Proceedings 2nd International PlantPower Symposium 2012

Our world is confronted with an energy crisis on a global scale. Our current energy supplies are polluting our environment and are not based on endless cycles. Clean technologies are needed that provide people and planet with safe, affordable and secure energy. PlantPower is a new additional source of electricity. It was in 1911 when the British botanist Michael C. Potter showed that bacteria can cause electrical effects accompanied by decomposition of organic matter. Nowadays, 101 years later, this electrical effect evolved into the development of a multitude of bio-electrochemical systems providing all kinds of services like wastewater treatment, electricity generation or chemical recovery. The Plant Microbial Fuel Cell (Plant-MFC) offers in-situ electricity production with living plants and bacteria. This unique combination was just invented 6 years ago and is already scaled-up to 25 square meters. The last 4 years a multidisciplinary research team explored the Plant-MFC in an EU research project. Exciting discoveries and great technological development took place over the last few years. New research questions came up and opportunities were identified to improve the system in the future. By bringing scientists, companies and entrepreneurs together we expect to bring PlantPower from the lab into the real world. The Plant-MFC is promising from a technical, environmental and economic perspective. The design criteria for the future are defined; but still development on several issues is needed. Besides the fundamental research, scaling-up the technology is the next challenge. Especially wetlands offer the opportunity to produce electricity on a large scale. World-wide 800,000,000 ha wetland are present, however they are often under pressure due to our need of arable land for food, feed or chemicals. Here the Plant-MFC can be a solution since Plant-MFCs can be combined with nature and in that sense make nature preservation economically feasible. This 2nd international PlantPower symposium will show exiting results of the EU PlantPower consortium and other researchers

Iron tolerance and the role of aerenchyma in wetland plants

The relative iron (II) tolerance of a range of wetland plants was determined and compared with some species characteristic of well drained soils. A wide range of tolerance occurred amongst the wetland species but they were generally more tolerant than those from well drained soils. No correlation was found between iron (II) tolerance and the amount of air space {% v/v) (aerenchyma) in the roots of these species. There was a significant negative correlation between air space and iron uptake by roots. This may have been caused by iron (II) oxidation in the rhizosphere resulting in decreased availability. There was evidence that differential iron (II) tolerance of excised root tips was maintained under aerobic and anaerobic conditions. It was thus suggested that iron (II) tolerance may not be dependent on iron exclusion or oxidation of iron (II) by oxygen diffusing through the aerenchyma. Levels of malic and citric acids in roots were altered by iron (II) sulphate, but the absolute levels and changes in levels had no correlation with the iron (II) tolerance, of the species. Peroxidase and catalase activities in root tips of plants gown in drained and flooded sand culture were measured and considered in relation to the oxidising power of roots. Activity was detected in all species examined but was generally I unaffected by flooding. Evidence from the literature suggested that these enzymes of peroxide metabolism are unlikely to be active in flooded roots and so could not mediate their oxidising power. The structure of root aerenchyma had great variability between species. The Cyperaceae had the most complex and well organised structure. Growth under flooded conditions increased air space in most species, but there were exceptions. In Eriophrum angustifolium and E. vaginatum air space was high under drained conditions and was not increased by flooding. In Filipendula ulmaria the small amount of air space was not increased by flooding. Low nutrient levels increased air space production in Nardus stricta. The function of aerenchyma and the influence of environmental factors on its production are discussed

Rusty rice - unravelling rice plant and microbial interactions in the paddy soil iron cycle

