Chemical characterization and identification of Pinaceae pollen by infrared microspectroscopy

Evaluating the Use of Fourier Transform Raman Spectroscopy for Pollen Chemical Characterization

Vibrational spectroscopy is gaining popularity for understanding ecological and evolutionary patterns in plants, particularly in relation to the analysis of pollen grains. So far, Fourier transform infrared spectroscopy (FT-IR) has been the main approach used to classify pollen grains based on chemical variations. However, FT-IR may be less suitable for detecting differences in the pollen grain exine, mainly composed of sporopollenin. In contrast, Raman spectroscopy has increased sensitivity for the main chemical components found within sporopollenins. We compare the classification performance and chemical information provided by FT-IR and FT-Raman using a large dataset of Quercus L. pollen, comprising five species in three sections: (i) Cerris: Q. suber, (ii) Ilex: Q. coccifera, Q. rotundifolia, and (iii) Quercus: Q. robur, Q. faginea). Here, we used multiblock sparse partial least squares discriminant analyses (MB-sPLS-DA) analyses to directly compare the two infrared methods. Both FT-IR and FT-Raman successfully classified Quercus pollen to section level (100% accuracy). At the species level our models achieved ∼90% accuracy for FT-Raman and FT-IR separately and in the combined multiblock model. The multiblock results showed an increased number of sporopollenin peaks observed in FT-Raman spectra as compared to FT-IR. These peaks are also of a higher importance for classification. Results also showed differences in the types of vibrations that are of diagnostic value for the two infrared methods. CH2 deformations are more important in FT-Raman, while C-O-C, C-O, and C = O stretches are more important for FT-IR-based identification of pollen. These vibrations are indicators of carbohydrates, proteins and lipids. FT-Raman provides equally successful diagnostic potential to FT-IR, but uses more chemical information based on variations in sporopollenin chemistry than FT-IR. We suggest that the combined analysis of FT-IR and FT-Raman using multiblock analysis has great potential for classification.

Analytical and experimental solutions for Fourier transform infrared microspectroscopy measurements of microparticles: A case study on Quercus pollen

BACKGROUND

FTIR microspectroscopy is a popular non-destructive technique for chemical analysis and identification of microparticles, such as microplastics, pollen, spores, microplankton organisms, sediments and microfossils. Unfortunately, measured spectra of microparticles are usually distorted by Mie-type scattering interferents thus hindering the analysis of spectral data. To retrieve chemical absorbance spectra, two different approaches are regularly employed: analytical (application of scatter-correction preprocessing methods), and experimental (measurement in an embedding matrix). The comparative studies of preprocessing spectral strategies are needed to determine pros and cons of these approaches, and when they are most suitable for use.

RESULTS

We conducted the first-ever comparative study on 12 different analytical and experimental approaches for FTIR measurements of microparticles, as demonstrated on classification and chemical characterisation of pollen of four Quercus species. Individual pollen grains were measured on 1) microscope slides and 2) embedded in a paraffin-polyethylene (PEP) matrix. For analytical approaches, we have applied simple model-based algorithm (EMSC: extended multiplicative signal correction), Mie-theory model-based algorithm (ME-EMSC: Mie-extinction EMSC) and deep learning-based algorithm (DCNN: deep convolutional neural network). Moreover, we applied algorithms for the correction of the embedded spectra: fringe-correction EMSC and two different paraffin-correction EMSC algorithms. The best classification accuracy is obtained for simple preprocessing, where scattering information is not completely removed, as well as for complex algorithms where scattering information is parameterized and retained. In chemical characterisation studies, strong scattering signals hinder valuable chemical information, and it is imperative to suppress them either by embedding or by an analytical approach.

SIGNIFICANCE

The results show that scattering spectral interferents are not necessarily detrimental for classification studies of biological microparticles. In fact, they have considerable diagnostic value even in closely related microorganisms due to species-specific physical properties. The results clearly show that analytical and experimental solutions for FTIR measurements of microparticles should be carefully selected, taking into account the origin of the microparticles (i.e., biological or artificial) and purpose of the study (classification or chemical characterisation).

Compounds composition of pollen tubes of Scots pine (Pinus sylvestris L.)

The aim of this research was to study the composition of pollen tubes of Scots pine (Pinus sylvestris L.). Pollen cultivation on deionized distilled water excluded the potential influence of the cultivation medium on the pollen tube growth and development. The fluorescent study indicated a gradual distribution of chemical compounds along the length of the tube. It was shown that the protoplast apical zone and the parietal layer near the tube's tip are most likely actively involved in the ion transport regulation in the growing pollen tube. The callose synthesis in the tip of matured pine tube completed the first stage of its active growth. Significant differences and pH gradients at the nucleus region and the parietal layer of the tube wall indicate that H+ gradient is the direct driving force of vesicle transport and can regulate the growth of pollen tubes. The distribution of amino acids, RNA, proteins and lipids was uniform throughout the length of the pine pollen tube. The content of amino acids, RNA, DNA and proteins slightly increased near the cell nucleus and drastically increased in the apical zone. At the very tip of the tube, a slight increase in the concentration of polysaccharides and a significant decrease in the content of amino acids, RNA, DNA, proteins and lipids were detected.

