A comparison of lipid storage in Phaeodactylum tricornutum and Tetraselmis suecica using laser scanning confocal microscopy

Visualisation of microalgal lipid bodies through electron microscopy

In this study, transmission electron microscopy (TEM) and cryo-scanning electron microscopy (cryo-SEM) were evaluated for their ability to detect lipid bodies in microalgae. To do so, Phaeodactylum tricornutum and Nannochloropsis oculata cells were harvested in both the mid-exponential and early stationary growth phase. Two different cryo-SEM cutting methods were compared: cryo-planing and freeze-fracturing. The results showed that, despite the longer preparation time, TEM visualisation preceded by cryo-immobilisation allows a clear detection of lipid bodies and is preferable to cryo-SEM. Using freeze-fracturing, lipid bodies were rarely detected. This was only feasible if crystalline layers in the internal structure, most likely related to sterol esters or di-saturated triacylglycerols, were revealed. Furthermore, lipid bodies could not be detected using cryo-planing. Cryo-SEM is also not the preferred technique to recognise other organelles besides lipid bodies, yet it did reveal chloroplasts in both species and filament-containing organelles in cryo-planed Nannochloropsis oculata samples.

Comparison of two Phaeodactylum tricornutum ecotypes under nitrogen starvation and resupply reveals distinct lipid accumulation strategies but a common degradation process

Introduction

Phaeodactylum tricornutum is a model species frequently used to study lipid metabolism in diatoms. When exposed to a nutrient limitation or starvation, diatoms are known to accumulate neutral lipids in cytoplasmic lipid droplets (LDs). Those lipids are produced partly de novo and partly from the recycle of plastid membrane lipids. Under a nitrogen resupply, the accumulated lipids are catabolized, a phenomenon about which only a few data are available. Various strains of P. tricornutum have been isolated around the world that may differ in lipid accumulation patterns.

Methods

To get further information on this topic, two genetically distant ecotypes of P. tricornutum (Pt1 and Pt4) have been cultivated under nitrogen deprivation during 11 days followed by a resupply period of 3 days. The importance of cytoplasmic LDs relative to the plastid was assessed by a combination of confocal laser scanning microscopy and cell volume estimation using bright field microscopy pictures.

Results and discussion

We observed that in addition to a basal population of small LDs (0.005 μm3 to 0.7 μm3) present in both strains all along the experiment, Pt4 cells immediately produced two large LDs (up to 12 μm3 after 11 days) while Pt1 cells progressively produced a higher number of smaller LDs (up to 7 μm3 after 11 days). In this work we showed that, in addition to intracellular available space, lipid accumulation may be limited by the pre-starvation size of the plastid as a source of membrane lipids to be recycled. After resupplying nitrogen and for both ecotypes, a fragmentation of the largest LDs was observed as well as a possible migration of LDs to the vacuoles that would suggest an autophagic degradation. Altogether, our results deepen the understanding of LDs dynamics and open research avenues for a better knowledge of lipid degradation in diatoms.

Revealing Lipid Body Formation and its Subcellular Reorganization in Oleaginous Microalgae Using Correlative Optical Microscopy and Infrared Nanospectroscopy

The purpose of this work is to develop an integrated imaging approach to characterize without labeling at the sub-cellular level the formation of lipid body droplets (LBs) in microalgae undergoing nitrogen starvation. First conventional optical microscopy approaches, gas chromatography, and turbidimetry measurements allowed to monitor the biomass and the total lipid content in the oleaginous microalgae Parachlorella kesslerii during the starvation process. Then a local analysis of the LBs was proposed using an innovative infrared nanospectroscopy technique called atomic force microscopy-based infrared spectroscopy (AFM-IR). This label-free technique assessed the formation of LBs and allowed to look into the LB composition thanks to the acquisition of local infrared spectra. Last correlative measurements using fluorescence microscopy and AFM-IR were performed to investigate the subcellular reorganization of LB and the chloroplasts.

