Quantitative proteomics of a B12 -dependent alga grown in coculture with bacteria reveals metabolic tradeoffs required for mutualism

Key Proteomics Tools for Fundamental and Applied Microalgal Research

Microscopic, photosynthetic prokaryotes and eukaryotes, collectively referred to as microalgae, are widely studied to improve our understanding of key metabolic pathways (e.g., photosynthesis) and for the development of biotechnological applications. Omics technologies, which are now common tools in biological research, have been shown to be critical in microalgal research. In the past decade, significant technological advancements have allowed omics technologies to become more affordable and efficient, with huge datasets being generated. In particular, where studies focused on a single or few proteins decades ago, it is now possible to study the whole proteome of a microalgae. The development of mass spectrometry-based methods has provided this leap forward with the high-throughput identification and quantification of proteins. This review specifically provides an overview of the use of proteomics in fundamental (e.g., photosynthesis) and applied (e.g., lipid production for biofuel) microalgal research, and presents future research directions in this field.

LFQRatio: A Normalization Method to Decipher Quantitative Proteome Changes in Microbial Coculture Systems

The value of synthetic microbial communities in biotechnology is gaining traction due to their ability to undertake more complex metabolic tasks than monocultures. However, a thorough understanding of strain interactions, productivity, and stability is often required to optimize growth and scale up cultivation. Quantitative proteomics can provide valuable insights into how microbial strains adapt to changing conditions in biomanufacturing. However, current workflows and methodologies are not suitable for simple artificial coculture systems where strain ratios are dynamic. Here, we established a workflow for coculture proteomics using an exemplar system containing two members, Azotobacter vinelandii and Synechococcus elongatus. Factors affecting the quantitative accuracy of coculture proteomics were investigated, including peptide physicochemical characteristics such as molecular weight, isoelectric point, hydrophobicity, and dynamic range as well as factors relating to protein identification such as varying proteome size and shared peptides between species. Different quantification methods based on spectral counts and intensity were evaluated at the protein and cell level. We propose a new normalization method, named "LFQRatio", to reflect the relative contributions of two distinct cell types emerging from cell ratio changes during cocultivation. LFQRatio can be applied to real coculture proteomics experiments, providing accurate insights into quantitative proteome changes in each strain.

The effects of extracellular algicidal compounds of Bacillus sp. B1 on Heterosigma akashiwo: a metabolomics approach

Heterosigma akashiwo (H. akashiwo), a harmful algal species, has been a global environmental problem. Extracellular algicidal compounds (EACs) extracted from Bacillus sp. B1 exhibited algicidal effects against H. akashiwo. However, little is known about the algicidal mechanism and metabolic process. In this study, metabolomics and physiological analyses were combined to investigate the cellular responses of H. akashiwo when treated with EACs. The results indicated that EACs at 10% (v_EACs/v_sample) showed more than 90% inhibition of H. akashiwo. EAC treatment resulted in excessive reactive oxygen species (ROS) production in algal cells, causing stress responses such as inhibition of photosynthetic pigment synthesis, reduction of sugar synthesis, imbalance of osmotic pressure in the cell membrane, disruption of cell size and morphology, and eventual cell death. The results reveal the underlying mechanism of the algicidal process and provide new insights into algae-bacteria interactions and the application of metabolomics to algal research.

Chlamydomonas reinhardtii, a Reference Organism to Study Algal-Microbial Interactions: Why Can't They Be Friends?

The stability and harmony of ecological niches rely on intricate interactions between their members. During evolution, organisms have developed the ability to thrive in different environments, taking advantage of each other. Among these organisms, microalgae are a highly diverse and widely distributed group of major primary producers whose interactions with other organisms play essential roles in their habitats. Understanding the basis of these interactions is crucial to control and exploit these communities for ecological and biotechnological applications. The green microalga Chlamydomonas reinhardtii, a well-established model, is emerging as a model organism for studying a wide variety of microbial interactions with ecological and economic significance. In this review, we unite and discuss current knowledge that points to C. reinhardtii as a model organism for studying microbial interactions.

