Microalgae and their effects on metal bioavailability in paddy fields

Abstract

Heavy metal contamination in paddy soils poses a serious threat to rice quality. However, these risks may be reduced by biological processes that sequester or transform metals into less mobile forms. Microalgae are commonly found in wetland ecosystems, but their effects on soil properties and metal uptake by rice remain unresolved. In this paper, we summarize the available literature to evaluate the role of microalgae in metal bioavailability in paddy fields. Up-to-date information was gathered and analyzed to provide a deeper understanding of metal pollution in paddy soils; factors controlling metal bioavailability; and the types of unicellular algae present, their mechanisms of metal accumulation and detoxification, and their effects on various properties such as pH, redox potential (Eh), and organic matter; as well as the bioavailability of different metal(loid)s in paddy soils. Metal accumulation in rice and its associated risks to human health do not depend on the total amount of metals in soils, but are controlled by metal bioavailability, which in turn is affected by soil chemical properties and biological processes. As an important component of the biota in paddy soils, microalgae not only contribute significantly to carbon and nitrogen fixation through photosynthesis, but also strongly modify key soil parameters including pH, Eh, dissolved oxygen, and organic matter. Such effects undoubtedly have considerable impacts on metal phytoavailability and biogeochemical cycles in paddy fields. Although many studies have been carried out to classify microalgae and describe their ecological functions, the effects of microalgae on metal bioavailability have not been extensively studied and no detailed mechanisms have been elucidated. Therefore, further investigations are warranted in this field. Future research should focus on identifying additional microalgae strains, characterizing their physiological activities, and quantifying their impacts on metal uptake in rice.