Influence of common palynological extraction treatments on ultraviolet absorbing compounds (UACs) in sub-fossil pollen and spores observed in FTIR spectra

Introduction

Biological life, atmospheric circulation and the Earth’s climate may be influenced by UV-B radiation. In plants, Ultraviolet Absorbing Compounds (UACs) are an indicator of UV-B exposure, and the abundance of UACs in pollen and spores of embryophytes is measurable using Fourier Transform Infrared (FTIR) micro-Spectroscopy. However, understanding the influence of common chemical pre-treatments on sub-fossil pollen and spores with a view to UV-B reconstruction still requires investigation.

Methods

Here, peat samples collected from a Late Holocene raised bog were treated with different chemicals (HCl, KOH, and acetolysis) for varying treatment times (up to 210 min). Pollen or spores of three common taxa (Alnus, Calluna and Sphagnum) were isolated and FTIR spectra obtained on individual grains. The spectra were compared to modern pollen and spore samples collected nearby.

Results

Spectra of modern and sub-fossil samples show several visible differences related to lipid and protoplast contents. The results of chemical treatments on sub-fossil pollen and spores reveal that HCl produced limited changes, while KOH and acetolysis altered several peaks, including the UAC-related aromatic peak at 1516 cm−1. We observe that all treatments modify the FTIR spectra to some degree, from weakest (HCl) to strongest (acetolysis). With respect to reduction of UAC peak area and treatment time, we observe in some cases a significant log-decay relationship, notably for KOH treatment on Calluna pollen and acetolysis on Sphagnum spores. Compared to untreated control samples, UAC peak area in Alnus, Calluna and Sphagnum reduced by 68%, 69% and 60% respectively, after only 3 min of acetolysis treatment. After 60 minutes of acetolysis treatment UAC peaks were reduced by 77%, 84% and 88%.

Discussion

Due to the potential for taxon-specific effects and significant reductions in UAC peak area even within short treatment times, our recommendation for future applications in palaeoecological studies on palynomorph chemistry is to avoid chemical digestions in the pollen extraction process in favour of separation methods including micro-sieving and density separation.

Attenuated Total Reflection (ATR) Micro-Fourier Transform Infrared (Micro-FT-IR) Spectroscopy to Enhance Repeatability and Reproducibility of Spectra Derived from Single Specimen Organic-Walled Dinoflagellate Cysts

The chemical composition of recent and fossil organic-walled dinoflagellate cyst walls and its diversity is poorly understood and analyses on single microscopic specimens are rare. A series of infrared spectroscopic experiments resulted in the proposition of a standardized attenuated total reflection micro-Fourier transform infrared-based method that allows the collection of robust data sets consisting of spectra from individual dinocysts. These data sets are largely devoid of nonchemical artifacts inherent to other infrared spectrochemical methods, which have typically been used to study similar specimens in the past. The influence of sample preparation, specimen morphology and size and spectral data processing steps is also assessed within this methodological framework. As a result, several guidelines are proposed which facilitate the collection and qualitative interpretation of highly reproducible and repeatable spectrochemical data. These, in turn, pave the way for a systematic exploration of dinocyst chemistry and its assessment as a chemotaxonomical tool or proxy.

Clustering approach for the analysis of the fluorescent bioaerosol collected by an automatic detector

Automatically operating particle detection devices generate valuable data, but their use in routine aerobiology needs to be harmonized. The growing network of researchers using automatic pollen detectors has the challenge to develop new data processing systems, best suited for identification of pollen or spore from bioaerosol data obtained near-real-time. It is challenging to recognise all the particles in the atmospheric bioaerosol due to their diversity. In this study, we aimed to find the natural groupings of pollen data by using cluster analysis, with the intent to use these groupings for further interpretation of real-time bioaerosol measurements. The scattering and fluorescence data belonging to 29 types of pollen and spores were first acquired in the laboratory using Rapid-E automatic particle detector. Neural networks were used for primary data processing, and the resulting feature vectors were clustered for scattering and fluorescence modality. Scattering clusters results showed that pollen of the same plant taxa associates with the different clusters corresponding to particle shape and size properties. According to fluorescence clusters, pollen grouping highlighted the possibility to differentiate Dactylis and Secale genera in the Poaceae family. Fluorescent clusters played a more important role than scattering for separating unidentified fluorescent particles from tested pollen. The proposed clustering method aids in reducing the number of false-positive errors.

