Effects of fundamental nutrient stresses on the lipid accumulation profiles in two diatom species Thalassiosira weissflogii and Chaetoceros muelleri

Transcriptomic analysis of Chaetoceros muelleri in response to different nitrogen concentrations reveals the activation of pathways to enable efficient nitrogen uptake

Nitrogen is the principal nutrient deficiency that increases lipids and carbohydrate content in diatoms but negatively affects biomass production. Marine diatom Chaetoceros muelleri is characterized by lipid and carbohydrate accumulation under low nitrogen concentration without affecting biomass. To elucidate the molecular effects of nitrogen concentrations, we performed an RNA-seq analysis of C. muelleri grown under four nitrogen concentrations (3.53 mM, 1.76 mM, 0.44 mM, and 0.18 mM of NaNO3). This research revealed that changes in global transcription in C. muelleri are differentially expressed by nitrogen concentration. "Energetic metabolism", "Carbohydrate metabolism" and "Lipid metabolism" pathways were identified as the most upregulated by N deficiency. Due to N limitation, alternative pathways to self-supply nitrogen employed by microalgal cells were identified. Additionally, nitrogen limitation decreased chlorophyll content and caused a greater response at the transcriptional level with a higher number of unigenes differentially expressed. By contrast, the highest N concentration (3.53 mM) recorded the lowest number of differentially expressed genes. Amt1, Nrt2, Fad2, Skn7, Wrky19, and Dgat2 genes were evaluated by RT-qPCR. In conclusion, C. muelleri modify their metabolic pathways to optimize nitrogen utilization and minimize nitrogen losses. On the other hand, the assembled transcriptome serves as the basis for metabolic engineering focused on improving the quantity and quality of the diatom for biotechnological applications. However, proteomic and metabolomic analysis is also required to compare gene expression, protein, and metabolite accumulation.

First regional reference database of northern Adriatic diatom transcriptomes

Marine microbial communities form the basis for the functioning of marine ecosystems and the conservation of biodiversity. With the application of metagenomics and metatranscriptomics in marine environmental studies, significant progress has been made in analysing the functioning of microbial communities as a whole. These molecular techniques are highly dependent on reliable, well-characterised, comprehensive and taxonomically diverse sequenced reference transcriptomes of microbial organisms. Here we present a set of 12 individual transcriptome assemblies derived from 6 representative diatom species from the northern Adriatic Sea grown under 2 environmentally relevant growth conditions (phosphate replete vs. phosphate deprived). After filtering the reads and assembly, an average number of 64,932 transcripts per assembly was obtained, of which an average of 8856 were assigned to functionally known proteins. Of all assigned transcripts, an average of 6483 proteins were taxonomically assigned to diatoms (Bacillariophyta). On average, a higher number of assigned proteins was detected in the transcriptome assemblies of diatoms grown under replete media condition. On average, 50% of the mapped proteins were shared between the two growth conditions. All recorded proteins in the dataset were classified into 24 COG categories, with approximately 25% belonging to the unknown function and the remaining 75% belonging to all other categories. The resulting diatom reference database for the northern Adriatic, focussing on the response to nutrient limitation as characteristic for the region and predicted for the future world oceans, provides a valuable resource for analysing environmental metatranscriptome and metagenome data. Each northern Adriatic transcriptome can also be used by itself as a reference database for the (meta)transcriptomes and gene expression studies of the associated species that will be generated in the future.

Influence of Nutrient Deprivation on the Antioxidant Capacity and Chemical Profile of Two Diatoms from Genus Chaetoceros

The limited availability of phosphate, nitrogen and silicon in the growth media affects the growth, cellular processes, and metabolism of diatoms. Silicon deficiency primarily affects diatom morphology, while phosphate deficiency reduces the production of nucleic acids and phospholipids. Differences in pigment and protein composition are mainly due to nitrogen deficiency. In this study, Chaetoceros socialis and Chaetoceros costatus were cultured under phosphate, nitrogen, and silicon deprivation conditions. The diatom biomass was collected during the stationary growth phase and extracted with 70% ethanol under ultrasonication. The chemical profiles of the extracts were analyzed by high-performance liquid chromatography with high-resolution mass spectrometry with electrospray ionisation (UHPLC-ESI-HRMS), while the antioxidant capacity was determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and oxygen radical absorbance capacity (ORAC) assays. Pigments, fatty acids, sterols, and derivatives were detected in both species. The total phenolic content in the extracts ranged from 46.25 ± 1.08 to 89.38 ± 6.21 mg of gallic acid equivalent (GAE)/L and from 29.58 ± 1.08 to 54.17 ± 1.18 mg GAE/L. for C. costatus and C. socialis, respectively. Antioxidant activity was higher in C. costatus extracts, especially those obtained from nitrogen-deprived media. The results of this study contribute to the existing knowledge and the ongoing efforts to overcome application and commercialization barriers of microalgae for wide-ranging potential in different industries.

