Characterization of the GPR1/FUN34/YaaH protein family in the green microalga Chlamydomonas suggests their role as intracellular membrane acetate channels

Effect of NH4Cl supplementation on growth, photosynthesis, and triacylglycerol content in Chlamydomonas reinhardtii under mixotrophic cultivation

AIM

Ammonium chloride (NH4Cl) is one of the nitrogen sources for microalgal cultivation. An excessive amounts of NH4Cl are toxic for microalgae. However, combining mixotrophic conditions and excessive quantities of NH4Cl positively affects microalgal biomass and lipid production. In this study, we investigated the impact of NH4Cl on the growth, biomass, and triglyceride (TAG) content of the green microalga Chlamydomonas reinhardtii especially under mixotrophic conditions.

METHODS AND RESULTS

Under photoautotrophic conditions (without organic carbon supplementation), adding 25 mM NH4Cl had no significant effect on microalgal growth or TAG content. However, under mixotrophic condition (with acetate supplementation), NH4Cl interfered with microalgal growth while inducing TAG content. To explore these effects further, we conducted a two-step cultivation process and found that NH4Cl reduced microalgal growth, but induced total lipid and TAG content, especially after 4-day cultivation. The photosynthesis performances showed that NH4Cl completely inhibited oxygen evolution on day 4. However, NH4Cl slightly reduced the Fv/Fm ratio indicating that the NH4Cl supplementation directly affects microalgal photosynthesis. To investigate the TAG induction effect by NH4Cl, we compared the protein expression profiles of microalgae grown mixotrophically with and without 25 mM NH4Cl using a proteomics approach. This analysis identified 1782 proteins, with putative acetate uptake transporter GFY5 and acyl-coenzyme A oxidase being overexpressed in the NH4Cl-treated group.

CONCLUSION

These findings suggested that NH4Cl supplementation may stimulate acetate utilization and fatty acid synthesis pathways in microalgae cells. Our study indicated that NH4Cl supplementation can induce microalgal biomass and lipid production, particularly when combined with mixotrophic conditions.

Proteomic and phosphoproteomic analysis of a Haematococcus pluvialis (Chlorophyceae) mutant with a higher heterotrophic cell division rate reveals altered pathways involved in cell proliferation and nutrient partitioning

Haematococcus pluvialis has been used to produce the ketocarotenoid antioxidant, astaxanthin. Currently, heterotrophic cultivation of H. pluvialis is limited by slow growth rates. This work aimed to address this challenge by exploring the mechanisms of acetate metabolism in Haematococcus. Chemical mutagenesis and screening identified H. pluvialis strain KREMS 23D-3 that achieved up to a 34.9% higher cell density than the wild type when grown heterotrophically on acetate. An integrative proteomics and phosphoproteomics approach was employed to quantify 4955 proteins and 5099 phosphorylation sites from 2505 phosphoproteins in the wild-type and mutant strains of H. pluvialis. Among them, 12 proteins were significantly upregulated and 22 significantly downregulated in the mutant while phosphoproteomic analysis identified 143 significantly upregulated phosphorylation sites on 106 proteins and 130 downregulated phosphorylation sites on 114 proteins. Upregulation of anaphase-promoting complex phosphoproteins and downregulation of a putative cell cycle division 20 phosphoprotein in the mutant suggests rapid mitotic progression, coinciding with higher cell division rates. Upregulated coproporphyrinogen oxidase and phosphorylated magnesium chelatase in the mutant demonstrated altered nitrogen partitioning toward chlorophyll biosynthesis. The large proportion of differentially expressed phosphoproteins suggests phosphorylation is a key regulator for protein expression and activity in Haematococcus. Taken together, this study reveals the regulation of interrelated acetate metabolic pathways in H. pluvialis and provides protein targets that may guide future strain engineering work.

