macrophyte’s position in L. undosum’s preference hierarchy. Highest total organic AEs were found for P. capillacea (61.2%) and H. dioica (59.4%); lowest AEs were detected for E. menziesii (34.9%), a preferred dietary item. Nitrogen was assimilated from red algae with higher efficiencies (74.9–84.3%) than from brown or green algae. These data suggest that the digestive capabilities of L. undosum are better suited for assimilating organic material and nitrogen from less-preferred, non-kelp foods. This supports the hypothesis that factors besides nutritional composition and digestive optimization have played a role in the evolution of feeding preferences in L. undosum and probably other herbivorous snails associated with northeastern Pacific kelp beds.     

Photosynthesis and carbon storage between tides in a brown alga, Fucus vesiculosus

Intertidal macroalgae may spend a significant part of their lives in air. During photosynthesis in air, they encounter much lower concentrations of inorganic carbon than in seawater. Because they accumulate inorganic carbon from seawater, we investigated whether they similarly accumulate it from air. We measured photosynthesis in the intertidal species Fucus vesiculosus L. during 1990 and 1991 with a gas-phase O₂ electrode or CO₂-exchange apparatus in air and with a liquid-phase O₂ electrode in seawater. Maximum rates were rapid and similar in air and seawater regardless of the method. Tissue from seawater could carry on photosynthesis in CO₂-free air, indicating that carbon was stored in the tissue. After 2 h, this store was depleted and photosynthesis ceased. Supplying CO₂ in air replenished the store. Under identical conditions, terrestrial C₃ and C₄ species showed no evidence of this store, but a CAM (crassulacean acid metabolism) species did. However, in contrast to the CAM behavior, F. vesiculosus did not store CO₂ significantly in the dark. We found a small acid-releasable pool of carbon in the tissue that disappeared as photosynthesis depleted the carbon store. However, the pool was too small to account for the total carbon stored. While CO₂ was being acquired or released from the store in the light, photosynthesis was not inhibited by 21% O₂. These results indicate that there are two parallel paths for the supply of CO₂ to photosynthesis. The first depends on inorganic carbon in seawater or in air and supports rapid photosynthesis. The second involves CO₂ slowly released from an organic intermediate. The release protects CO₂ fixation from the inhibitory effects of 21% O₂. Photosynthesis in F. vesiculosus thus appears to be C₃-like in its rapid fixation of CO₂ from a small inorganic pool into phosphoglycerate. However, it is C₄-like in its pre-fixation of carbon in an organic pool in the light, and is CAM-like in its ability to slowly use this pool as a sole source of CO₂. The organic pool may serve to protect photosynthetic CO₂ fixation against the inhibitory effects of O₂ in air and in the boundary layer in seawater. Received: 6 March 1998 / Accepted: 16 October 1998     

Diet and food preferences of the adult horseshoe crab Limulus polyphemus in Delaware Bay,New Jersey,USA

Adult horseshoe crabs Limulus polyphemus (L.) feed on a wide variety of infaunal and epifaunal invertebrates during their spring spawning migration in Delaware Bay, New Jersey, USA. Comparison of the gut contents with estimates of available prey showed that the most abundant potential prey item, the bivalve Gemma gemma, was avoided. The thinner shelled but comparatively scarce clam Mulinia lateralis was a preferred prey item. In the laboratory, crabs fed on G. gemma when it was the only available item but not when M. lateralis or soft-shell clams, Mya arenaria, were offered in conjunction. Large M. lateralis (>10mm) were preferred to small M. lateralis; there was no discrimination between M. lateralis and M. arenaria of the same size. Male and female horseshoe crabs had similar gut contents and laboratory feeding preferences, despite the fact that females are larger than males. Crabs spawning later in the summer contained more food than did crabs collected at the peak of spawning activity.     

Differences in embryo production between sympatric species of mysids (family Mysidae) in the shallow coastal waters off Vancouver Island, BC

The ecological importance of mysid (Crustacea: Mysidacea) populations in coastal food webs is not well understood. Although the 10 or more species of epibenthic mysids found in Clayoquot Sound, BC, Canada, form the primary prey resource for seasonally abundant gray whales (Eschrichtius robustus), little is known about their life history. Here, Holmesmysis sculpta dominates multi-species swarms, however there are several potential routes to this state, one of which is higher embryo production. Reproductive capacity is key to the resiliency of local mysid populations and species diversity, in this study I compare one aspect, brood size for the four most commonly found species, H. sculpta, Neomysis rayii, Exacanthomysis davisi, and Columbiaemysis ignota. The number of embryos per brood was found to vary significantly between species; however, individual length is a stronger determinant of brood size. Here, I report previously unknown life history attributes of coastal mysid species, with important consequences for community structure and local marine food webs.     