Metabolite Quantification by Fourier Transform Infrared Spectroscopy in Diatoms: Proof of Concept on Phaeodactylum tricornutum

Diatoms are feedstock for the production of sustainable biocommodities, including biofuel. The biochemical characterization of newly isolated or genetically modified strains is seminal to identify the strains that display interesting features for both research and industrial applications. Biochemical quantification of organic macromolecules cellular quotas are time-consuming methodologies which often require large amount of biological sample. Vibrational spectroscopy is an essential tool applied in several fields of research. A Fourier transform infrared (FTIR) microscopy-based imaging protocol was developed for the simultaneous cellular quota quantification of lipids, carbohydrates, and proteins of the diatom Phaeodactylum tricornutum. The low amount of sample required for the quantification allows the high throughput quantification on small volume cultures. A proof of concept was performed (1) on nitrogen-starved experimental cultures and (2) on three different P. tricornutum wild-type strains. The results are supported by the observation in situ of lipid droplets by confocal and brightfield microscopy. The results show that major differences exist in the regulation of lipid metabolism between ecotypes of P. tricornutum.

A method for studying lipid adsorption to silicone hydrogel contact lenses

The aim of this study was to develop an experimental methodology to measure lipid deposition with contact lenses. Contact lenses were incubated in a lipid solution. The amount and types of adsorbed lipids were assessed using mass spectrometry and confocal microscopy. The recovery of lipids from lenses varied with lipid and lens type. Most non-polar and polar lipids were desorbed from lenses during the first 5 min of extraction. Fluorescently labelled phosphatidylcholine bound within the matrix of Senofilcon A lenses but to the surface of Lotrafilcon B lenses, whereas fluorescently labelled cholesteryl ester was found throughout both lenses. The efficacy of extraction of lipids from contact lenses varies for different lipid classes and different lens materials. Differences in the amount and time of lipid desorption probably resulted from the strength of the bond between lipid and lens polymer and the depth of adsorption of lipid in the polymer.

Comparative Structural and Functional Analyses of the Fusiform, Oval, and Triradiate Morphotypes of Phaeodactylum tricornutum Pt3 Strain

The diatom Phaeodactylum tricornutum is a marine unicellular microalga that exists under three main morphotypes: oval, fusiform, and triradiate. Previous works have demonstrated that the oval morphotype of P. tricornutum Pt3 strain presents specific metabolic features. Here, we compared the cellular organization of the main morphotypes of the diatom P. tricornutum Pt3 strain through transmission electron and advanced light microscopies. The three morphotypes share similarities including spectral characteristics of the plastid, the location of the nucleus, the organization of mitochondria around the plastid as well as the existence of both a F-actin cortex, and an intracellular network of F-actin. In contrast, compared to fusiform and triradiate cells, oval cells spontaneously release proteins more rapidly. In addition, comparison of whole transcriptomes of oval versus fusiform or triradiate cells revealed numerous differential expression of positive and negative regulators belonging to the complex dynamic secretory machinery. This study highlights the specificities occurring within the oval morphotype underlying that the oval cells secrete proteins more rapidly.

Effects of Propranolol on Growth, Lipids and Energy Metabolism and Oxidative Stress Response of Phaeodactylum tricornutum

Simple Summary

In the past two decades, increasing attention has been directed to investigate the incidence and consequences of pharmaceuticals in the aquatic environment. Propranolol is a non-selective β-adrenoceptor blocker used in large quantities worldwide to treat cardiovascular conditions. Diatoms (model organism) exposed to this compound showed evident signs of oxidative stress, a significant reduction of the autotrophic O₂ production and an increase in the heterotrophic mitochondrial respiration. Additionally, diatoms exposed to propranolol showed a consumption of its storage lipids. In ecological terms this will have cascading impacts in the marine trophic webs, where these organisms are key elements, through a reduction of the water column oxygenation and essential fatty acid availability to the heterotrophic organisms that depend on these primary producers. In ecotoxicological terms, diatoms photochemical and fatty acid traits showed to be potential good biomarkers for toxicity assessment of diatoms exposed to this widespread pharmaceutical compound.