Moderate high temperature is beneficial or detrimental depending on carbon availability in the green alga Chlamydomonas reinhardtii

High temperatures impair plant growth and reduce agricultural yields, but the underlying mechanisms remain elusive. The unicellular green alga Chlamydomonas reinhardtii is an excellent model to study heat responses in photosynthetic cells due to its fast growth rate, many similarities in cellular processes to land plants, simple and sequenced genome, and ample genetic and genomics resources. Chlamydomonas grows in light by photosynthesis and with externally supplied acetate as an organic carbon source. Understanding how organic carbon sources affect heat responses is important for the algal industry but remains understudied. We cultivated wild-type Chlamydomonas under highly controlled conditions in photobioreactors at 25 °C (control), 35 °C (moderate high temperature), or 40 °C (acute high temperature) with or without constant acetate supply for 1 or 4 day. Treatment at 35 °C increased algal growth with constant acetate supply but reduced algal growth without sufficient acetate. The overlooked and dynamic effects of 35 °C could be explained by induced acetate uptake and metabolism. Heat treatment at 40 °C for more than 2 day was lethal to algal cultures with or without constant acetate supply. Our findings provide insights to understand algal heat responses and help improve thermotolerance in photosynthetic cells.

Towards advanced nutrient removal by microalgae-bacteria symbiosis system for wastewater treatment

In this study, the microalgae-bacteria symbiosis (ABS) system by co-culturing Chlorella sorokiniana with activated sludge was constructed for pollutants removal, and the according interaction mechanism was investigated. The results showed that the ABS system could almost completely remove ammonia nitrogen, and the removal efficiency of total nitrogen and total phosphorus could accordingly reach up to 65.3 % and 42.6 %. Brevundimonas greatly promoted microalgal biomass growth (maximum chlorophyll-a concentration of 9.4 mg/L), and microalgae contributed to the increase in the abundance of Dokdonella and Thermomonas in ABS system, thus facilitating nitrogen removal. The extended Derjaguin-Landau-Verwey-Overbeek theory indicated a repulsive potential barrier of 561.7 KT, while tryptophan-like proteins and tyrosine-like proteins were key extracellular polymeric substances for the formation of flocs by microalgae and activated sludge. These findings provide an in-depth understanding of interaction mechanism between microalgae and activated sludge for the removal of contaminants from wastewater.

Proteomics unveil a central role for peroxisomes in butyrate assimilation of the heterotrophic Chlorophyte alga Polytomella sp

Volatile fatty acids found in effluents of the dark fermentation of biowastes can be used for mixotrophic growth of microalgae, improving productivity and reducing the cost of the feedstock. Microalgae can use the acetate in the effluents very well, but butyrate is poorly assimilated and can inhibit growth above 1 gC.L-1. The non-photosynthetic chlorophyte alga Polytomella sp. SAG 198.80 was found to be able to assimilate butyrate fast. To decipher the metabolic pathways implicated in butyrate assimilation, quantitative proteomics study was developed comparing Polytomella sp. cells grown on acetate and butyrate at 1 gC.L-1. After statistical analysis, a total of 1772 proteins were retained, of which 119 proteins were found to be overaccumulated on butyrate vs. only 46 on acetate, indicating that butyrate assimilation necessitates additional metabolic steps. The data show that butyrate assimilation occurs in the peroxisome via the β-oxidation pathway to produce acetyl-CoA and further tri/dicarboxylic acids in the glyoxylate cycle. Concomitantly, reactive oxygen species defense enzymes as well as the branched amino acid degradation pathway were strongly induced. Although no clear dedicated butyrate transport mechanism could be inferred, several membrane transporters induced on butyrate are identified as potential condidates. Metabolic responses correspond globally to the increased needs for central cofactors NAD, ATP and CoA, especially in the peroxisome and the cytosol.

Photoprotective carbon redistribution in mixotrophic Haematococcus pluvialis under high light stress

Mixotrophy of Haematococcus pluvialis is a potential strategy for producing astaxanthin. However, this strategy has not been extensively commercialized because the mixotrophic mechanisms by which H. pluvialis overcomes high light stress are unclear. This study analyzed the biochemical compositions and differential proteomics of mixotrophic H. pluvialis under different light conditions. High light exposure substantially increased astaxanthin, carbohydrate, and fatty acid contents. A total of 119 and 81 proteins were significantly up- and down-regulated after two days of high light exposure. These proteins mainly enriched pathways for photosynthetic metabolism, glyoxylate cycle, and biosynthesis of secondary metabolites. This study proposed a regulatory model through which mixotrophic H. pluvialis copes with high light stress. The model includes pathways for modulating photosynthetic apparatus, increasing astaxanthin accumulation by enhancing photorespiration, pentose phosphate and Embden-Meyerhof-Parna pathways, while thickening the cell wall by malate-oxaloacetate shuttle.