Response of varying combined nutrients on biomass and biochemical composition of marine diatoms Chaetoceros gracilis and Thalassiosira weissflogii

Marine diatoms have high adaptability and are known to accumulate lipids under nutrient stress conditions. The present study involves determining the effect of varying macro and micronutrients on growth kinetics and metabolite production of oleaginous marine diatoms, Thalassiosira weissflogii and Chaetoceros gracilis. The results highlighted that C. gracilis and T. weissflogii showed maximum biomass yield of 0.86 ± 0.06 g/L and 0.76 ± 0.01 g/L in the 2f and f supplemented medium respectively. A 2.5-fold increase in cellular lipid content was recorded in the 2f culture setup of both strains ranging from 20 % to 26.7 % (w/w). The study also reveals that high eutrophic nutrient media (f, 2f and 4f) triggered biomass productivity as well as total protein and carbohydrate content in both strains. Thus, providing a reproducible insight of trophic flexibility of diatoms, concomitant with the increment in multiple commercially valuable products.

Enhancement of Biomass Production of Diatom Nitzschia sp. S5 through Optimisation of Growth Medium Composition and Fed-Batch Cultivation

The growing commercial application of microalgae in different industry sectors, including the production of bioenergy, pharmaceuticals, nutraceuticals, chemicals, feed, and food, demands large quantities of microalgal biomass with specific compositions produced at reasonable prices. Extensive studies have been carried out on the design of new and improvement of current cultivation systems and the optimisation of growth medium composition for high productivity of microalgal biomass. In this study, the concentrations of the main macronutrients, silicon, nitrogen and phosphorus, essential for the growth of diatom Nitzschia sp. S5 were optimised to obtain a high biomass concentration. The effect of main macronutrients on growth kinetics and cell composition was also studied. Silicon had the most significant effect on diatom growth during batch cultivation. The concentration of biomass increased 5.45-fold (0.49 g L-1) at 1 mM silicon concentration in modified growth medium compared to the original Guillard f/2 medium. Optimisation of silicon, nitrogen, and phosphorus quantities and ratios further increased biomass concentration. The molar ratio of Si:N:P = 7:23:1 mol:mol:mol yielded the highest biomass concentration of 0.73 g L-1. Finally, the fed-batch diatom cultivation of diatom using an optimised Guillard f/2 growth medium with four additions of concentrated macronutrient solution resulted in 1.63 g L-1 of microalgal biomass. The proteins were the most abundant macromolecules in microalgal biomass, with a lower content of carbohydrates and lipids under all studied conditions.

Optimum Temperature and Salinity Conditions for Growth, Lipid Contents, and Fatty Acids Composition of Centric Diatoms Chaetoceros Calcitrans and Thaassiosira Weissflogii

Microalgae are very important organisms as primary producers and have a wide range of applications in areas such as aquaculture, pharmaceuticals, medicine, biofuels, and others. In this study, the effect of temperature and salinity on growth, biomass, proximate composition, and lipid production of Chaetoceros calcitrans (Paulsen) and Thalassiosira weissflogii (Grunow) were investigated. The best growth rate (SGR) and highest biomass production were observed at 30°C and 30 ppt for C. calcitrans and at 30°C and 25 ppt for T. weissflogii. At these optimum temperature and salinity combinations, the maximum cell density was accomplished by day 12 for C. calcitrans (6.74 × 106 cells ml˗1) and by day 10 for T. weissflogii (3.45 × 106 cells ml˗1). The proximate composition during this period was 38.25 ± 0.99% protein, 16.96 ± 0.90% lipid, and 9.39 ± 0.59% carbohydrate in C. calcitrans, compared to 13.49 ± 0.28% protein, 10.43% ± 0.25% lipid and 16.49 ± 0.47% carbohydrate in T. weissflogii. Furthermore, over 35% of lipids in C. calcitrans were palmitic acid (C16), while in T. weissflogii, over 24% of lipids were myristic acid (C14). Although C. calcitrans exhibited higher lipid content than T. weissflogii, both species displayed higher levels of saturated (SFA) and monounsaturated (MUFAs) fatty acids and low levels of polyunsaturated fatty acids (PUFAs). The findings illustrated that under their optimum temperature and salinity combinations, both species might produce significant sources of lipids, which can be utilised in various activities such as aquaculture, pharmaceuticals, medicine, biofuels and others.

