Patrick Fink

Ecological functions of volatile organic compounds in aquatic systems

In terrestrial ecosystems, volatile organic compounds (VOCs) are widely acknowledged as an important group of infochemicals. They play a major role in pollinator attraction by terrestrial plants and as insect pheromones. Furthermore, they are the mediating agent of so-called ‘tritrophic interactions’. When plants are attacked by herbivorous insects, volatile signal substances are emitted, which act as attractants for parasitoids that kill the herbivores, thereby protecting the plant from herbivory. Despite the generally acknowledged importance of VOCs in terrestrial chemical ecology, their functions in aquatic food webs are largely unknown. VOCs produced by algae and cyanobacteria are a major concern in water processing, since aquatic primary producers are the reason for regularly encountered taste and odour problems in drinking water. Only very recently, research in aquatic chemical ecology has started to investigate possible ecological functions for the production of VOCs by algae and cyanobacteria. Volatile aldehydes released by wounded cells of marine planktonic diatoms seem to act as defensive compounds against herbivorous copepods on the population level. Just recently, it was found that VOCs released from benthic algae and cyanobacteria can be utilised as food and/or habitat finding cues by aquatic invertebrates such as freshwater gastropods and nematodes. Here, I review concepts and recent experimental studies on the ecological functions of such VOCs in aquatic ecosystems. Understanding the factors that lead to the liberation of volatile compounds is an essential prerequisite to properly assessing their ecological functions. It appears that (similar to terrestrial plant–herbivore interactions) VOCs can also play a steering role for both attraction and defence in aquatic ecosystems.

Stoichiometric mismatch between littoral invertebrates and their periphyton food

Abstract: Ecological stoichiometry is considered a key concept in understanding con-straints in energy transfer at the plant-herbivore interface. However, whether this con-cept is relevant for benthic freshwater ecosystems is not fully known. Therefore, afield survey was conducted in 2003 during the growing season in the littoral zone ofLake Constance, a large pre-alpine lake in central Europe. The aim was to assess tem-poral variation in the elemental stoichiometric composition in both herbivorous macro-invertebrates and their food resource, the periphyton in two different lakes. The peri-phyton showed large temporal and spatial variation in carbon, nitrogen, and phospho-rus content, with particularly high molar C:P ratios of up to 1225:1. Periphyton C:Pand C:N ratios were often high and constantly above the Redfield ratio that is consid-ered optimal for autotrophic growth. In contrast to the pronounced fluctuations in thenutrient ratios of their food resource, the herbivorous macroinvertebrates showed onlyvery little variation in their nutrient ratios, which indicated that they are homeostatic,i.e., physiologically restricted to a comparatively narrow range of C:P and C:N ratios.Distinct species-specific C : P and C : N ratios were found for different taxonomicgroups of macroinvertebrates, which indicated different requirements of optimal die-tary C:P and C:N ratios and which might influence the ability of the taxa to competefor limiting elemental nutrients. Considering the temporally very high C:P and C:Nratios of the periphytic resource and the very low ratios of the consumer body tissue,this stoichiometric mismatch is likely to constrain growth and reproduction of these lit-toral invertebrates. Therefore, the concept of stoichiometric food quality limitationmight also be applicable to the littoral food web in lakes.Key words: benthos, C:N:P ratio, ecological stoichiometry, herbivory, homeostasis,lake littoral, macroinvertebrates, mismatch, nutrient ratios, phosphorus.

Volatile foraging kairomones in the littoral zone : Attraction of an herbivorous freshwater gastropod to algal odors

Volatile organic compounds (VOCs) produced by algae and cyanobacteria are primarily responsible for odors in fresh waters. Among other functions, VOCs may serve as important infochemicals in biofilms of benthic primary producers. VOCs liberated by benthic, mat-forming cyanobacteria can be used as habitat-finding cues by insects, nematodes, and possibly other organisms. We developed a new gastropod behavioral assay that allows detection of food preference without offering food, thus allowing the distinction between taste, which requires direct contact with the food source, and the detection of odorous infochemicals, which work over distance. We demonstrated that VOCs released from disintegrated cells of a benthic, mat-forming, green alga (Ulothrix fimbriata) are food-finding cues (“foraging kairomones”) that attract the herbivorous freshwater snail Radix ovata. A mixture of three C5 lipoxygenase compounds and 2(E),4(E)-heptadienal that mimic the major VOCs released by U. fimbriata attracted the snails, whereas neither the mixture of C5 compounds nor 2(E),4(E)-heptadienal were effective when given alone. This study suggests that VOCs can play a steering role as infochemicals in freshwater benthic habitats, as has been established for many organismic interactions in terrestrial ecosystems.