Electrochemical Voltammogram Recording for Identifying Varieties of Ornamental Plants

An electrochemical voltammogram recording method for plant variety identification is proposed. Electrochemical voltammograms of Vistula, Andromeda, Danuta, Armandii 'Apple Blossom,' Proteus, Hagley Hybrid, Violet Elizabeth, Kiri Te Kanawa, Regina, and Veronica's Choice were recorded using leaf extracts with two solvents under buffer solutions. The voltametric data recorded under different conditions were derived as scatter plots, 2D density patterns, and hot maps for variety identification. In addition, the voltametric data were further used for genetic relationship studies. The dendrogram deduced from the voltammograms was used as evidence for relationship study. The dendrogram deduced from voltametric data suggested the Andromeda, Danuta, Proteus, Regina, and Hagley Hybrid were closely related, while Violet Elizabeth and Veronica's Choice were closely related. In addition, Vistula and Armandii 'Apple Blossom' could be considered outliers among the varieties.

Discrimination of grass pollen of different species by FTIR spectroscopy of individual pollen grains

Fourier-transform infrared (FTIR) spectroscopy enables the chemical characterization and identification of pollen samples, leading to a wide range of applications, such as paleoecology and allergology. This is of particular interest in the identification of grass (Poaceae) species since they have pollen grains of very similar morphology. Unfortunately, the correct identification of FTIR microspectroscopy spectra of single pollen grains is hindered by strong spectral contributions from Mie scattering. Embedding of pollen samples in paraffin helps to retrieve infrared spectra without scattering artifacts. In this study, pollen samples from 10 different populations of five grass species (Anthoxanthum odoratum, Bromus inermis, Hordeum bulbosum, Lolium perenne, and Poa alpina) were embedded in paraffin, and their single grain spectra were obtained by FTIR microspectroscopy. Spectra were subjected to different preprocessing in order to suppress paraffin influence on spectral classification. It is shown that decomposition by non-negative matrix factorization (NMF) and extended multiplicative signal correction (EMSC) that utilizes a paraffin constituent spectrum, respectively, leads to good success rates for the classification of spectra with respect to species by a partial least square discriminant analysis (PLS-DA) model in full cross-validation for several species. PLS-DA, artificial neural network, and random forest classifiers were applied on the EMSC-corrected spectra using an independent validation to assign spectra from unknown populations to the species. Variation within and between species, together with the differences in classification results, is in agreement with the systematics within the Poaceae family. The results illustrate the great potential of FTIR microspectroscopy for automated classification and identification of grass pollen, possibly together with other, complementary methods for single pollen chemical characterization.

Chemical Analysis of Pollen by FT-Raman and FTIR Spectroscopies

Pollen studies are important for the assessment of present and past environment, including biodiversity, sexual reproduction of plants and plant-pollinator interactions, monitoring of aeroallergens, and impact of climate and pollution on wild communities and cultivated crops. Although information on chemical composition of pollen is of importance in all of those research areas, pollen chemistry has been rarely measured due to complex and time-consuming analyses. Vibrational spectroscopies, coupled with multivariate data analysis, have shown great potential for rapid chemical characterization, identification and classification of pollen. This study, comprising 219 species from all principal taxa of seed plants, has demonstrated that high-quality Raman spectra of pollen can be obtained by Fourier transform (FT) Raman spectroscopy. In combination with Fourier transform infrared spectroscopy (FTIR), FT-Raman spectroscopy is obtaining comprehensive information on pollen chemistry. Presence of all the main biochemical constituents of pollen, such as proteins, lipids, carbohydrates, carotenoids and sporopollenins, have been identified and detected in the spectra, and the study shows approaches to measure relative and absolute content of these constituents. The results show that FT-Raman spectroscopy has clear advantage over standard dispersive Raman measurements, in particular for measurement of pollen samples with high pigment content. FT-Raman spectra are strongly biased toward chemical composition of pollen wall constituents, namely sporopollenins and pigments. This makes Raman spectra complementary to FTIR spectra, which over-represent chemical constituents of the grain interior, such as lipids and carbohydrates. The results show a large variability in pollen chemistry for families, genera and even congeneric species, revealing wide range of reproductive strategies, from storage of nutrients to variation in carotenoids and phenylpropanoids. The information on pollen's chemical patterns for major plant taxa should be of outstanding value for various studies in plant biology and ecology, including aerobiology, palaeoecology, forensics, community ecology, plant-pollinator interactions, and climate effects on plants.