Indoor mass cultivation of marine diatoms for biodiesel production using induction plasma synthesized nanosilica

In this work, two benthic marine diatoms Chaetoceros sp. and Thalassiosira sp. were grown in modified f/2 medium in which normal silica was replaced with inductively coupled plasma (ICP) nanosilica for indoor mass cultivation and its impact on growth, lipid content, lipid quality and metabolite production were monitored. Results indicate thatunder mass cultivation using ICP nano silica medium, Thalassiosirasp. reached 3.6 and Chaetoceros sp. reached 3.2-fold higher cell density compared to normal Si medium. The primary metabolite production and total lipid content was higher in Chaetoceros sp. (44.33 ± 2.51% DCW) compared to Thalassiosira sp. (29.66 ± 1.52% DCW). In mass cultivation, ICP synthesized nanosilica powder was effective in enhancing the cell density, production of metabolites, pigments, and lipids in the marine diatoms studied. This is the first report on the use of ICP nanosilica in carrying out indoor mass cultivation of marine diatom isolates as potential biodiesel and biomolecule feedstocks.

Biochemical and Molecular Aspects of Phosphorus Limitation in Diatoms and Their Relationship with Biomolecule Accumulation

Simple Summary

Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. It has been reported that P limitation in diatoms induces the synthesis of biomolecules and the accumulation of storage compounds, such as pigments, carbohydrates and lipids, with diverse biological activities, which can be used in diverse biotechnological applications. However, the molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. The integration of results obtained from omics sciences could provide a broad understanding of the response of diatoms to P limitation, and the information obtained could help to solve challenges such as biomass production, by-products yield and genetic improvement of strains.

Abstract

Diatoms are the most abundant group of phytoplankton, and their success lies in their significant adaptation ability to stress conditions, such as nutrient limitation. Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. Molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. Among the molecular responses that have been reported in diatoms cultured under P deficient conditions is the upregulation of genes encoding enzymes related to the transport, assimilation, remobilization and recycling of this nutrient. Regarding biochemical responses, due to the reduction of the requirements for carbon structures for the synthesis of proteins and phospholipids, more CO₂ is fixed than is consumed by the Calvin cycle. To deal with this excess, diatoms redirect the carbon flow toward the synthesis of storage compounds such as triacylglycerides and carbohydrates, which are excreted as extracellular polymeric substances. This review aimed to gather all current knowledge regarding the biochemical and molecular mechanisms of diatoms related to managing P deficiency in order to provide a wider insight into and understanding of their responses, as well as the metabolic pathways affected by the limitation of this nutrient.

Production of lipids by Chaetoceros affinis in media based on palm oil mill effluent

Biomass and lipid production by the marine diatom Chaetoceros affinis were characterized under continuous light with aeration. Media based on palm oil mill effluent (POME; 10, 20 and 30 % v/v in distilled water) were used together with a standard control medium. The maximum biomass concentration on day 12 of batch cultures in control medium was 821 ± 71 mg L^-1. Under identical conditions, in the best POME medium (20 % POME v/v in distilled water with other inorganic components), the biomass concentration was reduced by ∼11 % to 734 ± 66 mg L^-1. The lipid content of the biomass grown in the control medium was 50.8 ± 4.5 % by dry weight, but was a little lower (48.9 ± 4.1 % by dry wt) in the above specified best POME medium. In the best POME medium, oleic acid was the major fatty acid (72.3 ± 5.2 % by weight) in the total lipids extracted from the biomass and monounsaturated fatty acids were the main type of fatty acids (74.6 ± 5.2 %). POME levels of >20 % in the medium suppressed both biomass and lipid production relative to the medium with 20 % POME.

Enhanced Lipid Production in Chlamydomonas reinhardtii Caused by Severe Iron Deficiency

Microalgae are used as a source of lipids for the production of biofuels. Most algae produce neutral lipids under stress conditions. Here, lipid accumulation by the unicellular alga Chlamydomonas reinhardtii was examined during cultivation under iron-limiting conditions. Severe iron stress caused the cells to accumulate a significant amount of lipid, specifically triacylglycerols (TAGs), by compromising the growth. Semi-quantitative measurements by Fourier transform infrared (FTIR) spectroscopy showed an increase in both carbohydrate and lipid content in iron-stressed C. reinhardtii cells compared to control. Analysis by flow cytometry and thin layer chromatography confirmed that severe iron deficiency-induced TAG accumulation to fourfold higher than in iron-replete control cells. This accumulation of TAGs was mostly degraded from chloroplast lipids accompanied by overexpression of diacylglycerol acyltransferase (DGAT2A) protein. Furthermore, liquid chromatography-mass spectrometry (LC-MS) analysis demonstrated significantly enhanced levels of C16:0, C18:2, and C18:3 fatty acids (FAs). These results indicate that iron stress triggers the rapid accumulation of TAGs in C. reinhardtii cells. The enhanced production of these lipids caused by the iron deficiency may contribute to the efficient production of algal biofuels if we escalate to the photobioreactor's growth conditions.

