Molecular weight driving bioavailability and intrinsic degradation mechanisms of dissolved organic phosphorus in lake sediment

Influence of anti-seasonal inundation on geochemical processes of arsenic speciation in the water-level-fluctuation zone soil of the Three Gorges Reservoir, China

Since the completion of Three Gorges Dam, the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) experiences the periodic anti-seasonal inundation. However, knowledge for mechanisms of mobilization and transformation of arsenic (As) in WLFZ soils of the TGR remains scarce. To address this gap, a combination of field observation and simulated flooding experiments attempts to illustrate the As mobilization, the transformation between As(V) and As(III), and the factors driving these processes. The study revealed that anti-seasonal inundation (with a temperature at 13 ℃) mitigated As release from submerged soils. Interestingly, the total As and ratio of As(III) (the more toxic form of As) concentrations in porewater at 13 ℃ was lower, and the prevalence of As(III) occurred later than those at 32 °C (imitate the seasonal inundation condition). The results indicated that the As reduction and the corresponding toxic risks in submerged soils were alleviated under anti-seasonal inundation. The study proposes the reduction of As-bearing manganese (Mn) mineral assemblages and competitive adsorption of dissolved organic carbon (DOC) as primary mechanisms for As mobilization. Furthermore, microorganism-mediated detoxification/reduction processes involving DOC, nitrogen, and Mn (oxyhydr)oxides were identified as central pathways for As(III) enrichment under anti-seasonal inundation. This study enhances understandings of the biogeochemical processes and fate of As in WLFZ soils influenced by artificial regulation of the reservoir.

Particulate and water-mobilizable phosphorus from a watershed with a plain river network contributes equal amounts of algal available phosphorus to its downstream lake

This research was designed to estimate the contributions of phosphorus (P) in different factions from an upstream plain river network to algal growth in a downstream shallow eutrophic lake, Taihu Lake, in China. During three flow regimes, the P fractions in multiple phases (particulate, colloidal and dissolved phases) and their algal availabilities were assessed via bioassays with Dolichospermum flos-aquae as the test organism. The P partitioning patterns among multiple phases were strongly affected by the concentration of total suspended solids (TSS) that changed with the river flow regime, with stronger disturbance of sediments at lower water levels (low flow) and weaker disturbance of sediments at higher water levels (high flow) in the plain river network. The median TSS concentration across the river network decreased from 157.4 mg/L during low flow to 31.8 mg/L during high flow, and the median particulate P concentration decreased from 0.132 mg/L to 0.093 mg/L. The particulate P contributed equally to the amount of algal available P (AAP) as did the water-mobilizable P (colloidal plus dissolved phase) in the rivers flowing into Taihu Lake. The annual average concentrations of particulate algal available P (P-AAP), colloidal algal available P (C-AAP) and dissolved algal available P (D-AAP) were estimated to be 0.032 mg/L, 0.012 mg/L and 0.019 mg/L, respectively, during 2012-2018, accounting for 50.8 %, 19.0 % and 30.2 %, respectively, of the total AAP. At the watershed scale, controlling P drainage from downstream urbanized areas should be emphasized. Additionally, controlling sediment resuspension or reducing the TSS concentration in the inflowing rivers is important for decreasing the particulate P flux to downstream lakes.

Dissolved organic matter influences the indigenous bacterial community and polycyclic aromatic hydrocarbons biodegradation in soils

In polycyclic aromatic hydrocarbon (PAH) contaminated soils, bioremediation is superior to other strategies owing to its low cost and environmental friendliness. However, dissolved organic matter (DOM) and indigenous bacterial communities can affect the efficiency of PAH-degrading bacteria (PDB). This study found that exogenous PDB (C1) including the genera Acinetobacter, Stenotrophomonas, and Comamonas, decreased the bacterial diversity of Alfisol, Ultisol, Inceptisol, and Mollisol, and DOM enhanced the diffusion of PDB and the bioavailability of PAH. In addition, bacteria preferred to ingest low molecular weight DOM fractions, and the abundances of lipid-like and protein-like substances decreased by 0.12-3.03 % and 1.73-4.60 %. The DOM fractions had a more marked influence on the indigenous bacteria than the exogenous PDB, and PDB dominated the PAH biodegradation process in the soils. More COO functional groups promoted the utilization of higher molecular weight-related homologue fractions by bacteria, and lower molecular weight fractions carrying more CH2 functional groups declined during biodegradation. This study investigated the variations in bacterial communities during biodegradation and revealed the effects of DOM fractions on biodegradation in PAH-contaminated soils at the molecular level. These results will promote the development of bioremediation strategies for organics-contaminated soil and provide guidance for prediction models of soil biodegradation kinetics.

