On the rise: Development of a multi-tiered, indoor duckweed cultivation system.

Duckweed species (Lemnaceae) are suitable for remediation and valorization of agri-feed industry wastewaters and therefore can contribute to a more sustainable, circular economy where waste is a resource. Industrial applications will, however, require space efficient cultivation methods that are not affected by prevailing weather conditions. Here, the development and operation of a multi-tiered duckweed bioreactor is described. The developed prototype bioreactor depicted in this paper is composed of four cultivation layers (1 m2 each) with integrated LED lighting (generating up to 150 µmol m-2  s-1 ), a system of pumps and valves to manage the recirculatory flow (2.5 L min-1 ) of wastewater, and an automatic harvesting system. Using a nutrient poor medium, good growth of the duckweed species Lemna minor was achieved in the bioreactor, and this was matched by strong nutrient depletion from the medium, especially for phosphorus (45-mg total phosphorus [TP] removed per m-2  day-1 ). A fully automatic harvesting arm reliably captured similar amounts of duckweed biomass across multiple harvesting cycles, revealing a future scenario whereby labor and interventions by human operators are minimized. Further developments to advance the system towards fully automated operation will include, for example, the use of specific nutrient sensors to monitor and control medium composition. It is envisaged that multi-tiered, indoor bioreactors can be employed in the agri-feed industry where wastewaters are, in many cases, continuously generated throughout the year and need remediating immediately to avoid costly storage. Given the extensive use of automation technology in conventional wastewater treatment plants, multi-tiered duckweed bioreactors can be realistically integrated within the operating environment of such treatment plants. PRACTITIONER POINTS: Duckweed is suitable for remediation and valorization of agri-feed wastewater. Industrial duckweed applications require space efficient cultivation methods. Development and operation of a multi-tiered duckweed bioreactor is detailed. Flow dynamics and automatic harvesting in the bioreactor are optimized. It is concluded that a multi-tiered bioreactor can be used in industry.

UV Radiation Induces Specific Changes in the Carotenoid Profile of Arabidopsis thaliana.

UV-B and UV-A radiation are natural components of solar radiation that can cause plant stress, as well as induce a range of acclimatory responses mediated by photoreceptors. UV-mediated accumulation of flavonoids and glucosinolates is well documented, but much less is known about UV effects on carotenoid content. Carotenoids are involved in a range of plant physiological processes, including photoprotection of the photosynthetic machinery. UV-induced changes in carotenoid profile were quantified in plants (Arabidopsis thaliana) exposed for up to ten days to supplemental UV radiation under growth chamber conditions. UV induces specific changes in carotenoid profile, including increases in antheraxanthin, neoxanthin, violaxanthin and lutein contents in leaves. The extent of induction was dependent on exposure duration. No individual UV-B (UVR8) or UV-A (Cryptochrome or Phototropin) photoreceptor was found to mediate this induction. Remarkably, UV-induced accumulation of violaxanthin could not be linked to protection of the photosynthetic machinery from UV damage, questioning the functional relevance of this UV response. Here, it is argued that plants exploit UV radiation as a proxy for other stressors. Thus, it is speculated that the function of UV-induced alterations in carotenoid profile is not UV protection, but rather protection against other environmental stressors such as high intensity visible light that will normally accompany UV radiation.

Wastewater valorisation in an integrated multitrophic aquaculture system; assessing nutrient removal and biomass production by duckweed species.