Abstract

Present demographic trends suggest a rise in the contributions of human pharmaceuticals into coastal ecosystems, underpinning an increasing demand to evaluate the ecotoxicological effects and implications of drug residues in marine risk assessments. Propranolol, a non-selective β-adrenoceptor blocker, is used worldwide to treat high blood pressure conditions and other related cardiovascular conditions. Although diatoms lack β-adrenoceptors, this microalgal group presents receptor-like kinases and proteins with a functional analogy to the animal receptors and that can be targeted by propranolol. In the present work, the authors evaluated the effect of this non-selective β-adrenoceptor blocker in diatom cells using P. tricornutum as a model organism, to evaluate the potential effect of this compound in cell physiology (growth, lipids and energy metabolism and oxidative stress) and its potential relevance for marine ecosystems. Propranolol exposure leads to a significant reduction in diatom cell growth, more evident in the highest concentrations tested. This is likely due to the observed impairment of the main primary photochemistry processes and the enhancement of the mitochondrial respiratory activity. More specifically, propranolol decreased the energy transduction from photosystem II (PSII) to the electron transport chain, leading to an increase in oxidative stress levels. Cells exposed to propranolol also exhibited high-dissipated energy flux, indicating that this excessive energy is efficiently diverted, to some extent, from the photosystems, acting to prevent irreversible photoinhibition. As energy production is impaired at the PSII donor side, preventing energy production through the electron transport chain, diatoms appear to be consuming storage lipids as an energy backup system, to maintain essential cellular functions. This consumption will be attained by an increase in respiratory activity. Considering the primary oxygen production and consumption pathways, propranolol showed a significant reduction of the autotrophic O₂ production and an increase in the heterotrophic mitochondrial respiration. Both mechanisms can have negative effects on marine trophic webs, due to a decrease in the energetic input from marine primary producers and a simultaneous oxygen production decrease for heterotrophic species. In ecotoxicological terms, bio-optical and fatty acid data appear as highly efficient tools for ecotoxicity assessment, with an overall high degree of classification when these traits are used to build a toxicological profile, instead of individually assessed.

Virtual-freezing fluorescence imaging flow cytometry

By virtue of the combined merits of flow cytometry and fluorescence microscopy, imaging flow cytometry (IFC) has become an established tool for cell analysis in diverse biomedical fields such as cancer biology, microbiology, immunology, hematology, and stem cell biology. However, the performance and utility of IFC are severely limited by the fundamental trade-off between throughput, sensitivity, and spatial resolution. Here we present an optomechanical imaging method that overcomes the trade-off by virtually freezing the motion of flowing cells on the image sensor to effectively achieve 1000 times longer exposure time for microscopy-grade fluorescence image acquisition. Consequently, it enables high-throughput IFC of single cells at >10,000 cells s⁻¹ without sacrificing sensitivity and spatial resolution. The availability of numerous information-rich fluorescence cell images allows high-dimensional statistical analysis and accurate classification with deep learning, as evidenced by our demonstration of unique applications in hematology and microbiology.

High throughput imaging flow cytometry suffers from trade-offs between throughput, sensitivity and spatial resolution. Here the authors introduce a method to virtually freeze cells in the image acquisition window to enable 1000 times longer signal integration time and improve signal-to-noise ratio.