Oxylipins from freshwater diatoms act as attractants for a benthic herbivore

Lipoxygenase products (often called oxylipins) are degradation products of fatty acids. They are known to regulate many defensive and developmental pathways in plants. Among these lipoxygenase products are volatile aldehydes, that are released from wound activated diatom cells and that are potent inhibitors of mitotic prolifera- tion in sea urchin embryos. They have been shown to dramatically lower the hatching success of herbivorous copepods and therefore are considered to constitute an acti- vated defence strategy. Alternatively, lipoxygenase products might also serve as info- chemicals, especially in biofilms of benthic algae and cyanobacteria. Here, we demon- strate that the bouquet of volatile lipoxygenase products released from a benthic dia- tom (Achnanthes biasolettiana) is attractive to an herbivorous gastropod grazer (Radix ovata). The volatiles are released from the algae upon cell damage and can be utilized as food-finding cues by the herbivorous snails. In contrast to this observation, the odour bouquet from another benthic diatom species (Gomphonema parvulum) did not show any attractant activity to the snail, which is probably caused by differences in the bouquet of odorous substances released by the two diatom isolates. In contrast to G. parvulum, the odour bouquet of A. biasolettiana consisted of mono- and diunsaturated alcohols and ketones, primarily with a C8-skeleton. Most of the compounds have never before been described from diatoms. This study shows that diatom lipoxygenase prod- ucts cannot only be involved in activated defence, but can also be utilised as food-find- ing cues by herbivores. Thus, the ecological role of these volatile organic compounds can be very complex and will strongly depend on the ecological context of the chemi- cal interaction.

The Acyl-Acyl Carrier Protein Synthetase from Synechocystis sp. PCC 6803 Mediates Fatty Acid Import

The transfer of fatty acids across biological membranes is a largely uncharacterized process, although it is essential at membranes of several higher plant organelles like chloroplasts, peroxisomes, or the endoplasmic reticulum. Here, we analyzed loss-of-function mutants of the unicellular cyanobacterium Synechocystis sp. PCC 6803 as a model system to circumvent redundancy problems encountered in eukaryotic organisms. Cells deficient in the only cytoplasmic Synechocystis acyl-acyl carrier protein synthetase (SynAas) were highly resistant to externally provided α-linolenic acid, whereas wild-type cells bleached upon this treatment. Bleaching of wild-type cells was accompanied by a continuous increase of α-linolenic acid in total lipids, whereas no such accumulation could be observed in SynAas-deficient cells (Δsynaas). When SynAas was disrupted in the tocopherol-deficient, α-linolenic acid-hypersensitive Synechocystis mutant Δslr1736, double mutant cells displayed the same resistance phenotype as Δsynaas. Moreover, heterologous expression of SynAas in yeast (Saccharomyces cerevisiae) mutants lacking the major yeast fatty acid import protein Fat1p (Δfat1) led to the restoration of wild-type sensitivity against exogenous α-linolenic acid of the otherwise resistant Δfat1 mutant, indicating that SynAas is functionally equivalent to Fat1p. In addition, liposome assays provided direct evidence for the ability of purified SynAas protein to mediate α-[14C]linolenic acid retrieval from preloaded liposome membranes via the synthesis of [14C]linolenoyl-acyl carrier protein. Taken together, our data show that an acyl-activating enzyme like SynAas is necessary and sufficient to mediate the transfer of fatty acids across a biological membrane.

The smell of good food: volatile infochemicals as resource quality indicators

Foraging success generally depends on various environmental and physiological factors. Particularly for organisms with limited motility such as gastropods, food searching is a very cost-intensive process. As energy gain through foraging is dependent on both resource quality and quantity, consumers have to be able to differentiate between varying resource items. The effectiveness of food searching could be increased through the perception of diet-derived chemical signals that convey information about a food resources quality over a certain distance. This strategy would clearly help to optimize movement decisions. In this study, we investigated the foraging behaviour of a freshwater gastropod towards volatile signal substances released from benthic algae grown under high and low nutrient availability, representing high and low food quality, using behavioural assays in the laboratory. Our results demonstrate that volatile organic compounds (VOCs) serve as foraging kairomones for these aquatic, benthic herbivores. Further, we were able to show for the first time that snails are able to differentiate between high- and low-quality food sources, only by the perception of food odours alone (volatile infochemicals). Gas chromatography coupled with mass spectrometry demonstrated quantitative as well as qualitative differences in the chemical composition of the VOCs bouquet, dependent on algal nutrient content. Our results suggest that the recognition of resource quality via the reception of signal substances is likely to be adaptive for consumers with low mobility to maximize ingestion of high-quality resources.