Molecular insights into the microbial degradation of sediment-derived DOM in a macrophyte-dominated lake under aerobic and hypoxic conditions

The mineralization of dissolved organic matter (DOM) in sediments is an important factor leading to the eutrophication of macrophyte-dominated lakes. However, the changes in the molecular characteristics of sediment-derived DOM during microbial degradation in macrophyte-dominated lakes are not well understood. In this study, the microbial degradation process of sediment-derived DOM in Lake Caohai under aerobic and hypoxic conditions was investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and metagenomics. The results revealed that the microbial degradation of sediment-derived DOM in macrophyte-dominated lakes was more intense under aerobic conditions. The microorganisms mainly metabolized the protein-like substances in the macrophyte-dominated lakes, and the carbohydrate-active enzyme genes and protein/lipid-like degradation genes played key roles in sediment-derived DOM degradation. Organic compounds with high H/C ratios such as lipids, carbohydrates, and protein/lipid-like compounds were preferentially removed by microorganisms during microbial degradation. Meanwhile, there was an increase in the abundance of organic molecular formula with a high aromaticity such as tannins and unsaturated hydrocarbons with low molecular weight and low double bond equivalent. In addition, aerobic/hypoxic environments can alter microbial metabolic pathways of sediment-derived DOM by affecting the relative abundance of microbial communities (e.g., Gemmatimonadetes and Acidobacteria) and functional genes (e.g., ABC.PE.P1 and ABC.PE.P) in macrophyte-dominated lakes. The abundances of lipids, unsaturated hydrocarbons, and protein compounds in aerobic environments decreased by 58 %, 50 %, and 44 %, respectively, compared to in hypoxic environments under microbial degradation. The results of this study deepen our understanding of DOM biodegradation in macrophyte-dominated lakes under different redox environments and provide new insights into nutrients releases from sediment and continuing eutrophication in macrophyte-dominated lakes.

Revealing the characteristics of biofilms on different polypropylene plastic products: Comparison between disposable masks and takeaway boxes

The increasingly severe plastic pollution issue was intensified by the enormous plastic emissions into ecosystems during the Covid-19 pandemic. Plastic wastes entering the environment were swiftly exposed to microorganisms and colonized by biofilms, and the plastic-biofilm combined effects further influenced the ecosystem. However, the non-woven structure of disposable masks discarded carelessly during the COVID-19 pandemic was different from those of plastics with flat surface. To reveal the potential effects of plastic structure on colonized biofilms, white disposable surgical masks (DM) and transparent takeaway boxes (TB), both made of polyethylene, were selected for the incubation of organic conditioning films and biofilms. The results indicated that the non-woven structure of disposable mask was destroyed by the influence of water infiltration and biofilm colonization. The influence of surface structure on conditioning films led to a relatively higher proportion of tryptophan-like substances on DM than those on TB samples. Therefore, biofilms with significantly higher microbial biomass and carbon metabolic capacity were formed on DM than those on TB samples owing to the combined effects of their differences in surface structure and conditioning films. Moreover, abundant functional microorganisms associated with stress tolerance, carbon metabolism and biofilm formation were observed in biofilms on disposable mask. Combining with the results of partial least squares regression analysis, the selective colonization of functional microorganisms on disposable masks with uneven surface longitudinal fluctuation was revealed. Although the predicted functions of biofilms on disposable masks and takeaway boxes showed more similarity to each other than to those of free-living aquatic microorganisms owing to the existence of the plastisphere, biofilms on disposable masks may potentially trigger environmental risks different from those of takeaway boxes by unique carbon metabolism and abundant biomass.