Storage Compound Accumulation in Diatoms as Response to Elevated CO2 Concentration

Accumulation of reserve compounds (i.e., lipids and chrysolaminarin) in diatoms depends on the environmental conditions, and is often triggered by stress conditions, such as nutrient limitation. Manipulation of CO2 supply can also be used to improve both lipids and carbohydrates accumulation. Given the high diversity among diatoms, we studied the two marine model diatoms-Thalassiosira pseudonana and Phaeodactylum tricornutum, a freshwater diatom, Asterionella formosa, and Navicula pelliculosa-found in fresh- and sea-water environments. We measured the accumulation of reserve compounds and the activity of enzymes involved in carbon metabolism in these diatoms grown at high and atmospheric CO2. We observed that biomass and lipid accumulation in cells grown at high CO2 differ among the diatoms. Lipid accumulation increased only in P. tricornutum and N. pelliculosa grown in seawater in response to elevated CO2. Moreover, accumulation of lipids was also accompanied by an increased activity of the enzymes tested. However, lipid accumulation and enzyme activity decreased in N. pelliculosa cultured in fresh water. Chrysolaminarin accumulation was also affected by CO2 concentration; however, there was no clear relation with lipids accumulation. Our results are relevant to understand better the ecological role of the environment in the diatom adaptation to CO2 and the mechanisms underpinning the production of storage compounds considering diatom diversity.

Improved lipid production in oleaginous brackish diatom Navicula phyllepta MACC8 using two-stage cultivation approach

A two-stage cultivation method involving the initial growth in optimized conditions for biomass production followed by those for lipid production in oleaginous brackish diatom Navicula phyllepta MACC8 resulted in a proportional increase of lipid concentration along with biomass production. The diatom was further subjected to stress conditions by altering the nutrient components such as nitrate, phosphate, silicate, and temperature. Silicon deprivation resulted in the highest lipid percentage of 28.78% of weight at the end of the 18th day of the second stage. A significant increase in lipid content was observed on the complete removal of the nutrients silicon and urea one at a time, while the biomass showed a considerable reduction. The application of multiple nutrient stress conditions had a profound influence on the increased rate of lipid production. A combination of phosphate deprivation, silicate limitation and temperature reduction resulted in a significant increase in lipid percentage of 32.13% at the cost of reduced biomass (1.1 g L^-1), whereas phosphate deprivation, urea limitation, and temperature reduction resulted in lipid percentage of 27.58% with a biomass of 1.44 g L^-1 at the end of the second stage. Further, the results were supported by Nile red staining, FTIR, fatty acid profile and oxidative stress marker analyses. The changes in biochemical composition and oxidative stress parameters within the various stress conditions demonstrated the profound influence of the selected stress factors on the biodiesel productivity of the diatom, besides its stress tolerance. A two-phase culturing system, with multifactor stress application, especially nitrogen limitation along with phosphate starvation and temperature stress, would be the suitable method for gaining maximum biomass productivity and lipid content in diatom Navicula phyllepta MACC8 towards biofuel production.

Biodiesel production through algal cultivation in urban wastewater using algal floway

The aim of this work was to study algal floway (AFW) to treat urban wastewater and to evaluate biomass productivity, lipid contents and biodiesel production. The results indicated the seasonal average algae productivity of 34.83 g dry weight m² d^-1 with a nutrient removal rate of 2.52 g m² d^-1N and 1.25 g m² d^-1P while the lipid content ranged between 14 and 22% of dry cell weight with the highest lipid productivity of 9.29 g m^-2 d^-1 during summer. Biodiesel quality was superior during summer with high centane number and cold filter plugging point values. High eicosapentaenoic acid content was found during winter growth cycles. AFW algae community was dominated by pennate diatoms during all growing seasons. This study is one of its kinds in Indian wastewaters and it provides fundamental information for further optimization and use of AFW to treat domestic wastewater and to produce algae biofuel feedstock.