Reisfelder stellen eines der weltweit wichtigsten Agrargebiete dar und versorgen mehr als die Hälfte der Weltbevölkerung mit einem der wichtigsten Nahrungsmittel - Reis. Eisen (Fe) ist eines der am häufigsten vorkommenden redoxaktiven Metalle in Reisfeldböden. Meist liegt es als reduziertes Eisen (Fe(II)) oder oxidiertes Eisen (Fe(III)) vor. In wassergesättigten Reisfeldböden, die typischerweise frei von Sauerstoff (O2) sind, liegt Fe in seiner mobilen und gelösten Form als Fe(II) vor. Eisen stellt einerseits einen essentiellen Mikronährstoff für Reispflanzen dar. Eine zu hohe Eisenaufnahme über die Wurzeln kann jedoch zu einer Vergiftung führen und das Photosystem schädigen. Um die Aufnahme von Fe zu kontrollieren, können Reispflanzen O2 über die Wurzeln abgeben an den Boden abgeben. Dieser Prozess wird Radial Oxygen Loss (ROL) genannt. Durch ROL wird der Boden mit O2 angereichert, Fe(II) oxidiert und fällt auf der Wurzeloberfläche aus und wird als Eisen-Plaque-Minerale abgelagert. Die Bildung von Eisenplaque an den Wurzeln verringert nicht nur die Mobilität des Eisens, sondern dient auch als Adsorptionsmittel für Schadstoffe in Reisfeldern und kann deren Aufnahme in die Reiskörner verringern. Neben Reispflanzen können zahlreiche andere chemische und mikrobielle Redox-Prozesse die Speziation von Fe durch die Oxidation von Fe(II) und die Reduktion von Fe(III) beeinflussen. Dieser Eisen-Redox-Kreislauf in Reisböden ist an eine Vielzahl anderer Bodenelementkreisläufe gekoppelt, die die (Im)mobilisierung vieler Nähr- und Schadstoffe beeinflussen können. Insbesondere mikroaerophile Fe(II)-oxidierende Bakterien, die die Oxidation von Fe(II) enzymatisch an die Reduktion von O2 koppeln, spielen bei der Oxidation von Fe(II) eine wichtige Role. Ihr Einfluss auf den Eisenkreislauf in Reisfeldern ist bisher jeodch nur unzureichend verstanden. Darüber hinaus fehlt noch immer eine ganzheitliche Studie, die die räumlich-zeitliche Entwicklung von Eisenplaque an Reiswurzeln und die Wechselwirkungen mit Bodenbakterien in wassergesättigten Reisfeldern während des Pflanzenwachstums quantitativ untersucht. Insbesondere ist der Einfluss von mikrobieller Fe(III)-Reduktion auf die Mineralogie von Eisenplaque, welche als Senke oder Quelle für Schadstoffe fungieren kann, bisher kaum dokumentiert. In diesem Dissertationsprojekt haben wir neue Ansätze entwickelt, die eine dynamische, nicht-invasive Identifizierung von geochemischen Rhizosphärenparametern und eine Quantifizierung der Eisenminerale, die während des Wachstums von Reispflanzen gebildet werden, ermöglichen. Darüber hinaus leiteten wir Einflussgrößen mikrobieller Prozesse ab, die den Eisenkreislauf in Reisfeldböden und die (Im)mobilisierung von Schadstoffen, wie etwa Arsen, durch Fe(II)-Oxidation und Fe(III)-Reduktion während des vegetativen Wachstums von Reispflanzen beeinflussen. Obwohl mikroaerophile Fe(II)-oxidierende Bakterien in verschiedensten Ökogebieten, die durch entgegengesetzte Gradienten an O2 und Fe(II) geprägt sind, identifiziert wurden, ist wenig über ihren Einfluss auf den Eisenzyklus bekannt. Die schnelle Oxidation von Fe(II) mit O2 unter neutralen pH-Wert Bedingungen verdrängt diese Mikroorganismen in Nischen, in denen die O2 Konzentrationen niedrig genug sind, um mit der abiotischen Reaktion zu konkurrieren. Eine Quantifizierung der mikrobiellen Fe(II)-Oxidationsraten in klassischen Mikrokosmen stellt weiterhin eine Herausforderung dar, da abiotische und biotische Fe(II)-Oxidationsreaktionen bisher nicht separat quantifizierbar waren. Insbesondere die Akkumulation von Fe(III) (Bio)mineralen als Produkt der (a)biotischen Fe(II)-Oxidation steigert den Konkurrenzdruck auf die Bakterien durch die Beschleunigung abiotischer oberflächenkatalysierter heterogener Fe(II)-Oxidationsraten. In diesem Projekt wurde daher ein experimenteller Ansatz entwickelt, der eine Quantifizierung der mikrobiellen und abiotischen Fe(II)-Oxidationsraten ermöglicht. Bei O2 Konzentrationen von 20 μM O2 und der anfänglichen Abwesenheit von Fe(III)-Mineralien konnten mikroaerophile Bakterien (99,6% Ähnlichkeit mit Sideroxydans spp.), welche aus einem Reisfeld isoliert wurden, innerhalb von etwa 26 Stunden mit bis zu 40% zur gesamten Fe(II)-Oxidation beitragen. Eine Zelle oxidierte hierbei bis zu 3,6 × 10-15 mol Fe(II) pro Stunde. Bei O2 Konzentrationen von 5 bis 20 µM war der biologische Fe(II) Umsatz am höchsten, während niedrigere O2 Konzentrationen die biologische Fe(II)-Oxidation hemmten. Höhere O2-Konzentrationen beschleunigten wiederum die abiotische Fe(II)-Oxidation, die ab 30 µM O2 über die mikrobielle Fe(II)-Oxidation dominierte. Zusätzlich konnte gezeigt werden, dass Fe(III)-(Bio)minerale die oberflächenkatalytische heterogene abiotische Fe(II)-Oxidation induzieren und den mikrobiellen Einfluss auf Fe(II)-Oxidation