The aquaculture industry is considered a key sector for the supply of high quality, nutritious food. However, growth of the aquaculture sector has been slow, particularly in Europe, and this is amongst others linked to concerns about environmental impacts of this industry. Integrated Multitrophic Aquaculture (IMTA) has been identified as an important technology to sustainably improve freshwater fish production. In IMTA, economically valuable extractive species feed on waste produced by other species, remediating wastewater, and minimising the environmental impact of aquaculture. This study presents quantitative information on the nitrogen and phosphorus removal efficiency of a duckweed-based, pilot, semi-commercial IMTA system. Duckweed species are free-floating freshwater species belonging to the family of Lemnaceae. The aim of this study was to test the potential of duckweed-based IMTA under realistic environmental conditions. Three different approaches were used to assess remediation capacity; 1) assessment of water quality pre and post treatment with duckweed showed that the system can remove 0.78 and 0.38 T y-1 of Total Nitrogen (TN) and Total Phosphorus (TP), respectively 2) based on nitrogen and phosphorus content of newly grown duckweed biomass, it was shown that 1.71 and 0.22 T y-1 of TN and TP can be removed, respectively 3) extrapolation based on laboratory established nitrogen and phosphorus uptake rates determined that 0.88 and 0.08 T y-1 of TN and TP can be removed by the system. There is substantive agreement between the three assessments, and the study confirms that duckweed can maintain good quality water in an IMTA system, while yielding high protein content (21.84 ± 2.45%) biomass. The quantitative data on nitrogen and phosphorus removal inform the design of further IMTA systems, and especially create a scientific basis to determine the balance between aquaculture and extractive species.

Light intensity alters the phytoremediation potential of Lemna minor.

Lemnaceae, i.e. duckweed species, are attractive for phytoremediation of wastewaters, primarily due to their rapid growth, high nutrient uptake rates, tolerance to a broad range of growing conditions and ability to expeditiously assimilate a variety of pollutants. Light is essential for plant growth, and therefore, phytoremediation. Nevertheless, the effect of light intensity remains poorly understood in relation to phytoremediation, a knowledge gap that impedes the development of indoor, fully controlled, stacked remediation systems. In the present study, the effect of light intensity (10-850 µmol m-2 s-1) on the phytoremediation potential of Lemna minor was assessed. Plants were grown on either an optimal growth medium (half-strength Hutner's) or synthetic dairy processing wastewater, using stationary axenic (100 mL) or re-circulating non-sterile (11.7 L) systems. The relative growth rate (RGR) of L. minor grown on half-strength Hutner's increased proportionally with increasing light intensity. In contrast, the RGR of L. minor grown on synthetic dairy wastewater did not increase with light over an intensity range from 50 to 850 µmol m-2 s-1. On synthetic dairy wastewater, total nitrogen and total phosphorous removal also remained unchanged between 50 and 850 µmol m-2 s-1, although L. minor protein content (% fresh weight) increased from 1.5 to 2% at higher light intensities. Similar results were obtained with the larger re-circulating system. The results demonstrate interactive effects of light intensity and wastewater composition on growth and phytoremediation potential of L. minor. The data imply that light intensities above 50 µmol m-2 s-1 may not necessarily confer benefits in duckweed wastewater remediation, and this informs engineering of stacked, indoor remediation systems.

Effects of natural solar UV-B radiation on three Arabidopsis accessions are strongly affected by seasonal weather conditions.

Large numbers of studies have reported on the responses of plants that are exposed to a specific dose of ultraviolet-B (UV-B) radiation. However, in the natural environment UV-B is a highly dynamic variable with UV-B intensities depending on, amongst others, geographic, temporal, weather and climatic factors. Furthermore, UV-B effects on plants can potentially be modulated by other environmental variables, and vice versa. This study aimed to characterize UV-B effects on plant morphology and accumulation of UV-screening pigments within the context of an oceanic climate and to assess the potential seasonality of plant UV-B responses. Arabidopsis thaliana was grown outdoors under UV-blocking or transmitting filters. Genotypic differences in the adaptive response to UV-B were assessed at seven time-points over a 12 month period and involved the Arabidopsis accessions Ler, Col-0, and Bur-0. Strong seasonal effects were found on rosette morphology and total UV-screening pigment concentrations across the three accessions. Low temperatures were the main determinant of accumulation of UV-absorbing pigments, with no clear UV-B effect observed at any time throughout the year. There was a significant UV effect on morphology during the summer months, and this was most likely associated with stress. This study shows that UV-effects need to be analysed in the context of weather, and other co-occurring natural factors, and emphasizes the importance of a holistic, multifactorial approach for the investigation of environmentally relevant UV-effects.

The hydroxycinnamic acid content of barley and brewers' spent grain (BSG) and the potential to incorporate phenolic extracts of BSG as antioxidants into fruit beverages.

