Paul Ziegler

Duckweeds are a family of small floating macrophytes (the Lemnaceae) that inhabit quiet freshwaters worldwide. They have long been employed to determine toxicity to higher plants in the aquatic environment, and standardized national and international protocols have been developed for this purpose using two representative species. While these protocols, which assess the growth of the leaf-like fronds of the tested duckweed, are indeed suitable and still frequently used for detecting the toxicity of water-borne substances to aquatic higher plant life, they are cumbersome and lengthy, determine endpoints rather than depict toxicity timelines, and provide no information as to the mechanisms involved in the indicated toxicity. Progress has been made in downscaling, shortening and improving the standardized assay procedures, and the use of alternative duckweed species, protocols and endpoints for detecting toxicity has been explored. Biomarkers of toxic effect have long been determined concomitantly with testing for toxicity itself, and their potential for the assessment of toxicity has recently been greatly expanded by transcriptomic, proteomic and metabolomic techniques complemented by FITR spectroscopy, transformation and genotoxicity and timescale toxicity testing. Improved modern biomarker analysis can help to both better understand the mechanisms underlying toxicity and facilitate the identification of unknown toxins.

Duckweeds are widely distributed small, simply constructed aquatic higher plants (the Lemnaceae) found on quiet freshwater surfaces. Species inhabiting temperate climates may have to cope with long periods of severe cold during the winter season. Several duckweeds form compact resting structures from the assimilatory fronds of the growing season that can bridge inhospitable conditions in a quiescent state. Of these, turions separate from the mother fronds and overwinter on the water body bottom in a dormant state. They can surface, germinate, and sprout to resume active growth upon warming in the spring. The turions of the largest duckweed, Spirodela polyrhiza, have been intensively examined as to ultrastructure, the factors governing their formation and release from dormancy, and the signals driving their germination and sprouting and the accompanying starch degradation. Comparative transcriptomics of assimilatory fronds and dormant turions are revealing the molecular features of this developmental cycle. The results illustrate an elegant sequence of reactions that ensures aquatic survival of even severe winters by frost avoidance in a vegetative mode. Since little is known about other duckweed resting fronds, the S. polyrhiza turion developmental cycle cannot be considered to be representative of duckweed resting fronds in general but can serve as a reference for corresponding investigations.

Duckweeds (Lemnaceae) are small, simply constructed aquatic higher plants that grow on or just below the surface of quiet waters. They consist primarily of leaf-like assimilatory organs, or fronds, that reproduce mainly by vegetative replication. Despite their diminutive size and inornate habit, duckweeds have been able to colonize and maintain themselves in almost all of the world’s climate zones. They are thereby subject to multiple adverse influences during the growing season, such as high temperatures, extremes of light intensity and pH, nutrient shortage, damage by microorganisms and herbivores, the presence of harmful substances in the water, and competition from other aquatic plants, and they must also be able to withstand winter cold and drought that can be lethal to the fronds. This review discusses the means by which duckweeds come to grips with these adverse influences to ensure their survival. Important duckweed attributes in this regard are a pronounced potential for rapid growth and frond replication, a juvenile developmental status facilitating adventitious organ formation, and clonal diversity. Duckweeds have specific features at their disposal for coping with particular environmental difficulties and can also cooperate with other organisms of their surroundings to improve their survival chances.

Starch can accumulate in both actively growing vegetative fronds and over-wintering propagules, or turions of duckweeds, small floating aquatic plants belonging to the family of the Lemnaceae. The starch synthesizing potential of 36 duckweed species varies enormously, and the starch contents actually occurring in the duckweed tissues are determined by growth conditions, various types of stress and the action of growth regulators. The present review examines the effects of phytohormones and growth retardants, heavy metals, nutrient deficiency and salinity on the accumulation of starch in duckweeds with a view to obtaining high yields of starch as a feedstock for biofuel production. Biotechnological approaches to degrading duckweed starch to its component sugars and the fermentation of these sugars to bio-alcohols are also discussed.

Zusammenfassung: Das rasche Zuwachsen von Gewässeroberflächen macht die unscheinbaren, kleinen Wasserlinsengewächse (Lemnaceae) oft unbeliebt. Doch ihr Wachstumspotential, zusammen mit ihren besonderen morphologischen, klonalen und genomischen Eigenschaften sowie ihrer leichten Kultivierbarkeit machen Wasserlinsen zu wertvollen Modellpflanzen für die Untersuchung von pflanzenwissenschaftlichen Fragestellungen. Zudem wurden sie als Indikatorpflanzen in der aquatischen Ökotoxikologie etabliert. Wasserlinsen können zu der biologischen Reinigung von Abwässern beitragen, und die Biomasse, die ihr Wachstum dabei liefert, kann vielen Zwecken dienen, wie Energie‐ und Treibstoffproduktion sowie Verwendung als Dünger und Futtermittel. Wasserlinsen aus kontrollierter Anzucht können Nahrungsmittel für Menschen darstellen, und sie können auch als biologische Fabriken für die heterologe Proteinexpression dienen. Das vielfältige Nutzungspotential von Wasserlinsen ist noch sehr unvollständig umgesetzt worden: Dies zu tun, ist eine Aufgabe für die Zukunft.

Surface or ground waters can be contaminated with numerous toxic substances. The duckweeds Lemna minor and Lemna gibba are widely used for assaying waterborne toxicity to higher plants in terms of growth inhibition and photosynthetic pigment reduction. These tests cannot, however, in themselves determine the nature of the agents responsible for toxicity. Morphological, developmental, physiological, biochemical, and genetic responses of duckweeds to exposure to toxic water contaminants constitute biomarkers of toxic effect. In principle, the very detection of these biomarkers should enable the contaminants having elicited them (and being responsible for the toxicity) to be identified. However, in practice, this is severely compromised by insufficient specificity of biomarkers for their corresponding toxicants and by the lack of documentation of biomarker/toxin relationships. The present contribution illustrates the difficulties of using known water contaminant-related duckweed biomarkers to identify toxins, and discusses possibilities for achieving this goal.

The presence of toxic substances in wastewaters and outdoor bodies of water is an important ecotoxicological issue. The aim of this review is to illustrate how duckweeds, which are small, simply constructed, floating aquatic plants, are well suited to addressing this concern. The ability of duckweeds to grow rapidly on nutrient-rich water and to facilitate the removal of many substances from aqueous solution comprises the potential of these macrophytes for the remediation of wastewater and polluted aqueous reservoirs, while producing usable biomass containing the unwanted substances having been taken up. Their ease of cultivation under controlled and even sterile conditions makes duckweeds excellent test organisms for determining the toxicity of water contaminants, and duckweeds are important as model aquatic plants in the assessment of ecotoxicity. Duckweeds are also valuable for establishing biomarkers for the toxic effects of water contaminants on aquatic higher plants, but the current usefulness of duckweed biomarkers for identifying toxicants is limited. The recent sequencing of a duckweed genome holds the promise of combining the determination of water contaminant toxicity with toxicant diagnostics by means of gene expression profiling via DNA microarrays.