Several observations and model calculations suggest that chemically mediated interactions can structure planktonic food webs. However, only recently have improvements in chemical methods, coupled with ecological assays, led to the characterization of chemical cues that affect the behaviour and/or physiology of planktonic organisms. We are currently beginning to elucidate if or how chemical signals can directly affect the interactions between species and even shape complex community structures in aquatic systems. Here, we highlight recent research on the nature and action of chemical signals in the pelagic marine and freshwater environments, with an emphasis on kairomones and defence metabolites.

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Chemical cues, defence metabolites and the shaping of pelagic interspecific interactions

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Are alkyne reductions chemo-, regio-, and stereoselective enough to provide pure (Z)-olefins in polyfunctionalized bioactive molecules?

Many organisms use subtle chemical cues not only to find partners and food, but also to sense the presence of natural enemies and to avoid predation. As we discuss here, an increasing number of studies now show that low, non-toxic concentrations of chemicals, ranging from heavy metals and pesticides to seemingly harmless substances such as surfactants, can disrupt the transfer of chemical information, inducing maladaptive responses in both the signaller and the receiver. Similar to endocrine disruptors, these 'info disruptors' form a new class of chemical threats, which could have far-reaching implications for ecosystem functioning and conservation management.

https://www.cell.com/trends/ecology-evolution/pdf/S0169-5347(07)00108-5.pdf

Info-disruption: pollution and the transfer of chemical information between organisms

Morphological evolution from a unicellular to multicellular state provides greater opportunities for organisms to attain larger and more complex living forms. As the most common freshwater cyanobacterial genus, Microcystis is a unicellular microorganism, with high phenotypic plasticity, which forms colonies and blooms in lakes and reservoirs worldwide. We conducted a systematic review of field studies from the 1990s to 2017 where Microcystis was dominant. Microcystis was detected as the dominant genus in waterbodies from temperate to subtropical and tropical zones. Unicellular Microcystis spp. can be induced to form colonies by adjusting biotic and abiotic factors in laboratory. Colony formation by cell division has been induced by zooplankton filtrate, high Pb2+ concentration, the presence of another cyanobacterium (Cylindrospermopsis raciborskii), heterotrophic bacteria, and by low temperature and light intensity. Colony formation by cell adhesion can be induced by zooplankton grazing, high Ca2+ concentration, and microcystins. We hypothesise that single cells of all Microcystis morphospecies initially form colonies with a similar morphology to those found in the early spring. These colonies gradually change their morphology to that of M. ichthyoblabe, M. wesenbergii and M. aeruginosa with changing environmental conditions. Colony formation provides Microcystis with many ecological advantages, including adaption to varying light, sustained growth under poor nutrient supply, protection from chemical stressors and protection from grazing. These benefits represent passive tactics responding to environmental stress. Microcystis colonies form at the cost of decreased specific growth rates compared with a unicellular habit. Large colony size allows Microcystis to attain rapid floating velocities (maximum recorded for a single colony, ∼ 10.08 m h-1 ) that enable them to develop and maintain a large biomass near the surface of eutrophic lakes, where they may shade and inhibit the growth of less-buoyant species in deeper layers. Over time, accompanying species may fail to maintain viable populations, allowing Microcystis to dominate. Microcystis blooms can be controlled by artificial mixing. Microcystis colonies and non-buoyant phytoplankton will be exposed to identical light conditions if they are evenly distributed over the water column. In that case, green algae and diatoms, which generally have a higher growth rate than Microcystis, will be more successful. Under such mixing conditions, other phytoplankton taxa could recover and the dominance of Microcystis would be reduced. This review advances our understanding of the factors and mechanisms affecting Microcystis colony formation and size in the field and laboratory through synthesis of current knowledge. The main transition pathways of morphological changes in Microcystis provide an example of the phenotypic plasticity of organisms during morphological evolution from a unicellular to multicellular state. We emphasise that the mechanisms and factors influencing competition among various close morphospecies are sometimes paradoxical because these morphospecies are potentially a single species. Further work is required to clarify the colony-forming process in different Microcystis morphospecies and the seasonal variation in this process. This will allow researchers to grow laboratory cultures that more closely reflect field morphologies and to optimise artificial mixing to manage blooms more effectively.

Colony formation in the cyanobacterium Microcystis

Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism(s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.

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Induced defences in marine and freshwater phytoplankton: a review.

Aliphatic sulfates released from Daphnia induce morphological defense of phytoplankton: isolation and synthesis of kairomones

2,6-Dimethylheptyl sulfate (1) and 6-methyloctyl sulfate (3) were isolated from Daphnia pulex as the Daphnia kairomones that induced morphological defense of a freshwater phytoplankton Scenedesmus gutwinskii var. heterospina (NIES-802). The absolute stereochemistry at C2 of 1 was determined by (1)H-NMR analysis of the (R)-MTPA ester of alcohol 2. The absolute configuration at C6 of 3 was determined by Ohrui's method applied to alcohol 4.

https://www.jstage.jst.go.jp/article/cpb/56/1/56_1_129/_pdf

Isolation and absolute configuration determination of aliphatic sulfates as the Daphnia kairomones inducing morphological defense of a phytoplankton.

New aliphatic sulfates and sulfamates were isolated from Daphnia pulex as the Daphnia kairomones that induced morphological defense of a freshwater phytoplankton Scenedesmus gutwinskii var. heterospina (NIES-802). Their structures were determined by spectroscopic and synthetic studies.

Isolation of new aliphatic sulfates and sulfamate as the Daphnia Kairomones inducing morphological change of a phytoplankton Scenedesmus gutwinskii.

Isolation and Absolute Configuration Determination of Aliphatic Sulfates as the Daphnia Kairomones Inducing Morphological Defense of a Phytoplankton.

Daphnia kairomones induce morphological change to green alga. An active compound (8-methylnonyl sulfate), which was originally isolated and determined from Daphnia pulex body, was identified from a...

Akinori Nishigami

Papers

Aliphatic sulfates released from Daphnia induce morphological defense of phytoplankton: isolation and synthesis of kairomones

Isolation and absolute configuration determination of aliphatic sulfates as the Daphnia kairomones inducing morphological defense of a phytoplankton.

Isolation of new aliphatic sulfates and sulfamate as the Daphnia Kairomones inducing morphological change of a phytoplankton Scenedesmus gutwinskii.

Isolation and Absolute Configuration Determination of Aliphatic Sulfates as the Daphnia Kairomones Inducing Morphological Defense of a Phytoplankton.

Time-of-Flight LC/MS Identification and Confirmation of a Kairomone in Daphnia magna Cultured Medium