von 40% auf nur 10% reduzieren. Dieser neu entwickelte Ansatz kann für die Kultivierung verschiedenster mikroaerophiler Kulturen Verwendung finden und dabei helfen mikrobielle Fe(II) Umsatzraten zu ermitteln. Die Ergebnisse können dann dazu beitragen, die Auswirkungen der mikroaerophilen Fe(II)-Oxidation auf den biogeochemischen Eisenzyklus in zahlreichen natürlichen und anthropogenen Ökosystemen besser einzuschätzen. Neben der mikrobiellen Fe(II)-Oxidation kann die Reispflanze selbst durch die Oxidation von Fe(II) mittels ROL zahlreiche Bodenparameter beeinflussen und mikrobielle Gemeinschaften verändern. Die O2 Abgabe über die Wurzeln führt nicht nur zur chemischen Oxidation von Fe(II), sondern kann auch von zum Beispiel mikroaerophilen Fe(II)-oxidierenden Bakterien als den Elektronenakzeptor benutzt werden. In der Rhizosphäre der Reispflanze können so sowohl mikrobielle als auch chemische Prozesse zur Bildung von Eisenmineralen auf und um die Wurzeloberfläche beitragen. Die Identifikation von potentiellen Nischen für mikroaerophile Fe(II)-oxidierende Bakterien in der Rhizosphäre von Reispflanzen blieb jedoch bisher aus. Im Rahmen dieser Dissertation wurden die räumliche Ausdehnung von ROL an Reiswurzeln währen des Wachstums von Reispflanzen zeitlich hochaufgelöst quantifiziert und dessen Einfluss auf andere Bodenparameter identifiziert. Durch die Anwendung nicht-invasiver Techniken in transparenten künstlichen Böden konnte so zum ersten Mal festgestellt werden, dass sich gegengesetzte Gradienten an O2 und Fe(II) rund um die Wurzeloberflöche ausbilden, welche sich von der Oberfläche der Reiswurzel bis zu 10-25 mm in die Rhizosphäre erstrecken. Diese mikrooxische Zone dehnte sich in der gesamten Rhizosphäre exponentiell aus und schuf optimale Nischen für mikroaerophile Fe(II)-oxidierende Bakterien. Durch die nicht-invasive Quantifizierung von Eisenmineralen, der Identifikation von Boden-pH-Wert Veränderungen und dem Bestimmen der Fe(II)-Oxidationskinetik in der gesamten Rhizosphäre, konnten abschließend festgestellt werden, dass ROL maßgeblich die Bildung von Eisenplaque und lokale pH-Wert Veränderungen steuert. Die gesammelten Ergebnisse unterstreichen die dynamischen geochemischen Wechselwirkungen an Reiswurzeln, während die neu entwickelten Methoden dazu beitragen können, den räumlich und zeitlich hoch dynamischen Eisenkreislauf in der Rhizosphäre zu verfolgen. Ergänzend wurde festgestellt, dass Wurzelspitzen die mitunter höchste Variation an lokalen O2 Konzentrationen aufweisen. Radial oxygen loss führte hier zu tageszeitlichen Schwankungen der O2 Konzentrationen zwischen 5-50 µM O2. Die gesamte Wurzelmasse wurde hierbei zu etwa 30% mit Eisenplaque bedeckt, was 60-180 mg Fe(III) pro Gramm getrockneter Wurzel entspricht. Darüber hinaus wurde festgestellt, dass sich die Mineralogie der Eisenplaqueminerale während des Pflanzenwachstums veränderte. Innerhalb von 40 Tagen wurden frisch gebildete niedrigkristalline Eisenminerale (z.B. Ferrihydrit) an den Wurzelspitzen in höherkristalline Eisenminerale (z.B. Goethit) umgewandelt. Eisen(III)-reduzierende Bakterien (Geobacter spp.), waren hierbei in der Lage, bis zu 30% Fe(II) durch reduktive Auflösung von Eisenplaque zu remobilisieren. Mehr als 50% der Eisenplaqueminerale haben sich in Fe(II)-Minerale (z.B. Siderit, Vivianit und Fe-S-Phasen) umgewandelt während etwa 15% als Fe(III)-Mineralien zurückblieben. Auf der Grundlage gesammelter Daten konnte abschließend quantitativ festgestellt werden, dass die ROL-induzierte Bildung von Eisenplaque an der Wurzel und die mikrobielle reduktive Auflösung mehr als 5% des gesamten Eisenhaushalts der Rhizosphäre beeinflussen, was Auswirkungen auf die (Im-)mobilisierung von beispielsweise Bodennährstoffen und Schadstoffen haben kann. In diesem Zusammenhang ist es allgemein anerkannt, dass die Eisenminerale auf Reiswurzeln Schadstoffe, wie beispielsweise Arsen (As) durch Sorption oder Komplexierung immobilisieren können. Das verringert nicht nur die Nettoaufnahme in die Pflanze, sondern vermindert auch deren Mobilität in kontaminierten Böden. Wenig ist jedoch über die Auswirkungen von Fe(III)-reduzierenden Bakterien auf die Schadstoff-Immobilisierungskapazität von Eisenplaque-Mineralen an Reiswurzeln bekannt. Im Rahmen dieser Dissertation wurde die Bildung von sekundär gebildeten Eisenmineralen (70% Siderit, 30% Ferrihydrit, Fh & Goethit, Gt ) als Produkt der mikrobiellen Eisenplaque-Reduktion identifiziert. Diese mikrobiell reduzierten Eisenplaqueminerale immobilisierten bis zu 2,5-mal mehr As, als vollständig oxidierte Eisenplaqueminerale (Fh & Gt). Bei Untersuchungen mit 