The hydroxycinnamic acid (HA) content of starting barley for brewers' spent grains (BSG), whole BSG and phenolic extracts from BSG was measured using high performance liquid chromatography (HPLC) and correlated with antioxidant potential. The effect of BSG phenolic extracts on antioxidant activity of fruit beverages was also assessed (using the total phenolic content (TPC), 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays). The concentration of HA present in barley extract and BSG was in the order of ferulic acid (FA), p-coumaric acid (p-CA) derivatives, FA derivatives, p-CA, caffeic acid (CA) and CA derivatives. Results suggested that brewing and roasting decreased the HA content. Antioxidant activity was significantly (P<0.05) correlated with caffeic acid (R(2)=0.8309) and total HA (R(2)=0.3942) concentrations. Addition of extracts to fruit beverages resulted in a significant (P<0.05) increase in antioxidant activity of cranberry juice, measured by the FRAP assay. In vitro digestion significantly (P<0.05) reduced TPC, DPPH and FRAP activity of the fruit beverages.

Alumina nanoparticles enhance growth of Lemna minor.

The industrial use of nanoparticles is rapidly increasing, and this has given rise to concerns about potential biological impacts of engineered particles released into the environment. So far, relatively little is known about uptake, accumulation and responses to engineered nanoparticles by plants. In this study, the effects of alumina nanoparticles on growth, morphology and photosynthesis of Lemna minor were quantified. It was found that alumina nanoparticles substantially increase biomass accumulation of L. minor. Such a stimulatory effect of alumina nanoparticles on growth has not been reported previously. Enhanced biomass accumulation was paralleled by morphological adjustments such as increased root length and number of fronds per colony, and by increased photosynthetic efficiency. Metal nanoparticles have previously been shown to enhance the energy transfer efficiency of isolated reaction centres; therefore it is proposed that the mechanism underlying the alumina mediated enhancement of biomass accumulation in L. minor is associated with increased efficiencies in the light reactions of photosynthesis.

Modulation of chlorogenic acid biosynthesis in Solanum lycopersicum; consequences for phenolic accumulation and UV-tolerance.

Chlorogenic acid (CGA) is one of the most abundant phenolic compounds in tomato (Solanum lycopersicum). Hydroxycinnamoyl CoA quinate transferase (HQT) is the key enzyme catalysing CGA biosynthesis in tomato. We have studied the relationship between phenolic accumulation and UV-susceptibility in transgenic tomato plants with altered HQT expression. Overall, increased CGA accumulation was associated with increased UV-protection. However, the genetic manipulation of HQT expression also resulted in more complex alterations in the profiles of phenolics. Levels of rutin were relatively high in both HQT gene-silenced and HQT-overexpressing plants raised in plant growth tunnels. This suggests plasticity in the flux along different branches of phenylpropanoid metabolism and the existence of regulatory mechanisms that direct the flow of phenolic precursors in response to both metabolic parameters and environmental conditions. These changes in composition of the phenolic pool affected the relative levels of UV-tolerance. We conclude that the capability of the phenolic compounds to protect against potentially harmful UV radiation is determined both by the total levels of phenolics that accumulate in leaves as well as by the specific composition of the phenolic profile.

Complementary interactions between oxidative stress and auxins control plant growth responses at plant, organ, and cellular level.

Plant stress responses are a key factor in steering the development of cells, tissues, and organs. However, the stress-induced signal transduction cascades that control localized growth and cell size/differentiation are not well understood. It is reported here that oxidative stress, exerted by paraquat or alloxan, induced localized cell proliferation in intact seedlings, in isolated root segments, and at the single cell level. Analysis of the stress-induced mitotic activity revealed that oxidative stress enhances auxin-dependent growth cycle reactivation. Based on the similarities between responses at plant, tissue, or single cell level, it is hypothesized that a common mechanism of reactive oxygen species enhanced auxin-responsiveness underlies the stress-induced re-orientation of growth, and that stress-induced effects on the protoplast growth cycle are directly relevant in terms of understanding whole plant behaviour.