Species-Specific Biodegradation of Sporopollenin-Based Microcapsules

Sporoderms, the outer layers of plant spores and pollen grains, are some of the most robust biomaterials in nature. In order to evaluate the potential of sporoderms in biomedical applications, we studied the biodegradation in simulated gastrointestinal fluid of sporoderm microcapsules (SDMCs) derived from four different plant species: lycopodium (Lycopodium clavatum L.), camellia (Camellia sinensis L.), cattail (Typha angustifolia L.), and dandelion (Taraxacum officinale L.). Dynamic image particle analysis (DIPA) and field-emission scanning electron microscopy (FE-SEM) were used to investigate the morphological characteristics of the capsules, and Fourier-transform infrared (FTIR) spectroscopy was used to evaluate their chemical properties. We found that SDMCs undergo bulk degradation in a species-dependent manner, with camellia SDMCs undergoing the most extensive degradation, and dandelion and lycopodium SDMCs being the most robust.

Fossil pollen and spores as a tool for reconstructing ancient solar-ultraviolet irradiance received by plants: an assessment of prospects and challenges using proxy-system modelling

Ultraviolet-B radiation (UV-B, 280-315 nm) constitutes less than 1% of the total solar radiation that reaches the Earth's surface but has a disproportional impact on biological and ecological processes from the individual to the ecosystem level. Absorption of UV-B by ozone is also one of the primary heat sources to the stratosphere, so variations in UV-B have important relationships to the Earth's radiation budget. Yet despite its importance for understanding atmospheric and ecological processes, there is limited understanding about the changes in UV-B radiation in the geological past. This is because systematic measurements of total ozone and surface UV-B only exist since the 1970s, so biological or geochemical proxies from sediment archives are needed to reconstruct UV-B irradiance received at the Earth surface beyond the experimental record. Recent developments have shown that the quantification of UV-B-absorbing compounds in pollen and spores have the potential to provide a continuous record of the solar-ultraviolet radiation received by plants. There is increasing interest in developing this proxy in palaeoclimatic and palaeoecological research. However, differences in interpretation exist between palaeoecologists, who are beginning to apply the proxy under various geological settings, and UV-B ecologists, who question whether a causal dose-response relationship of pollen and spore chemistry to UV-B irradiance has really been established. Here, we use a proxy-system modelling approach to systematically assess components of the pollen- and spore-based UV-B-irradiance proxy to ask how these differences can be resolved. We identify key unknowns and uncertainties in making inferences about past UV-B irradiance, from the pollen sensor, the sedimentary archive, and through the laboratory and experimental procedures in order to target priority areas of future work. We argue that an interdisciplinary approach, modifying methods used by plant ecologists studying contemporary responses to solar-UV-B radiation specifically to suit the needs of palaeoecological analyses, provides a way forward in developing the most reliable reconstructions for the UV-B irradiance received by plants across a range of timescales.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) shows adaptation of grass pollen composition

MALDI time-of-flight mass spectrometry (MALDI-TOF MS) has become a widely used tool for the classification of biological samples. The complex chemical composition of pollen grains leads to highly specific, fingerprint-like mass spectra, with respect to the pollen species. Beyond the species-specific composition, the variances in pollen chemistry can be hierarchically structured, including the level of different populations, of environmental conditions or different genotypes. We demonstrate here the sensitivity of MALDI-TOF MS regarding the adaption of the chemical composition of three Poaceae (grass) pollen for different populations of parent plants by analyzing the mass spectra with partial least squares discriminant analysis (PLS-DA) and principal component analysis (PCA). Thereby, variances in species, population and specific growth conditions of the plants were observed simultaneously. In particular, the chemical pattern revealed by the MALDI spectra enabled discrimination of the different populations of one species. Specifically, the role of environmental changes and their effect on the pollen chemistry of three different grass species is discussed. Analysis of the group formation within the respective populations showed a varying influence of plant genotype on the classification, depending on the species, and permits conclusions regarding the respective rigidity or plasticity towards environmental changes.

Identification of birch pollen species using FTIR spectroscopy

In this study, the morphology and chemical composition of pollen grains of six birch species (Betula utilis Doorenbos, B. dahurica, B. maximowicziana, B. pendula, B. pubescens and B. humilis) were examined to verify which of these features allow distinguishing them in a more unambiguous way. For this purpose, scanning electron microscopy and light microscopy, as well as Fourier transform infrared (FTIR) spectroscopy and curve-fitting analysis of amide I profile, were performed. The microscopy images show that the pollen grains of B. pubescens, B. pendula and B. humilis are similar in diameter and significantly smaller than those of others species, with the largest diameter observed for B. utilis Doorenbos. However, the results obtained from FTIR spectroscopy indicate that the chemical compositions of B. pubescens and B. pendula are similar, but B. humilis is outlaying. Summarizing, it is not possible to unambiguously state, which feature or which technique is the best for differentiating between the six chosen birch species. However, the study showed that both techniques have potential for identification of birch pollen species.