3 verschieden hohen As-Konzentrationen in Eisenplaque, wurde festgestellt, dass >1 mg As pro 10 mg Eisenplaque die mikrobielle Reduktionsrate um 50% negativ beeinflusst. Während der reduktiven Auflösung von Eisenplaque wurde As zunächst remobilisiert, aber nach etwa 7 Tagen wieder an sekundär gebildete Eisenplaqueminerale adsorbiert. Etwa 20% des ursprünglichen Arsen(V) wurden an der redoxaktiven Oberfläche der sekundären Eisenplaqueminerale zu As(III) reduziert. Die Immobilisierung auf sekundären Eisenplaquemineralen war selektiv für As(V) und erhöhte den relativen Massenanteil von As(III) in Lösung. Diese Beobachtungen helfen dabei, den Einfluss mikrobieller Fe(III)-Reduktion und deren Auswirkungen auf die Schadstoff-(Im)mobilisierung in belasteten Reisfeldböden abzuschätzen. Ermöglicht wurden die im Rahmen dieser Doktorarbeit zusammengestellten Beobachtungen durch neu entwickelte Ansätze und Methodenkombinationen. Die Möglichkeit, bisher unsichtbare Eisen-Redox-Prozesse in der gesamten Rhizosphäre der Reispflanze zu quantifizieren und den Einfluss mikroaerophiler Fe(II)-oxidierender und Fe(III)-reduzierender Mikroorganismen zu entschlüsseln, ermöglicht eine neue Sicht auf den biogeochemischen Eisen-Redox-Zyklus in wassergesättigten Reisfeldböden. Das Wechselspiel zwischen Pflanzen, Bodenorganismen und Parametern beeinflusst nicht zuletzt die physikalisch-chemischen Eigenschaften von Eisenplaquemineralen, welche wiederum die Verfügbarkeit von Bodennährstoffen und die (Im)mobilität von Schadstoffen steuern kann.Paddy fields represent one of the most important agricultural areas and serve more than half of World’s population with a major food stock – rice. Iron (Fe) is one of the most abundant redox active metals in paddy soils, mostly present as reduced ferrous iron, Fe(II) or oxidized ferric iron, Fe(III). Water-logged paddy soils, typically depleted in oxygen (O2), allow Fe to be abundant as mobile and dissolved Fe(II). Rice plants demand Fe as essential micronutrient. However, high uptake of Fe through roots can lead to a toxification and damage the photosystem. In order to control the uptake of Fe, rice plants diffusively release O2 from their roots by radial oxygen loss (ROL) which locally oxygenates the anoxic paddy soil, oxidizes Fe(II) and forms ferric non-mobile iron plaque minerals on the root surface. The formation of iron plaque on roots not only diminishes the mobility of iron but also serves as an adsorbent for contaminants in paddy fields and can reduce the uptake into rice grains. Besides rice plants, numerous other chemical and microbial redox processes can impact the speciation and appearance of Fe by the oxidation of Fe(II) and the reduction of Fe(III). This iron redox cycle in paddy soils is coupled to a large variety of other soil element cycles which can influence the fate and the (im)mobilization of many nutrients and contaminants. Representing the bottle neck for a translocation of soil contaminants into the food chain, it is crucial to better understand processes involved in the biogeochemical iron cycle in paddy fields. In particular microaerophilic Fe(II)-oxidizing bacteria that enzymatically couple the oxidation of Fe(II) to the reduction of O2 under micro-oxic conditions and their role in the paddy field iron cycle remains so far poorly understood. Moreover, a holistic study that quantitatively investigated the spatiotemporal development of root iron plaque and interactions with Fe-cycling bacteria in water-logged paddy soils during plant growth is still lacking. Specifically, the consequences of microbial Fe(III) reduction for root iron plaque minerals to serve as a sink or source for contaminants are so far scarcely documented. In this PhD thesis project, we developed new approaches that allowed a dynamic non-invasive identification of geochemical rhizosphere parameters and a quantification of root iron plaque minerals forming during the growth of rice plants. Further, we derived an enumerative understanding for microbial processes impacting the paddy field iron cycle and contaminant (im)mobility (i.e. arsenic) by Fe(II) oxidation and Fe(III) reduction over the vegetative growth of rice plants. Although microaerophilic Fe(II)-oxidizing bacteria are found in numerous environments with opposing gradients of O2 and Fe(II), little is known about their contribution to the oxidative side of the iron cycle. The rapid autocatalytic oxidation of Fe(II) with O2 at neutral pH displaces these microorganisms into niches where O2 concentrations are low enough to compete with the abiotic reaction. Concomitantly, a quantification of microbial