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This study has thoroughly uncovered the characteristics of the marine euphotic zone system in the Beibu Gulf, featuring adjacent semiconducting minerals and electrochemically active microbes, via mineralogical characterization, photoelectrochemical measurements, and microbiological analysis techniques. Notably, results of Raman and ESEM with EDS indicated that anatase, the semiconducting mineral with prominent photocatalytic performance, exists in large quantities in the Beibu Gulf. Photoelectrochemical measurements tested photoelectric response and redox activity of suspended particulate minerals, demonstrating the suspended particulates dominated by titanium and iron oxide semiconducting minerals presented excellent photo-response property and distinct photoelectric catalytic potential under visible light irradiation. Its significant property of photoelectric response with 110.483% increment ratio of the average photocurrent density relative to the dark current density. 16S amplicon sequence analysis revealed that the euphotic zone was dominated by Proteobacteria, Actinobacteria, and Cyanobacteria at the phylum level, and by Synechococcus CC9902, Cyanobium PCC-6307 and unclassified Ilumatobacteraceae at the genus level. Halomonas (0.2–1.2%), Vibrio (0.7–1.8%), and Nautella (0.1–1.1%), closely related to semiconducting minerals reduction reaction, were also presented in YL and RGS samples of the Beibu Gulf. This work firstly revealed that the neglected characteristics of the marine euphotic zone system in the Beibu Gulf of the South China Sea, which is an important natural ecosystem for microorganisms to uptake energy sources. It will provide a new perspective for further research on novel ways for microorganisms to acquire energy sources through semiconducting minerals in the marine euphotic zone system. [ABSTRACT FROM AUTHOR]

Dolomite exhibits a wide distribution in geological strata. The metabolic activities of microorganisms in marine sediments play a crucial role in the formation of dolomite. Semiconducting minerals, such as hematite, goethite, and rutile, generate photoelectrons when exposed to sunlight, which can impact the community structure and metabolic activities of microorganisms. In this study, a simulated photoelectron system was conducted to investigate the response of the microbial community, as well as the regulation of sulfate reduction, to photoelectrons using high-throughput sequencing of the 16S rRNA gene. The regulatory effect of semiconducting mineral photoelectrons on the induction of carbonate precipitation by sulfate-reducing bacteria was explored. X-ray diffraction and Raman spectroscopy were used to characterize carbonate precipitation. During cultivation, the pH values of the system increased from 8.0 to approximately 8.5 and the rate of sulfate reduction was significantly enhanced under the influence of simulated photoelectrons. The alpha diversity of the microbial community decreased, and the semiconducting mineral photoelectronic system had a promoting effect on the enrichment of sulfate-reducing bacteria, mainly Desulfovibrio. Under the regulation of photoelectrons, sulfate-reducing bacteria can effectively oxidize organic matter and reduce sulfate in the environment, and proto-dolomite can be formed at a low Mg/Ca ratio. This process has important implications for carbon and sulfur element cycling in estuarine and oceanic photic zones, and provides a new explanation for the formation of large amounts of dolomite in geological history. [ABSTRACT FROM AUTHOR]

In the estuary and nearshore environments, suspended particulate matter (SPM) plays a particularly important role. This article presents a study on the suspended particulate matter and microbial communities in the euphotic zone of China's nearshore and estuarine regions. The study used various analytical techniques, including ICP-OES, SR-XRD, confocal Raman microscopy, ESEM, and EDX, to investigate the spatial distribution and elemental and mineral compositions of the suspended particulate matters. The study found that semiconducting minerals, such as iron oxide, sulfide minerals (hematite, goethite, and pyrrhotite), and titanium oxide minerals (rutile and anatase), were widely present in the suspended particulate matter. This discovery highlights the photochemical activity of suspended particulate matter in the euphotic zone. Furthermore, a correlation analysis of microbial communities revealed that the content of suspended particulate matter was positively correlated with denitrifying bacteria and metal-reducing bacteria in seawater. This study provides valuable insight into the ecosystem dynamics and biogeochemical cycling of estuaries and coastal seas, which are critical for sustaining the biodiversity and productivity of these ecosystems. [ABSTRACT FROM AUTHOR]

Marine euphotic zone is the pivotal region for interplay of light-mineral–microorganism and elements cycle, in which semiconducting minerals exist widely and iron-bearing goethite is a typical and widespread one. In this work, we have conducted in-depth researches on the effect of ferrous [Fe(II)] ions dissolved by photoreduction of goethite on microbial community structure and diversity. The mineral phase, structure and morphology of synthesized goethite were characterized by Raman, X-ray diffraction (XRD), energy disperse spectroscopy (EDS), environmental scanning electron microscope (ESEM), and atomic force microscope (AFM). Photoelectrochemical measurements tested photoelectric response and redox activity of goethite, having proved its significant property of photoelectric response with 44.11% increment of the average photocurrent density relative to the dark current density. The photoreduction experiments of goethite were conducted under light condition in simulated seawater. It has suggested the photoreduction of goethite could occur and Fe(III) was reduced to Fe(II). The dissolved Fe(II) from the photoreduction of goethite under light condition was nearly 11 times than that group without light after a 10-day reaction. Furthermore, results of microbial community sequencing analysis indicated that dissolved Fe(II) could affect the structure and regulate the decrease of microbial community diversity. The emergence of dominant bacteria associated with iron oxidation and transport protein has suggested their obvious selectivity and adaptability in the environment with adding dissolved Fe(II). This work revealed the photoreduction process of semiconducting goethite was remarkable, giving rise to a non-negligible dissolved Fe(II) and its selective effect on the structure, diversity, as well as the function of microbial community. This light-induced interaction between minerals and microorganisms may also further regulate correlative metabolic pathways of carbon cycle in the marine euphotic zone. [ABSTRACT FROM AUTHOR]