Molecular Advancements Establishing Chlamydomonas as a Host for Biotechnological Exploitation

Chlamydomonas reinhardtii is emerging as a production platform for biotechnological purposes thanks to recent achievements, which we briefly summarize in this review. Firstly, robust nuclear transgene expression is now possible because several impressive improvements have been made in recent years. Strains allowing efficient and stable nuclear transgene expression are available and were recently made more amenable to rational biotechnological approaches by enabling genetic crosses and identifying their causative mutation. The MoClo synthetic biology strategy, based on Golden Gate cloning, was developed for Chlamydomonas and includes a growing toolkit of more than 100 genetic parts that can be robustly and rapidly assembled in a predefined order. This allows for rapid iterative cycles of transgene design, building, testing, and learning. Another major advancement came from various findings improving transgene design and expression such as the systematic addition of introns into codon-optimized coding sequences. Lastly, the CRISPR/Cas9 technology for genome editing has undergone several improvements since its first successful report in 2016, which opens the possibility of optimizing biosynthetic pathways by switching off competing ones. We provide a few examples demonstrating that all these recent developments firmly establish Chlamydomonas as a chassis for synthetic biology and allow the rewiring of its metabolism to new capabilities.

Systems-wide analysis revealed shared and unique responses to moderate and acute high temperatures in the green alga Chlamydomonas reinhardtii

Different intensities of high temperatures affect the growth of photosynthetic cells in nature. To elucidate the underlying mechanisms, we cultivated the unicellular green alga Chlamydomonas reinhardtii under highly controlled photobioreactor conditions and revealed systems-wide shared and unique responses to 24-hour moderate (35°C) and acute (40°C) high temperatures and subsequent recovery at 25°C. We identified previously overlooked unique elements in response to moderate high temperature. Heat at 35°C transiently arrested the cell cycle followed by partial synchronization, up-regulated transcripts/proteins involved in gluconeogenesis/glyoxylate-cycle for carbon uptake and promoted growth. But 40°C disrupted cell division and growth. Both high temperatures induced photoprotection, while 40°C distorted thylakoid/pyrenoid ultrastructure, affected the carbon concentrating mechanism, and decreased photosynthetic efficiency. We demonstrated increased transcript/protein correlation during both heat treatments and hypothesize reduced post-transcriptional regulation during heat may help efficiently coordinate thermotolerance mechanisms. During recovery after both heat treatments, especially 40°C, transcripts/proteins related to DNA synthesis increased while those involved in photosynthetic light reactions decreased. We propose down-regulating photosynthetic light reactions during DNA replication benefits cell cycle resumption by reducing ROS production. Our results provide potential targets to increase thermotolerance in algae and crops.

A systems-wide analysis of the single-cell green alga Chlamydomonas reinhardti reveals shared and unique responses to moderate and acute high temperatures using multiple-level investigation of transcriptomics, proteomics, cell physiology, photosynthetic parameters, and cellular ultrastructure.

A comprehensive review on carbon source effect of microalgae lipid accumulation for biofuel production