Fe(II) oxidation rates in classical microcosms remained challenging, because abiotic and biotic Fe(II) oxidation reactions remained indecipherable so far. In particular the accumulation of ferric (bio)minerals, as a product of (a)biotic Fe(II) oxidation increases the competition by stimulating abiotic surface-catalyzed heterogeneous Fe(II) oxidation rates. In this project, we therefore developed an experimental approach that allows a quantification of microbial and abiotic Fe(II) oxidation rates in the presence or initial absence of ferric (bio)minerals. At dissolved O2 concentrations of 20 μM O2 and the initial absence of Fe(III) minerals, an Fe(II)-oxidizing culture (99.6% similarity to Sideroxydans spp.), isolated from a paddy field, contributed 40% to the total Fe(II) oxidation within approximately 26 hours and oxidized up to 3.6 × 10–15 mol Fe(II) cell–1 h–1. We found that this culture could enzymatically compete with the abiotic Fe(II) oxidation within an optimum range from 5 to 20 μM dissolved O2. Lower O2 levels limited the biotic Fe(II) oxidation, while higher O2 concentrations accelerated the abiotic Fe(II) oxidation which dominated over the microbial impact. Additionally, we could demonstrate that the initial presence of ferric (bio)minerals induced the surface-catalytic heterogeneous abiotic Fe(II) oxidation and reduced the microbial contribution to Fe(II) oxidation from 40% to only 10% at levels with 10 μM O2. We hypothesize that this newly-developed approach can be used for a large variety of microaerophilic Fe(II)-oxidizing cultures, while the obtained results can help to better assess the impact of microaerophilic Fe(II) oxidation on the biogeochemical iron cycle in numerous environmental natural and anthropogenic settings. Besides a microbial Fe(II) oxidation, also direct plant-mediated oxidation of Fe(II) by ROL can significantly influence numerous paddy field soil parameters and alter microbial communities. Especially the biogeochemistry of water-logged rice paddies, which are typically characterized by anoxic and reducing conditions, can dramatically be impacted by the temporal oxygenation of the rhizosphere with O2 from ROL. The local availability of ROL-borne O2 not only triggers the autocatalytic abiotic oxidation of Fe(II) but also provides the electron acceptor for microaerophilic Fe(II)-oxidizing bacteria. In the rice plant rhizosphere, both processes can contribute to the formation of ferric iron plaque minerals on and around the root surface. However, the identification of potential niches in the rice plant rhizosphere remained speculative so far. In this project, we temporally resolved spatial changes in ROL in the entire rice plant rhizosphere and identified the impact on the redoximorphic paddy soil biogeochemistry. By applying a series of non-invasive techniques in a transparent artificial soil, we could visualize for the first time opposing gradients of O2 and Fe(II) that extend from the rice root surface between 10−25 mm into the rhizosphere. This microoxic zone expanded exponentially in size throughout the entire rhizosphere creating optimum niches for microaerophilic Fe(II)-oxidizing bacteria during rice plant growth over 45 days. By non-invasively following and quantifying iron mineral formation, identifying changes in soil pH, and determining Fe(II) oxidation kinetics in the entire rhizosphere, we could demonstrate that root-related ROL induced iron redox transformations on and around the root surface which correlates to an acidification of the rhizosphere. These findings highlight the dynamic nature of roots in the rice plant rhizosphere while this newly-developed combination of methods spatiotemporally resolved their impact on iron redox chemistry and the formation of dynamic niches for microaerophilic Fe(II)-oxidizing bacteria in the rice plant rhizosphere. Complementing the above-reported studies, we found, that while radial oxygen loss (ROL) is the main driver for rhizosphere iron oxidation, roots tips showed the highest spatio-temporal variation in ROL (30% of the total root surface corresponding to 60-180 mg Fe(III) per gram dried root. Moreover, we found that root iron plaque minerals gradually transformed from freshly formed low-crystalline minerals (e.g. ferrihydrite) on root tips, to >20% higher-crystalline minerals (e.g. goethite) within 40 days. A culture of Fe(III)-reducing bacteria (Geobacter spp.), isolated from a rice paddy, was capable of remobilizing up to 30% Fe(II) from root iron plaque by reductive dissolution, while >50% iron plaque minerals transformed to Fe(II) minerals (e.g. siderite, vivianite