The effect of heavy metals on photosynthesis and loss of cell potassium in two species of marine algae,Dunaliella tertiolecta and Phaeodactylum tricornutum

Observations on sixth-stage nauplius (pre-cyprid) of the barnacle Balanus eberneus Gould treated with L-proline-3,4-H³ suggest that the cement gland in this stage is a functional organ with considerable biosynthetic activity; the activity of the cement glands in the cyprid similarly treated is relatively low, ceasing gradually during, and completely after, metamorphosis. In the early phases of this stage, the glands become highly vacuolated (degenerate), and disappear completely in 3 days. The origin of the cement apparatus in the newly metamorphosed barnacle is discussed.     

Biomass partitioning of benthic microbes in a Baltic inlet: relationships between bacteria, algae, heterotrophic flagellates and ciliates

 The structure of a benthic microbial food web and its seasonal changes were studied in the shallow brackish waters of the island of Hiddensee, northeastern Germany, at two sites in close proximity by monthly or bimonthly sampling from July 1995 to June 1996. Abundance and biomass of phototrophic and non-phototrophic bacteria, heterotrophic flagellates (HF) and ciliates as well as the biomass of microphytobenthos were determined in the upper 0.3 cm sediment layer. Abundance of organisms showed strong positive correlation with water temperature, with the exception of the bacteria. Non-phototrophic bacterial numbers ranged from 7 × 10⁸ to 6.7 × 10⁹ cells cm⁻³ and phototrophic bacterial abundance from 4 × 10⁷ to 2.7 × 10⁸. Heterotrophic protist abundance ranged from 8 × 10³ to 104 × 10³ ind cm⁻³ for HF and from 39 to 747 ind cm⁻³ for ciliates. The biomass partitioning demonstrated the primary importance of non-phototrophic bacteria (min. 0.83, max. 84.87 μg C cm⁻³), followed by the microphytobenthos (min. 1.32, max. 50.93 μg C cm⁻³). The heterotrophic protists contributed roughly the same fraction to the total microbial biomass, with the biomass of the HF being slightly higher (HF 0.23 to 1.76 μg C cm⁻³, ciliates 0.04 to 1.17 μg C cm⁻³). Taxonomic classification of the benthic HF revealed the euglenids to be the most important group in terms of abundance and biomass, followed by thaumatomastigids and kinetoplastids. Other important groups were apusomonads, cercomonads, pedinellids and cryptomonads. The structure of the HF assemblage showed strong seasonal changes with euglenids being the most abundant taxa in summer, while apusomonads and thaumatomastigids were predominant in winter. Similar to the pelagic microbial food web, benthic picophototrophic bacteria were occasionally abundant, and the feeding modes of heterotrophic protists exhibited a great variety (predominantly omnivores, bacterivores, herbivores or predators). Filter-feeding HF were of little importance. Contrary to the pelagic environment, a top-down control on total benthic bacterial numbers by HF seemed unlikely at the studied stations which were characterised by muddy sand. Received: 6 January 1999 / Accepted: 21 October 1999     

Chemical defenses, nutritional quality, and structural components in three sponge species: Ircinia felix, I. campana, and Aplysina fulva

Allocating chemical defenses to regions or tissues most at risk for predatory attack may provide protection while simultaneously minimizing associated metabolic costs. Chemical defense allocation patterns were investigated in the aspiculate sponges Ircinia felix, I. campana, and Aplysina fulva collected between July 2005 and April 2006 from J Reef off the coast of Georgia, U.S.A. It was predicted that chemical defenses would be (1) higher in the outermost 2 mm layer of the sponge; (2) positively correlated with tissue nutritional quality; and (3) correlated with structural components such as spongin fibers. Whereas defensive chemicals were concentrated in the outer 2 mm of A. fulva, the Ircinia species had higher concentrations in deeper tissue layers. Furthermore, no significant positive or negative correlation between chemical defenses and nutritional quality or levels of structural components was observed in these sponges. Overall, these results do not support the prediction that predation pressure by fish and large mobile invertebrates significantly impacts chemical defense allocation in these sponges.