Lake sedimentary biogenic silica from diatoms constitutes a significant global sink for aluminium

Origin, formation, and transformation of different forms of silica in Xuanwei Formation coal, China, and its' emerging environmental problem

The study on the origin of quartz and silica in Xuanwei Formation coal in Northwest Yunnan, China, is helpful to understand the relationship between quartz and silica and the high incidence of lung cancer from the root. To address these questions, the mineralogy and microscopic studies of silica in Xuanwei Formation coal were performed. The following results were obtained: (1) silica in the late Permian Xuanwei Formation coal seams originated from detrital input, early diagenesis, and late diagenesis. (2) A more significant contribution comes from early diagenesis, which contains abundant authigenic quartz and amorphous silica. (3) Quartz and silica from inorganic silicon are more symbiotic with kaolinite and from biogenic silicon with chamosite. (4) Three silica polymorphs in coal samples have been identified: opal-A (amorphous silica), opal-CT/-C (cristobalite/tridymite), and α quartz. (5) Opal-A is ubiquitous, while opal-CT/-C and α quartz are rare. (5) Opal-A is an amorphous and nontoxic ordinary silica. (6) Since the toxicity of amorphous silica and its presence in coal is an emerging topic, it should be continuously monitored.

Review of Recent Advances in Gas-Assisted Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS)

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful chemical characterization technique allowing for the distribution of all material components (including light and heavy elements and molecules) to be analyzed in 3D with nanoscale resolution. Furthermore, the sample's surface can be probed over a wide analytical area range (usually between 1 µm² and 10⁴ µm²) providing insights into local variations in sample composition, as well as giving a general overview of the sample's structure. Finally, as long as the sample's surface is flat and conductive, no additional sample preparation is needed prior to TOF-SIMS measurements. Despite many advantages, TOF-SIMS analysis can be challenging, especially in the case of weakly ionizing elements. Furthermore, mass interference, different component polarity of complex samples, and matrix effect are the main drawbacks of this technique. This implies a strong need for developing new methods, which could help improve TOF-SIMS signal quality and facilitate data interpretation. In this review, we primarily focus on gas-assisted TOF-SIMS, which has proven to have potential for overcoming most of the aforementioned difficulties. In particular, the recently proposed use of XeF₂ during sample bombardment with a Ga⁺ primary ion beam exhibits outstanding properties, which can lead to significant positive secondary ion yield enhancement, separation of mass interference, and inversion of secondary ion charge polarity from negative to positive. The implementation of the presented experimental protocols can be easily achieved by upgrading commonly used focused ion beam/scanning electron microscopes (FIB/SEM) with a high vacuum (HV)-compatible TOF-SIMS detector and a commercial gas injection system (GIS), making it an attractive solution for both academic centers and the industrial sectors.

Can biogenic n-heptadecane be utilized to represent algae cell density dynamics in water environment? Evidences from field investigation and laboratory validation

The monitoring and prediction of algae cell density are the fundamental supports for eutrophication management. As the molecular marker of algae, n-heptadecane has been successfully utilized to reflect algae biomass in sedimentary studies. However, whether biogenic n-heptadecane (bio C17) can be utilized to represent algae cell density dynamics in water environment still remains an issue. Current study aims to provide novel evidences from both field investigation and laboratory validation for it. Firstly, we found a strinkingly positive correlation between algae cell density dynamics and bio C17 variations (p = 4.34 × 10-10) via meta-analysis using field dataset in Lake Chaohu. Then, we selected Microcystis aeruginosa, Chlorella vulgaris and Melosira sp. as model species of cyanobacteria, green algae and diatom, respectively, for laboratory validation. Our results illustrated that n-heptadecane was synchronized with the growth for cyanobacteria and green algae but not for diatom. The association between bio C17 and algae cell density was species-dependent, and the relationship between bio C17 and cell density was linear within 107 cells∙mL-1. Therefore, we established and optimized a generalized additive model to fit observed algae cell density in Lake Chaohu. In the optimal model, bio C17, Pielou evenness index J and Shannon-Wiener index H' were included, totally explaining 66% of the variance of algae cell density. Model comparisons revealed that considering algae community could indeed improve the interpretation of algae cell density in natural environment. In conclusion, our study provided novel evidences that bio C17 can be utilized to represent the cell density dynamics of cyanobacteria and green algae in the environment.

Diatom assemblage responses to multiple environmental stressors in a deep brackish plateau lake, SW China

Eutrophication, climate change, water level fall, fish introduction, and salinity have been widely recorded to impact lake ecosystems around the world. However, the combined responding pattern in the lake ecosystem to the above multiple environmental stressors is not well understood. Here, we present diatom assemblage and geochemical indicators (total organic carbon (TOC), total nitrogen (TN), and total phosphorous (TP)) in lake sediment to investigate the long-term trends in the aquatic ecosystem in response to multiple environmental stressors in Lake Chenghai, a deep brackish lake located on Yunnan-Guizhou Plateau, SW China, during the past 80 years. We identified 8 genera and 15 species of diatom reaching a relative abundance of ≥ 2% in at least one sample, with the dominant taxa such as Cyclotella rhomboideo-elliptica and Aulacoseira alpigena through the sediment core. There was a clear shift in the diatom community from oligotrophic species of C. rhomboideo-elliptica to eutrophic species such as Cyclotella meneghiniana and Cyclostephanos dubius becoming dominant since ca. 1998 CE. In addition, the changes in the fish introduction, water level, temperature, and salinity also resulted in the variation in abundance of planktonic and benthic diatoms. The increase in the abundance of diatoms of C. rhomboideo-elliptica since ca. 1986 CE may be related to the decline in water level and increasing fish production. For one thing, the decline in water level reduces the concentrations of benthic diatoms but increases the relative biomass of planktonic diatoms. For another, the increasing fish production results in the decrease in the zooplankton biomass, thus reducing the predation pressure on planktonic algae. Besides, some specific species such as C. meneghiniana may respond to the increased nutrient release and increased salinity since ca. 1998 CE. Combined, our findings demonstrate that trophic level is the main driver of diatom assemblage changes, and other environmental variables including water level, fish introduction, and climate warming also contribute to diatom community variation in this brackish plateau lake during the last 80 years.

Modifying the thickness, pore size, and composition of diatom frustule in Pinnularia sp. with Al3+ ions

Diatoms are unicellular photosynthetic algae that produce a silica exoskeleton (frustule) which exposes a highly ordered nano to micro scale morphology. In recent years there has been a growing interest in modifying diatom frustules for technological applications. This is achieved by adding non-essential metals to the growth medium of diatoms which in turn modifies morphology, composition, and resulting properties of the frustule. Here, we investigate the frustule formation in diatom Pinnularia sp., including changes to overall morphology, silica thickness, and composition, in the presence of Al³⁺ ions at different concentrations. Our results show that in the presence of Al³⁺ the total silica uptake from the growth medium increases, although a decrease in the growth rate is observed. This leads to a higher inorganic content per diatom resulting in a decreased pore diameter and a thicker frustule as evidenced by electron microscopy. Furthermore, ²⁷Al solid-state NMR, FIB-SEM, and EDS results confirm that Al³⁺ becomes incorporated into the frustule during the silicification process, thus, improving hydrolysis resistance. This approach may be extended to a broad range of elements and diatom species towards the scalable production of silica materials with tunable hierarchical morphology and chemical composition.