and Fe–S phases) or persisted by >15% as Fe(III) minerals. Based on the obtained data, we estimated that ROL-induced root iron plaque formation and microbial reductive dissolution impact more than 5% of the total rhizosphere iron budget which can severely impact the (im)mobilization of soil contaminants and nutrients. In this context, it is generally accepted that root iron plaque on rice roots can immobilize As by sorption or coprecipitation which decreases the net uptake into the plant and diminishes its mobility in contaminated soils. However, little is known about the role of bacteria in the reduction of As-bearing Fe(III) plaque minerals or the efficiency of reduced iron plaque in As immobilization. In this project, we demonstrate the formation of secondary root iron plaque minerals (70% siderite, 30% ferrihydrite, Fh & goethite, Gt ) during microbial iron plaque reduction, that can immobilize 2.5 times more As than fully oxidized iron plaque (Fh & Gt). By comparing 3 different As-loads in iron plaque minerals, we found that >1mg As per 10 mg iron plaque can negatively affect microbial reduction rates by 50%. During reductive dissolution, As was first remobilized but re-adsorbed onto secondary iron plaque minerals after 7 days. Abiotic reduction of dissolved As(V) occurred on redox-active surfaces of secondary iron plaque minerals and produced >20% As(III) out of the initial As(V) pool. The later immobilization onto secondary iron plaque minerals was selective for As(V) and increased the relative abundance of As(III) in solution. These findings suggest that the obtained results can help to assess the role of microbial iron plaque reduction and to enumerate the consequences for the fate of As in contaminated paddy fields. In general, the reported findings in this PhD project identified a series of yet spatio-dynamically unresolved biological mechanisms that influence the iron cycle in rice paddies using newly-developed approaches and combinations of methods. The capability to quantify so far invisible iron redox processes in the entire rice plant rhizosphere and to decipher the role of microaerophilic Fe(II)-oxidizing and Fe(III)-reducing microorganisms provides a novel vision on the paddy soil biogeochemical iron redox cycle in which highly spatio-dynamic physico-chemical features can control the (im)mobility of iron, nutrients and contaminants. Overall, this PhD project provides a tangible example that yet invisible processes can be visualized by developing and combining classical and state-of-the art techniques

Microbial Transformations of Nitrogen, Sulfur, and Iron Dictate Vegetation Composition in Wetlands: A Review

The majority of studies on rhizospheric interactions focus on pathogens, mycorrhizal symbiosis, or carbon transformations. Although the biogeochemical transformations of N, S, and Fe have profound effects on vegetation, these effects have received far less attention. This review, meant for microbiologists, biogeochemists, and plant scientists includes a call for interdisciplinary research by providing a number of challenging topics for future ecosystem research. Firstly, all three elements are plant nutrients, and microbial activity significantly changes their availability. Secondly, microbial oxidation with oxygen supplied by radial oxygen loss from roots in wetlands causes acidification, while reduction using alternative electron acceptors leads to generation of alkalinity, affecting pH in the rhizosphere, and hence plant composition. Thirdly, reduced species of all three elements may become phytotoxic. In addition, Fe cycling is tightly linked to that of S and P. As water level fluctuations are very common in wetlands, rapid changes in the availability of oxygen and alternative terminal electron acceptors will result in strong changes in the prevalent microbial redox reactions, with significant effects on plant growth. Depending on geological and hydrological settings, these interacting microbial transformations change the conditions and resource availability for plants, which are both strong drivers of vegetation development and composition by changing relative competitive strengths. Conversely, microbial composition is strongly driven by vegetation composition. Therefore, the combination of microbiological and plant ecological knowledge is essential to understand the biogeochemical and biological key factors driving heterogeneity and total (i.e., microorganisms and vegetation) community composition at different spatial and temporal scales

Increasing plant availability of selenium in rice soils under variable redox conditions.