Energy is a major driving force for the economic development. Due to the scarcity of fossil fuels and negative impact on the environment, it is important to develop renewable and sustainable energy sources for humankind. Microalgae as the primary feedstock for biodiesel has shown great application potential. However, lipid yield from microalgae is limited by the upstream cost, which restrain the realization of large-scale biofuel production. The modification of lipid-rich microalgae cell has become the focus over the last few decades to improve the lipid content and productivity of microalgae. Carbon is a vital nutrient that regulates the growth and metabolism of microalgae. Different carbon sources are assimilated by microalgae cells via different pathways. Inorganic carbon sources are mainly used through the CO₂-concentrating mechanisms (CCMs), while organic carbon sources are absorbed by microalgae mainly through the Pentose Phosphate (PPP) Pathway and the Embden-Meyerhof-Pranas (EMP) pathway. Therefore, the addition of carbon source has a significant impact on the production of microalgae biomass and lipid accumulation. In this paper, mechanisms of lipid synthesis and carbon uptake of microalgae were introduced, and the effects of different carbon conditions (types, concentrations, and addition methods) on lipid accumulation in microalgal biomass production and biodiesel production were comprehensively discussed. This review also highlights the recent advances in microalgae lipid cultivation with large-scale commercialization and the development prospects of biodiesel production. Current challenges and constructive suggestions are proposed on cost-benefit concerns in large-scale production of microalgae biodiesel.

Dynamic light- and acetate-dependent regulation of the proteome and lysine acetylome of Chlamydomonas

The green alga Chlamydomonas reinhardtii is one of the most studied microorganisms in photosynthesis research and for biofuel production. A detailed understanding of the dynamic regulation of its carbon metabolism is therefore crucial for metabolic engineering. Post-translational modifications can act as molecular switches for the control of protein function. Acetylation of the ɛ-amino group of lysine residues is a dynamic modification on proteins across organisms from all kingdoms. Here, we performed mass spectrometry-based profiling of proteome and lysine acetylome dynamics in Chlamydomonas under varying growth conditions. Chlamydomonas liquid cultures were transferred from mixotrophic (light and acetate as carbon source) to heterotrophic (dark and acetate) or photoautotrophic (light only) growth conditions for 30 h before harvest. In total, 5863 protein groups and 1376 lysine acetylation sites were identified with a false discovery rate of <1%. As a major result of this study, our data show that dynamic changes in the abundance of lysine acetylation on various enzymes involved in photosynthesis, fatty acid metabolism, and the glyoxylate cycle are dependent on acetate and light. Exemplary determination of acetylation site stoichiometries revealed particularly high occupancy levels on K175 of the large subunit of RuBisCO and K99 and K340 of peroxisomal citrate synthase under heterotrophic conditions. The lysine acetylation stoichiometries correlated with increased activities of cellular citrate synthase and the known inactivation of the Calvin-Benson cycle under heterotrophic conditions. In conclusion, the newly identified dynamic lysine acetylation sites may be of great value for genetic engineering of metabolic pathways in Chlamydomonas.

Subcellular Localizations of Catalase and Exogenously Added Fatty Acid in Chlamydomonas reinhardtii

Fatty acids are important biological components, yet the metabolism of fatty acids in microalgae is not clearly understood. Previous studies found that Chlamydomonas reinhardtii, the model microalga, incorporates exogenously added fatty acids but metabolizes them differently from animals and yeast. Furthermore, a recent metabolic flux analysis found that the majority of lipid turnover in C. reinhardtii is the recycling of acyl chains from and to membranes, rather than β -oxidation. This indicates that for the alga, the maintenance of existing acyl chains may be more valuable than their breakdown for energy. To gain cell-biological knowledge of fatty acid metabolism in C. reinhardtii, we conducted microscopy analysis with fluorescent probes. First, we found that CAT1 (catalase isoform 1) is in the peroxisomes while CAT2 (catalase isoform 2) is localized in the endoplasmic reticulum, indicating the alga is capable of detoxifying hydrogen peroxide that would be produced during β-oxidation in the peroxisomes. Second, we compared the localization of exogenously added FL-C16 (fluorescently labelled palmitic acid) with fluorescently marked endosomes, mitochondria, peroxisomes, lysosomes, and lipid droplets. We found that exogenously added FL-C16 are incorporated and compartmentalized via a non-endocytic route within 10 min. However, the fluorescence signals from FL-C16 did not colocalize with any marked organelles, including peroxisomes. During triacylglycerol accumulation, the fluorescence signals from FL-C16 were localized in lipid droplets. These results support the idea that membrane turnover is favored over β-oxidation in C. reinhardtii. The knowledge gained in these analyses would aid further studies of the fatty acid metabolism.