Suspended phosphorus sustains algal blooms in a dissolved phosphorus-depleted lake

The expansion of algal bloom in surface waters is a global problem in the freshwater ecosystem. Differential reactivity of organic phosphorus (P_o) compounds from organic debris, suspended particulate matter (SPM), and sediment towards hydrolysis can dictate the extent of supply often limited inorganic P (P_i) for algal growth, thereby controlling the extent of bloom. Here, we combined solution P-31 nuclear magnetic resonance (³¹P NMR), sequential extraction, enzymatic hydrolysis, and 16S rRNA measurements to characterize speciation and biogeochemical cycling of P in Lake Erhai, China. Lower ratios of diester-P/monoester-P in SPM in January (mean 0.09) and July (0.14) than that in April (0.29) reflected the higher degree of diester-P remineralization in cold and warm months. Both H₂O-P_i and P_o were significantly higher in SPM (mean 1580 mg ·kg^-1 and 1618 mg ·kg^-1) than those in sediment (mean 8 mg ·kg^-1 and 387 mg ·kg^-1). In addition, results from enzymatic hydrolysis experiments demonstrated that 61% P_o in SPM and 58% in sediment in the H₂O, NaHCO₃, and NaOH extracts could be hydrolyzed. These results suggested that H₂O-P_i and P_o from SPM were the primarily bioavailable P sources for algae. Changes of P_i contents (particularly H₂O-P_i) in algae and alkaline phosphatase activity (APA) during the observation periods were likely to be controlled by the strategies of P uptake and utilization of algae. P remobilization/remineralization from SPM likely resulted from algae and bacteria (e.g., Pseudomonas). Collectively, these results provide important insights that SPM P could sustain the algal blooms even if the dissolved P was depleted in the water column.

Regulation of the Nutrient Cycle Pathway and the Microbial Loop Structure by Different Types of Dissolved Organic Matter Decomposition in Lakes

To explore the effect of different types of dissolved organic matter (DOM) decomposition on nutrient cycling pathways and the microbial loop, four lakes with different DOM sources were investigated monthly. In Lake Tangxun, Dolichospermum decay released highly labile dissolved organic nitrogen into the water column. This induced bacterial organic nitrogen decomposition, as indicated by the increased abundance of gltB, gltD, gdh, and glnA as well as aminopeptidase activity. Genes associated with dissimilatory nitrate reduction to ammonium further fueled ammonium accumulation, driving Microcystis blooms in the summer. In Lake Zhiyin, fish bait deposits (high nitrogen, similar to Dolichospermum detritus) also caused Microcystis blooms. Detritus from Microcystis decomposition then produced high levels of labile dissolved organic phosphorus, inducing phosphatase activity and increasing soluble reactive phosphorus concentrations from September to April in Lakes Tangxun and Zhiyin. The high refractory DOM from macrophytes in Lake Houguan led to insufficient nutrient availability, leading to nutrient mutualism between algae and bacteria. The high levels of labile dissolved organic carbon from terrestrial detritus in Lake Yandong increased bacterial biomass and production, resulting in low chlorophyll content due to the competitive relationship between algal and bacterial nutrient requirements. Therefore, different DOM compositions induce unique connections among available nutrients, algae, and bacteria in the microbial loop.

Cognizing and characterizing the organic phosphorus in lake sediments: Advances and challenges

Organic phosphorus (OP) is one of the main forms of phosphorus in lake ecosystems. Mounting evidence has shown that sediment OP has become a major but underestimated issue in addressing lake eutrophication and algal bloom. However, a holistic view of sediment OP remains missing. This review aims to provide an overview of progress on the studies of OP in lake sediments, focusing on the contribution of OP to internal P loading, its potential role in algal bloom, and the migration and transformation. In addition, this work systematically summarized current methods for characterizing OP content, chemical fraction, composition, bioavailability, and assessment of OP release in sediment, with the pros and cons of each method being discussed. In the end, this work pointed out following efforts needed to deepen the understanding of sediment OP, namely: (1) In-depth literature review from a global perspective regarding the contribution of sediment OP to internal P loading with further summary about its pattern of distribution, accumulation and historical changes; (2) better mathematical models for describing drivers and the linkages between the biological pump of algal bloom and the replenishment of sediment OP; (3) fully accounting the composition and molecular size of OP for better understanding its transformation process and mechanism; ; (4) developing direct, high-sensitivity and combined techniques to improve the precision for identifying OP in sediments; (5) establishing the response of OP molecular properties and chemical reactivity to OP biodegradability and designing a comprehensive and accurate composite index to deepen the understanding for the bioavailability of OP; and (6) integrating fundamental processes of OP in current models to better describe the release and exchange of P in sediment-water interface (SWI). This work is expected to provide critical information about OP properties and deliver perspectives of novel characterization methods.