Food matrix and processing modulate in vitro protein digestibility in soybeans

Soybeans represent the largest source of plant proteins on the planet but their proteins are associated with low digestibility. Although several studies addressed the limiting factors affecting the rate and extent of soy protein digestion, the net effect of the food matrix, especially of an intact cell wall, has been poorly investigated so far. The purpose of the present study was to examine the relationship between the cell matrix and protein hydrolysis during simulated in vitro digestion of soybean particles of different sizes prepared from unheated and boiled cotyledons. In addition, intact cells were isolated from unheated and autoclaved cotyledons and then digested with and without lipase inhibitors to understand the impact of an intact cell wall and the presence of oil bodies on soybean protein digestibility. Protein digestibility was the highest in the particles prepared after boiling of previously milled cotyledons compared to particles of the same size obtained by milling previously cooked cotyledons as well as of uncooked cotyledons. Protein digestibility in isolated intact cells was lower than that of extracted proteins regardless of the thermal load applied whereas inhibition of pancreatic lipase reduces protein digestibility only slightly. The data indicated that the cell wall could contribute to limit protein digestion in soybean tissues; however, it is not an absolute barrier to pancreatic proteases. An accurate design of the milling and cooking process could be instrumental to modulate the digestion kinetics of soybean proteins.

Structure and properties of oil bodies in diatoms

Diatoms accumulate triacylglycerols in spherical organelles called oil bodies when exposed to nutrient deprivation conditions. Oil body biology in diatoms has attracted significant attention due to the complexity of the intracellular organelles and the unique combination of genes generated by the evolutionary history of secondary endosymbiosis. The demand for biofuel production has further increased the interest in and importance of a better understanding of oil body biology in diatoms, because it could provide targets for genetic engineering to further enhance their promising lipid accumulation. This review describes recent progress in studies of the structure and properties of diatom oil bodies. Firstly, the general features of diatom oil bodies are described, in particular, their number, size and morphology, as well as the quantity and quality of lipids they contain. Subsequently, the diatom oil body-associated proteins, which were recently discovered through oil body proteomics, are introduced. Then, the metabolic pathways responsible for the biogenesis and degradation of diatom oil bodies are summarized. During biogenesis and degradation, oil bodies interact with other organelles, including chloroplasts, the endoplasmic reticulum and mitochondria, suggesting their dynamic nature in response to environmental changes. Finally, the functions of oil bodies in diatoms are discussed.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Imaging the accumulated intracellular microalgal lipids as a response to temperature stress

Over the last few decades, many scientists considered microalgae as promising actors for future biofuels because of the high lipid productivity inside their cells. Moreover, much attention has been paid to algal lipids as they can be used in biodiesel production. In this study, we optimized the different suitable conditions such as incubation time, incubation temperature, Dimethylesulfoxide and Nile red concentrations of the lipophilic fluorescence dye Nile red as an excellent and fast vital stain to detect and quantify intracellular lipids. This was achieved using the green alga Nannochloropsis salina. In addition, investigating the accumulation of lipid vesicles inside different isolated microalgal species as a response to temperature stress. Furthermore, the confocal laser scanning microscopy (LS510) for imaging and measuring the size and volume of the accumulated lipid vesicles was used.

Isolation, Identification and High-Throughput Screening of Neutral Lipid Producing Indigenous Microalgae from South African Aquatic Habitats

Exploring indigenous microalgae capable of producing significant amounts of neutral lipids through high-throughput screening is crucial for sustainable biodiesel production. In this study, 31 indigenous microalgal strains were isolated from diverse aquatic habitats in KwaZulu-Natal, South Africa. Eight superior lipid-producing strains were selected for further analysis, based on Nile red fluorescence microscopy screening. The microalgal isolates were identified to belong to the genera Chlorella, Neochloris and Chlamydomonas via morpho-taxonomic and molecular approach by 18S rRNA gene sequencing. Chlorella vulgaris PH2 had the highest specific growth rate (μ) and lowest doubling time of 0.24 day^-1 and 2.89 ± 0.05 day^-1, respectively. Chlorella vulgaris T4 had the highest biomass productivity of 35.71 ± 0.03 mg L^-1day^-1. Chlorella vulgaris PH2 had the highest lipid content of 34.28 ± 0.47 and 38 ± 9.2% (dcw) as determined by gravimetric analysis and the sulfo-phospho-vanillin (SPV) method, respectively. Chlorella vulgaris PH2 exhibited a high content of saturated fatty acids, while Chlorella sp. T4 exhibited a high total content of saturated and monounsaturated fatty acids with a low content of polyunsaturated fatty acids. The preponderance of neutral lipids suggests that Chlorella sp. T4 is a suitable candidate for biomass feedstock for biodiesel production.