This research focused on increasing plant availability of Se in Se-deficient paddy soils by agronomic biofortification. However, Se fertilization is complicated by the fact that the margin of safety between levels of Se compounds that will cause dietary deficiency and those that result in toxicity is small. The study was designed to understand the changes in Se availability in paddy rice soils under different moisture conditions and management practices. This knowledge was then used to develop an effective Se fertilization strategy. Because Se exists in several redox states and these can be transformed in soil by processes of oxidation or reduction, the study evaluated the effectiveness of different redox species of Se as fertilizers. In addition, methods of application and times of application were also evaluated. As the speciation of Se in rice grain also affects bioavailability to humans, Se speciation in rice grain was also evaluated. Isotopic dilution techniques were used to understand the potential availability of selenite (SeO₃⁻² ), selenate (SeO₄⁻² ) and elemental Se (Se (0)) applied to soils subjected to different water regimes – field capacity or submerged soil conditions. The availability of fertilizer Se (0), as measured by concentrations of labile Se species in soil, was low because of limited oxidation to SeO₃⁻² or SeO₄⁻². Elemental Se is therefore not suitable for pre-plant Se fertilization of lowland rice because it is not readily oxidized in paddy rice soils. In the submerged soils, concentrations of labile SeO₃⁻² and SeO₄⁻² were also low. More than, 80% of the Se added as either SeO₃⁻² or SeO₄⁻² was fixed into non-labile pools, likely through reduction to Se (0). Rates of oxidation of Se (0) will play a critical role either in determining whether reduced Se (0), which likely formed in submerged soils after fertilization, will contribute to plant Se uptake through oxidation during field drainage before harvest or in the rice rhizosphere. Kinetics of Se transformations occurring when a paddy soil is fertilized, flooded, and then re-oxidised were investigated. The results showed applied SeO₃⁻² was very quickly and completely transformed to non-labile pools under flooded soil conditions, with no detectable oxidation to SeO₄⁻² during the drainage period (7 days). Applied SeO₄⁻² was much more labile than SeO₃⁻², but the lability also decreased with time under submerged conditions and did not increase markedly during the drainage phase. These results indicate that SeO₃⁻² would not be an effective pre-plant fertilizer for rice production. Selenate is likely to be more effective, but losses to non-labile forms during the submerged phase of production also means that efficiency of pre-plant SeO₄⁻² fertilization is also compromised. The results of the first pot study indicated that most accumulation of Se in the grain occurred with SeO₄⁻² fertilizer when applied at heading; SeO₄⁻² -enriched urea applied at heading increased grain Se concentrations 5 to 6 fold (450 to 600 μg kg⁻ ¹) compared to the control (no Se fertilizer) in all three moisture treatments. Foliar SeO₃⁻² at heading and fluid SeO₄⁻² applied at heading in field capacity treatments increased grain Se 3.5- and 6-fold respectively compared to the control. The majority of Se in rice grains was identified in all treatments to be selenomethionine (SeM) which comprised over 90% of total grain Se. Selenate-enriched urea was the most effective Se fertilization strategy for paddy rice. A rice-growth experiment carried out with ⁷⁵Se radioisotope spiked fertilizer granule confirmed that Se-enriched urea granules applied at either tillering or heading produced significantly higher grain Se concentrations compared to any other Se application method. There were also higher concentrations of SeO₄⁻² in flood and pore water samples following the co-application of Se and urea compared to the other treatments. These results showed that urea applied in combination and co-located with SeO₄⁻² had a significant effect on the uptake and accumulation of Se in rice grains. Selenate-enriched urea granules applied to floodwater sank to the well-developed mat of adventitious roots on the soil surface. Oxygen release from the rice roots promotes production of NO₃⁻ from applied urea, which may have inhibited SeO₄⁻² reduction. The ability to maintain high concentration of SeO₄⁻² in soil solution with minimum speciation changes could be the reason for the higher uptake and accumulation of Se in rice grains in SeO₄⁻² -enriched urea treatment. Farmers well adapted to applying urea at tillering or heading are in a position to quickly and easily adopt these management practices. Future research should be conducted to verify these results under field conditions with other rice varieties and on different soil types.Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 201

A review on regulatory control of chromium stress in plants

Chromium (Cr) is a non-biodegradable heavy metal that persists long in aquatic and terrestrial ecosystems and enters the food chain. It is cytotoxic even at low concentrations and reduces the yield of plants. Plants also have cellular mechanisms to manage the accumulation of metal ions inside the cell to diminish the possible injury from non-essential metal ions. This paper reviews current information on plant response to Cr, a key environmental pollutant. The harmful effects together with absorption, transfer, and aggregation of Cr are discussed. The roles of the cell wall, plasma membrane, and plant microbes as the primary hindrances for Cr ingression into the cell, along with sequestration and compartmentalization process, have also been discussed. Cr-generated oxidative injury is also regarded as the main deliberated effect of Cr toxicity.  It interferes with NADPH oxidases (plasma membrane) and the electron transport chains, which develop electron leakage. Some genes related to Cr stress in plants get expressed, and suppression produces protective effects by activating the signal transduction pathways. The expression of genes like BnaCnng69940D and BnaC08g49360D is increased, which is involved in protein kinase activity, signal transduction, and oxidoreductase activity. The increased mRNA levels of Cr stress response proteins, including HSP90-1 and MT-1, have been reported in the Brassica napus plant. The stressed environment around the plants may stimulate the biosynthesis of phytochelatins and metal-binding proteins, which have a protective role in plant’s growth and development.