Consumer patchiness explained by volatile infochemicals in a freshwater ecosystem

Many animal species show considerable spatial variation in abundance within their habitats. Since they live in patchy environments where food resources are often heterogeneously distributed, it would be adaptive for them to have efficient chemoreceptive mechanisms to locate food resources over distances. In the current study we investigated whether gastropod consumers perceive and pursue odorant signals in three choice experiments which were conducted under semi-natural conditions. We tested the foraging behavior of freshwater snails to targets containing odorant stimuli and whether the recognition of food finding signals subsequently leads to the aggregation of grazers on the corresponding resource patch. We demonstrate that freshwater gastropods are able to recognize odor bouquets as foraging infochemicals and that grazing of conspecifics causes an aggregation of grazers under natural conditions. Further, they appear to be able to distinguish between high- and low-quality food resources based on resourc...

The Effect of Diet Mixing on a Nonselective Herbivore.

The balanced-diet hypothesis states that a diverse prey community is beneficial to consumers due to resource complementarity among the prey species. Nonselective consumer species cannot differentiate between prey items and are therefore not able to actively regulate their diet intake. We thus wanted to test whether the balanced-diet hypothesis is applicable to nonselective consumers. We conducted a laboratory experiment in which a nonselective model grazer, the freshwater gastropod Lymnaea stagnalis, was fed benthic green algae as single species or as a multi-species mixture and quantified the snails’ somatic growth rates and shell lengths over a seven-week period. Gastropods fed the mixed diet were found to exhibit a higher somatic growth rate than the average of the snails fed single prey species. However, growth on the multi-species mixture did not exceed the growth rate obtained on the best single prey species. Similar results were obtained regarding the animals’ shell height increase over time. The mixed diet did not provide the highest growth rate, which confirms our hypothesis. We thus suggest that the balanced-diet hypothesis is less relevant for non-selective generalist consumers, which needs to be considered in estimates of secondary production.

Do insect repellents induce drift behaviour in aquatic non-target organisms?

Synthetic insect repellents are compounds applied to surfaces to discourage insects, mainly mosquitoes, from landing on those surfaces. As some of these repellents have repeatedly been detected in surface waters at significant concentrations, they may also exert repellent effects on aquatic non-target organisms. In running water systems, aquatic invertebrates actively enter downstream drift in order to avoid unfavourable environmental conditions. We thus tested the hypothesis that the widely used insect repellents DEET (N,N-Diethyl-m-toluamide), EBAAP (3-[N-butyl-N-acetyl]-aminopropionic acid ethyl ester) and Icaridin (1-piperidinecarboxylic acid 2-(2-hydroxyethyl)-1-methylpropyl ester) induce downstream drift behaviour in the aquatic invertebrates Gammarus pulex (Crustacea, Amphipoda) and Cloeon dipterum (Insecta, Ephemeroptera), using a laboratory-scale drift assay. We found no clear increase in the drift behaviour of both invertebrate species across a concentration gradient of eight orders of magnitude and even beyond maximum environmental concentrations for any of the three repellents. We found no evidence for a direct drift-inducing activity of insect repellents on aquatic non-target organisms.

The best of both worlds: a combined approach for analyzing microalgal diversity via metabarcoding and morphology-based methods

An increasing number of studies use next generation sequencing (NGS) to analyze complex communities, but is the method sensitive enough when it comes to identification and quantification of species? We compared NGS with morphology-based identification methods in an analysis of microalgal (periphyton) communities. We conducted a mesocosm experiment in which we allowed two benthic grazer species to feed upon benthic biofilms, which resulted in altered periphyton communities. Morphology-based identification and 454 (Roche) pyrosequencing of the V4 region in the small ribosomal unit (18S) rDNA gene were used to investigate the community change caused by grazing. Both the NGS-based data and the morphology-based method detected a marked shift in the biofilm composition, though the two methods varied strongly in their abilities to detect and quantify specific taxa, and neither method was able to detect all species in the biofilms. For quantitative analysis, we therefore recommend using both metabarcoding and microscopic identification when assessing the community composition of eukaryotic microorganisms.