Based on the most active hydrosphere on the earth, the interplay between adjacent semiconducting minerals and electrochemically active microorganisms was scientifically researched in the marine euphotic zone of the Yellow Sea, China. Fe and Ti were found in suspended minerals by ICP-MS and EDS, probing the element composition of suspended minerals from macro to micro. Moreover, SR-XRD and Raman indicated that semiconducting minerals of iron and titanium metal oxide in situ such as anatase, rutile, brookite, and goethite consisted in suspended minerals, notably, which retained prominent photo-response property and distinct photoelectric catalytic potential under visible light. 16S rRNA gene systematically emerged that electrochemically active microorganisms, such as Pseudomonas and Paraclostridium, were pivotal in the solution and preponderant on the electrode respectively after a period of cultivation. Whereupon a dual-chamber reactor was established to probe into the mechanism of redox reaction and electron transfer process between microorganisms and suspended minerals. Importantly, compared with a graphite electrode, the maximum power density of the reaction was 2.78 times with the increase of suspended minerals as the electron acceptor under dark conditions, as well as 236.53% promotion with visible light. Experiments gave an index that semiconducting minerals could interact with electroactive microorganisms, effectively promoting the process of extracellular electron transfer in the euphotic zone, which will make a foundation for further research on electron energy transfer based on the interactions of semiconducting minerals and electrochemically active microorganisms, as well as the regulation mechanism of elements cycling. [ABSTRACT FROM AUTHOR]

The photoelectrochemical characteristic of semiconducting minerals plays an imperative role in supporting the growth of many electroactive microorganisms. In order to study the effects of photoelectrons triggered from semiconducting mineral by solar energy on the marine microbial growth, metabolism, and community structure, a dual-chamber electrochemical system was established by incubating seawater microbial community with varying simulated electrochemical conditions. The absolute quantifications of 16S rRNA and 18S rRNA genes suggested that photoelectrons could support the growth and reproduction of bacteria with an increase of two magnitudes from 1.34 ± 0.76 × 1012 to 1.41 ± 0.93 × 1014, and archaea with a slight increase from 0.50–8.74 × 1011 to 2.37–14.80 × 1011, whereas not for the growth of eukaryote in consideration of the complicated regulating mechanism of eukaryotic cells. The bacterial and archaeal communities were shaped by the supplement of photoelectron, also not for the eukaryotic community. Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria accounted for over 80% of the total bacterial community. Photoelectrons with appropriate energy could stimulate the diversity and maintain the community structure of bacteria and archaea while lower or higher electric potential caused lower diversity and more divergent microbial assemblages. The abundance of genes related to the electron transport chain was significantly higher in photoelectron regulated setups than that in open circuit ones, indicating that the extracellular photoelectrons experienced the transmembrane transition and entered the bacterial electron transport chain coupling cell anabolism and catabolism. This study proposed the potential impact of photoelectrons produced by semiconducting minerals on marine microorganisms and the putative electron flow under sunlight in the natural environments. [ABSTRACT FROM AUTHOR]

The solar light response and photoelectrons produced by widespread semiconducting mineral play important roles in biogeochemical cycles on Earth's surface. To explore the potential influence of photoelectrons generated by semiconducting mineral particles on nitrate-reducing microorganisms in the photic zone, a marine heterotrophic denitrifier Halomonas sp. strain 3727 was isolated from seawater in the photic zone of the Yellow Sea, China. This strain was classified as a Halomonadaceae. Whole-genome analysis indicated that this strain possessed genes encoding the nitrogen metabolism, i.e., narG, nasA, nirBD, norZ, nosB, and nxr, which sustained dissimilatory nitrate reduction, assimilatory nitrate reduction, and nitrite oxidation. This strain also possessed genes related to carbon, sulfur, and other metabolisms, hinting at its substantial metabolic flexibility. A series of microcosm experiments in a simulative photoelectron system was conducted, and the results suggested that this bacterial strain could use simulated photoelectrons with different energy for nitrate reduction. Nitrite, as an intermediate product, was accumulated during the nitrate reduction with limited ammonia residue. The nitrite and ammonia productions differed with or without different energy electron supplies. Nitrite was the main product accounting for 30.03% to 68.40% of the total nitrogen in photoelectron supplement systems, and ammonia accounted for 3.77% to 8.52%. However, in open-circuit systems, nitrite and ammonia proportions were 26.77% and 11.17%, respectively, and nitrogen loss in the liquid was not observed. This study reveals that photoelectrons can serve as electron donors for nitrogen transformation mediated by Halomonas sp. strain 3727, which reveals an underlying impact on the nitrogen biogeochemical cycle in the marine photic zone. [ABSTRACT FROM AUTHOR]