Advances in techniques for assessment of microalgal lipids

Microalgae are a varied group of organisms with considerable commercial potential as sources of various biochemicals, storage molecules and metabolites such as lipids, sugars, amino acids, pigments and toxins. Algal lipids can be processed to bio-oils and biodiesel. The conventional method to estimate algal lipids is based on extraction using solvents and quantification by gravimetry or chromatography. Such methods are time consuming, use hazardous chemicals and are labor intensive. For rapid screening of prospective algae or for management decisions (e.g. decision on timing of harvest), a rapid, high throughput, reliable, accurate, cost effective and preferably nondestructive analytical technique is desirable. This manuscript reviews the application of fluorescent lipid soluble dyes (Nile Red and BODIPY 505/515), nuclear magnetic resonance (NMR), Raman, Fourier transform infrared (FTIR) and near infrared (NIR) spectroscopy for the assessment of lipids in microalgae.

Recombinant endo-mannanase (ManB-1601) production using agro-industrial residues: Development of economical medium and application in oil extraction from copra

Expression of pRSETA manb-1601 construct in Hi-Control Escherichia coli BL21 (DE3) cells improved recombinant endo-mannanase (ManB-1601) production by 2.73-fold (1821±100U/ml). A low-cost, agro-industrial residue supplemented industrial medium for enhanced and economical production of ManB-1601 was developed in two mutual phases. Phase-I revealed the potential of various pre- (induction time: 5h, induction mode: lactose 0.5mM) and post-induction [peptone supplementation: 0.94%(w/v), glycerol 0.123%(v/v)] parameters for enhanced production of ManB-1601 and resulted in 4.61-fold (8406±400U/ml) and 2.53-fold (3.30g/l) higher ManB-1601 and biomass production, respectively. Under phase-II, economization of phase-I medium was carried out by reducing/replacing costly ingredients with solubilized-defatted flax seed meal (S-DFSM), which resulted in 3.25-fold (5926U/ml) higher ManB-1601 production. Industrial potential of ManB-1601 was shown in oil extraction from copra as enzyme treatment led to cracks, peeling, fracturing and smoothening of copra, which facilitated higher (18.75%) oil yield.

Application of high-content image analysis for quantitatively estimating lipid accumulation in oleaginous yeasts with potential for use in biodiesel production

Biodiesel from oleaginous microorganisms is a viable substitute for a fossil fuel. Current methods for microorganism lipid productivity evaluation do not analyze lipid dynamics in single cells. Here, we described a high-content image analysis (HCA) as a promising strategy for screening oleaginous microorganisms for biodiesel production, while generating single-cell lipid dynamics data in large cell density. Rhodotorula slooffiae yeast were grown in standard (CTL) or lipid trigger medium (LTM), and lipid droplet (LD) accumulation was analyzed in deconvolved confocal microscopy images of cells stained with the lipophilic fluorescent Nile red (NR) dye using automated cell and LD segmentation. The 'vesicle segmentation' method yielded valid morphometric results for limited lipid accumulation in smaller LDs (CTL samples) and for high lipid accumulation in larger LDs (LTM samples), and detected LD localization changes. Thus, HCA can be used to analyze the lipid accumulation patterns likely to be encountered in screens for biodiesel production.

