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Abstract
The extracellular electron transfer (EET) of the phytoplankton is of great interest due to its significance to a variety of biogeochemical redox processes, including the cycling of iron, and the generation of reactive oxygen species (ROS) and organic exudate. In order to improve our understanding of the impact of algogenic organic matters on the iron redox chemistry under conditions of typical oxic natural waters, the kinetics of iron redox transformations in presence of exudate of Microcystis aeruginosa was investigated at both pH 4 and pH 8. Our experimental results show that the organic exudate secreted by Microcystis aeruginosa contains short-lived Fe(III) reducing moieties possibly to facilitate the iron uptake by algae. Furthermore, our results support that these organic exudates have ability to complex Fe, thereby resulting in increased Fe(II) oxygenation and Fe(III) reduction rates. On irradiation, these organic exudates generate ROS like superoxide that further enhances the Fe(III) reduction rate. Based on our experimental results, we have developed a mathematical kinetic model to explain the ROS generation and Fe redox transformation under non-irradiated and irradiated conditions. We have also investigated the impact of the algal exudate on the formation, aggregation and reactivity of iron oxides. Our results show that these organic exudates have stabilizing effect on the particle growth and also impact the aggregate structure. Furthermore, these exudates coated particles show higher reactivity than the bare iron oxide particles.
The biological generation of ROS is regarded to be associated with the algae toxicity that causes fish mortality. In the second part of this project, the oxidants generation by a toxic raphidophycean, Chattonella marina, was studied using formate as an organic probe. The alga has ability to generate oxidant under irradiated condition at a rate of 7.2 nM h−1 cell-1. Our results further show that this oxidant is not hydroxyl radical, superoxide and/or peroxynitrite; however further work needs to determine the exact identity of this oxidant(s).
To show the EET of phytoplankton more directly, the current output of a mediator less photosynthetic microbial fuel cell inoculated with Chattonella marina was observed. Electricity output by suspended culture is higher than that in biolfilm, suggesting the soluble electron carriers secreted by Chattonella marina can facilitate the electron transport, which is also supported by the electrochemical characterization with two pairs of redox peaks.