• Reveal the microorganism-mineral electron transfer in the euphotic zone. • Extracellular electron transfer from P. aeruginosa PAO1 to semiconducting anatase. • Anatase with photoelectric activity promotes extracellular electron transfer. • Both direct and indirect electron transfer appear between anatase and PAO1. • Pyocyanin and pili of PAO1 dominate extracellular electron transfer process. Focusing the marine euphotic zone, which is the pivotal region for interaction of solar light-mineral-microorganism and the elements cycle, we have conducted the research on the mechanism of semiconducting minerals promoting extracellular electron transfer with microorganisms in depth. Therein, anatase which is one of the most representative semiconducting minerals in marine euphotic zone was selected. The mineralogical characterization of anatase was identified by ESEM, AFM, EDS, Raman, XRD, and its semiconducting characteristics was determined by UV–Vis and Mott-Schottky plots. Determined by the electrochemical measurement of I-t curves, the photocurrent density of anatase was more prominent than dark current density. Pseudomonas aeruginosa PAO1 was widely distributed in the euphotic zone, and its mutants of operons deficient in biosynthesis pyocyanin (Δ phz1 Δ phz2) and pili deficient (Δ pilA) were employed in this study. I-t curves indicated that both direct and indirect extracellular electron transfer processes occurred between anatase and PAO1. The indirect electron transfer depending on pyocyanin secreted by PAO1 was the main electron transfer mode. This work demonstrated the light-driven extracellular electron transfer and further revealed the photo-catalyzed mechanisms between anatase and PAO1 in marine euphotic zone. [ABSTRACT FROM AUTHOR]

The photo-enhanced dissolution of semiconducting birnessite by organic reductants prevalently occurs in the euphotic zones of natural aquatic systems. However, the underlying photoelectrons transfer mechanisms and kinetic factors are not fully understood. In this study, the anoxic photoreduction of δ-MnO 2 (a phase analogue for natural birnessite) by citrate under simulated solar irradiation at pH 4.0, 5.5 and 7.0 was investigated, and the structural evolution of δ-MnO 2 during the reaction was examined. The photoreduction of δ-MnO 2 was observed at all three pHs. ~86.0% and ~32.7% Mn2+ was released into the solution at pH 4.0 and 5.5, respectively; while negligible amount of dissolved Mn2+ was produced at pH 7.0. Mn average oxidation state (AOS) in both the bulk and surface of δ-MnO 2 lowered significantly with decreasing pH (3.39/2.56, 3.57/3.07 and 3.78/3.19 in bulk/surface at pH 4.0, 5.5 and 7.0, respectively). The increase in the photoreduction rate and extent of δ-MnO 2 at lower pHs indicated the photoelectron transfer was pH-dependent. From the amount of reduced Mn(III/II) in structure and Mn2+ in solution, the average photoelectron transfer amount per mol Mn was calculated as 1.81, 0.95 and 0.22 at pH 4.0, 5.5 and 7.0, respectively. The retention of Mn(II) (~13% in content) in the structure of reacted δ-MnO 2 was only observed at pH 4.0 rather than higher pHs. The photoreduction of Mn(IV) involved two successive steps of single-electron transfer to produce Mn(III) and further Mn(II), and the above results suggested the second-step photoelectron transfer was more effective under acidic conditions. The potential difference between the conduction band (CB) and MnO 2 /Mn2+ half-reaction was enlarged as pH decreased, which conduced to the optimal effectiveness of photoelectron transfer and thorough reduction of Mn(IV/III) under lower pH conditions. The pH-dependent Mn photoreduction occurred throughout the structure of δ-MnO 2 , whereas for the dark reaction, the influence of pH on the reduction extent only took effect on the surface. These observations implied the dark-reaction electron transfer predominantly occurred at the surface sites. By contrast, according to the semiconductor energy band theory, the excited CB photoelectrons were shared by all atoms in the lattice of δ-MnO 2 , and each Mn atom had equal chance to be reduced by photoelectrons. Furthermore, the accumulation of photo-reduced and large-size Mn(III/II) in the layer caused the spatial expansion of the in-plane crystallization, and could possibly alter the layer symmetry or even promote the long-term structural transformation towards tunneled Mn oxides. This work provides insights into the photocatalytic geochemical performance of the naturally-abundant birnessite, which is influenced by varied pH conditions on Earth's surface. Unlabelled Image • The Mn(IV) reduction of δ-MnO 2 by citrate is enhanced under light at pH 4.0–7.0. • The photoreduction is a H+-promoted two-step single-photoelectron transfer process. • The rate controlling step for δ-MnO 2 photoreduction is the second electron transfer. • Citrate-mediated Mn(IV) photoreduction in δ-MnO 2 occurs in the whole structure. [ABSTRACT FROM AUTHOR]