The use of fluorescent Nile red and BODIPY for lipid measurement in microalgae

Microalgae are currently emerging as one of the most promising alternative sources for the next generation of food, feed, cosmetics and renewable energy in the form of biofuel. Microalgae constitute a diverse group of microorganisms with advantages like fast and efficient growth. In addition, they do not compete for arable land and offer very high lipid yield potential. Major challenges for the development of this resource are to select lipid-rich strains using high-throughput staining for neutral lipid content in microalgae species. For this purpose, the fluorescent dyes most commonly used to quantify lipids are Nile red and BODIPY 505/515. Their fluorescent staining for lipids offers a rapid and inexpensive analysis tool to measure neutral lipid content, avoiding time-consuming and costly gravimetric analysis. This review collates and presents recent advances in algal lipid staining and focuses on Nile red and BODIPY 505/515 staining characteristics. The available literature addresses the limitations of fluorescent dyes under certain conditions, such as spectral properties, dye concentrations, cell concentrations, temperature and incubation duration. Moreover, the overall conclusion of the present review study gives limitations on the use of fluorochrome for screening of lipid-rich microalgae species and suggests improved protocols for staining recalcitrant microalgae and recommendations for the staining quantification.

Imaging of lipids in microalgae with coherent anti-stokes Raman scattering microscopy

Microalgae have great prospects as a sustainable resource of lipids for refinement into nutraceuticals and biodiesel, which increases the need for detailed insights into their intracellular lipid synthesis/storage mechanisms. As an alternative strategy to solvent- and label-based lipid quantification techniques, we introduce time-gated coherent anti-Stokes Raman scattering (CARS) microscopy for monitoring lipid contents in living algae, despite strong autofluorescence from the chloroplasts, at approximately picogram and subcellular levels by probing inherent molecular vibrations. Intracellular lipid droplet synthesis was followed in Phaeodactylum tricornutum algae grown under (1) light/nutrient-replete (control [Ctrl]), (2) light-limited (LL), and (3) nitrogen-starved (NS) conditions. Good correlation (r(2) = 0.924) was found between lipid volume data yielded by CARS microscopy and total fatty acid content obtained from gas chromatography-mass spectrometry analysis. In Ctrl and LL cells, micron-sized lipid droplets were found to increase in number throughout the growth phases, particularly in the stationary phase. During more excessive lipid accumulation, as observed in NS cells, promising commercial harvest as biofuels and nutritional lipids, several micron-sized droplets were present already initially during cultivation, which then fused into a single giant droplet toward stationary phase alongside with new droplets emerging. CARS microspectroscopy further indicated lower lipid fluidity in NS cells than in Ctrl and LL cells, potentially due to higher fatty acid saturation. This agreed with the fatty acid profiles gathered by gas chromatography-mass spectrometry. CARS microscopy could thus provide quantitative and semiqualitative data at the single-cell level along with important insights into lipid-accumulating mechanisms, here revealing two different modes for normal and excessive lipid accumulation.

Synergistic effect of fermentable and non-fermentable carbon sources enhances TAG accumulation in oleaginous yeast Rhodosporidium kratochvilovae HIMPA1

Novel strategy for enhancing TAG accumulation by simultaneous utilization of fermentable and non-fermentable carbon sources as substrate for cultivation of oleaginous yeast Rhodosporidium kratochvilovae HIMPA1 were undertaken in this investigation. The yeast strain showed direct correlation between the size of lipid bodies, visualized by BODIPY stain (493-515 nm) and TAG accumulation when examined on individual fermenting and non-fermenting carbon sources and their mixtures. Maximum TAG accumulation (μm) in glucose (2.38 ± 0.52), fructose (4.03 ± 0.38), sucrose (4.24 ± 0.45), glycerol (4.35 ± 0.54), xylulose (3.94 ± 0.12), and arabinose (2.98 ± 0.43) were observed. Synergistic effect of the above carbon sources (fermentable and non-fermentable) in equimolar concentration revealed maximum lipid droplet size of 5.35 ± 0.76 μm and cell size of 6.89 ± 0.97 μm. Total lipid content observed in mixed carbon sources was 9.26 g/l compared to glucose (6.2g/l). FAME profile revealed enhanced longer chain (C14:0-C24:0) fatty acids in mix carbon sources.