Plant microbial fuel cell in paddy field : A power source for rural area

As an energy carrier, electricity access is one of important aspect for human development. There is a positive correlation between electricity consumption per capita and human development index (HDI) and also gross domestic product (GDP).  However, the world electrification is not equally distributed. Most of those who do not have electricity access live in rural areas and located in developing countries. In these area, some people use polluted kerosene lamps as their light source or expensive gasoline generator as their electricity source. Other than that, battery is also widely used as a power source. In addition to the unequal electrification, the world electricity generation is still dominated by fossil fuel sources that have a negative impact on the environment, increased health risk and global climate change. Therefore, it is important to shift from conventional energy source to low-carbon renewable electricity sources. This thesis “Plant Microbial Fuel Cell in Paddy Field: a power source for rural area“ aims to assess the applicability of the plant  microbial fuel cell (Plant-MFC) as a low power off-grid power source in a rural area for a theoretical Indonesian case. To achieve this, a technical design was made for a household in rural area of Indonesia based on the latest research developments. Then, the applicability was assessed on technical, social, and environmental safety and health criteria as well as economics and some scenarios were suggested which could improve the real application. Values for a plant-MFC system to fulfil basic electricity needs were calculated. The main highlights and findings on this work are summarized in accordance with the chapters outlined in this thesis as following. Chapter 2 “Marine Sediment Mixed with Activated Carbon Allows Electricity Production and Storage from Internal and External Energy Sources: A New Rechargeable Bio-Battery with Bi-Directional Electron Transfer Properties” investigates the abilities of marine sediment and activated carbon to store and generate electricity in a bio-battery. In this work, several mixture of marine sediment and activated carbon were studied in a bio electrochemical system (BES). When operated in the MFC mode, the system generated electricity with solely marine sediment as the anode electron donor, resulted in the creation of a bio-battery. The results show that by usage of marine sediment and activated carbon (AC) electricity was generated and stored. The internal electrical storage density is 0.3 mWh/kg AC marine anode.  These insights give opportunities to apply such BES systems as e.g. ex-situ bio-battery to store and use electricity for off-grid purpose in remote areas. Chapter 3 “Activated Carbon Mixed with Marine Sediment is Suitable as Bioanode Material for Spartina anglica Sediment/Plant Microbial Fuel Cell: Plant Growth, Electricity Generation, and Spatial Microbial Community Diversity” aims to investigate the suitability of a mixture of activated carbon and marine sediment as a bioanode in a plant-MFC system with Spartina anglica. This work focused on study how different mixtures of the activated carbon (AC) and the marine sediment (MS) as an anode material affected the plant vitality, electricity generation and spatial microbial community. Results show that Spartina anglica grew in all of the plant-MFCs, although the growth was less fertile in the 100% activated carbon Plant-MFC. On long-term (2 weeks) performance, mixture of 33% and 67% marine sediment outperformed other Plant-MFCs in terms of current density (16.1 mA/m2 plant growth area) and power density (1.04 mW/m2 plant growth area). Results also show a high diversity of microbial communities dominated by Proteobacteria and indicates that the bacterial communities were affected by the anode composition. These findings show that the mixture of activated carbon and marine sediment are suitable material for bioanodes and could be useful for the application of Plant-MFC in a real wetland. Chapter 4 “Performance and Long Distance Data Acquisition via LoRa Technology of a Tubular Plant Microbial Fuel Cell Located in a Paddy Field in West Kalimantan, Indonesia” provide an insight about the field performance of tubular Plant-MFC. In this study, one-meter tubular Plant-MFC with graphite felt anode and cathode were installed in triplicates in a paddy field for four rice growth seasons. An online data acquisition using LoRa technology was developed to investigate the performance of the tubular Plant-MFC over the final whole rice paddy growing season. The result revealed that the Plant-MFC do not negatively affect the rice growth. A continuous electricity generation was achieved during a wet period in the crop season. On average the Plant-MFC generated power of 6.6 mW/m2 plant growth area (0.4mW per meter tube). The Plant-MFC also shows a potential to be used as a bio sensor, e.g. rain event indicator, during a dry period between the crop seasons. Chapter 5 “A Thin Layer of Activated Carbon Deposited on Polyurethane Cube Leads to New Conductive Bioanode for (Plant) Microbial Fuel Cell” exploits the potential of electrochemically active self-assembled biofilms to fabricate three-dimensional bio electrodes for of (plant) microbial fuel cells with minimum use of electrode materials. For this purpose, polyurethane foams coated with activated carbon was prepared and studied as platform bio anodes for harvesting electric current in lab microbial fuel cells (MFCs) and field Plant-MFCs. Results show that electric conductivity of the PU/AC electrode enhance over time during bioanode development. The maximum current and power density of an acetate fed MFC reached 3mA/m2 projected surface area of anode compartment and 22mW/m3 anode compartment. The field test of the Plant-MFC reached a maximum performance of 0.9 mW/m2 plant growth area at a current density of 5.6 mA/ m2 PGA. A rice paddy field test showed that the PU/AC electrode was suitable as anode material in combination with a graphite felt cathode.  Finally, the main findings of this thesis are summarized and discussed in Chapter 6, “General Discussion”. In this chapter, a theoretical available power for Plant-MFC system from a paddy field is presented to give an insight how far performance of current Plant-MFC meets theoretical understanding. Based on the experimental results, this chapter answers the thesis goal to discuss the applicability of the Plant-MFC as an off-grid power source in a rural area by assessing its technical, economic, social, and environmental safety and health criteria. Finally, an outlook for future Plant-MFC application is provided