Associated bacterial microbiome responds opportunistic once algal host Scenedesmus vacuolatus is attacked by endoparasite Amoeboaphelidium protococcarum

The interactions of microalgae and their associated microbiomes have come to the fore of applied phycological research in recent years. However, the functional mechanisms of microalgal interactions remain largely unknown. Here, we examine functional protein patterns of the microalgae Scenedesmus vacuolatus and its associated bacterial community during algal infection by the endoparasite Amoeboaphelidium protococcarum. We performed metaproteomics analyses of non-infected (NI) and aphelid-infected (AI) S.vacuolatus cultures to investigate underlying functional and physiological changes under infectious conditions. We observed an increase in bacterial protein abundance as well as a severe shift of bacterial functional patterns throughout aphelid-infection in comparison to NI treatment. Most of the bacterial proteins (about 55%) upregulated in AI were linked to metabolism and transport of amino acids, lipids, coenzymes, nucleotides and carbohydrates and to energy production. Several proteins associated with pathogenic bacterial-plant interactions showed higher protein abundance levels in AI treatment. These functional shifts indicate that associated bacteria involved in commensalistic or mutualistic interactions in NI switch to opportunistic lifestyles and facilitate pathogenic or saprotrophic traits in AI treatment. In summary, the native bacterial microbiome adapted its metabolism to algal host die off and is able to metabolize nutrients from injured cells or decompose dead cellular material.

Sharing Vitamin B12 between Bacteria and Microalgae Does Not Systematically Occur: Case Study of the Haptophyte Tisochrysis lutea

Haptophyte microalgae are key contributors to microbial communities in many environments. It has been proposed recently that members of this group would be virtually all dependent on vitamin B12 (cobalamin), an enzymatic cofactor produced only by some bacteria and archaea. Here, we examined the processes of vitamin B12 acquisition by haptophytes. We tested whether co-cultivating the model species Tisochrysis lutea with B12-producing bacteria in vitamin-deprived conditions would allow the microalga to overcome B12 deprivation. While T. lutea can grow by scavenging vitamin B12 from bacterial extracts, co-culture experiments showed that the algae did not receive B12 from its associated bacteria, despite bacteria/algae ratios supposedly being sufficient to allow enough vitamin production. Since other studies reported mutualistic algae–bacteria interactions for cobalamin, these results question the specificity of such associations. Finally, cultivating T. lutea with a complex bacterial consortium in the absence of the vitamin partially rescued its growth, highlighting the importance of microbial interactions and diversity. This work suggests that direct sharing of vitamin B12 is specific to each species pair and that algae in complex natural communities can acquire it indirectly by other mechanisms (e.g., after bacterial lysis).

Exploring the onset of B12 -based mutualisms using a recently evolved Chlamydomonas auxotroph and B12 -producing bacteria

Cobalamin (vitamin B₁₂ ) is a cofactor for essential metabolic reactions in multiple eukaryotic taxa, including major primary producers such as algae, and yet only prokaryotes can produce it. Many bacteria can colonize the algal phycosphere, forming stable communities that gain preferential access to photosynthate and in return provide compounds such as B₁₂ . Extended coexistence can then drive gene loss, leading to greater algal-bacterial interdependence. In this study, we investigate how a recently evolved B₁₂ -dependent strain of Chlamydomonas reinhardtii, metE7, forms a mutualism with certain bacteria, including the rhizobium Mesorhizobium loti and even a strain of the gut bacterium E. coli engineered to produce cobalamin. Although metE7 was supported by B₁₂ producers, its growth in co-culture was slower than the B₁₂ -independent wild-type, suggesting that high bacterial B₁₂ provision may be necessary to favour B₁₂ auxotrophs and their evolution. Moreover, we found that an E. coli strain that releases more B₁₂ makes a better mutualistic partner, and although this trait may be more costly in isolation, greater B₁₂ release provided an advantage in co-cultures. We hypothesize that, given the right conditions, bacteria that release more B₁₂ may be selected for, particularly if they form close interactions with B₁₂ -dependent algae.

Community Interaction Co-limitation: Nutrient Limitation in a Marine Microbial Community Context

The simultaneous limitation of productivity by two or more nutrients, commonly referred to as nutrient co-limitation, affects microbial communities throughout the marine environment and is of profound importance because of its impacts on various biogeochemical cycles. Multiple types of co-limitation have been described, enabling distinctions based on the hypothesized mechanisms of co-limitation at a biochemical level. These definitions usually pertain to individuals and do not explicitly, or even implicitly, consider complex ecological dynamics found within a microbial community. However, limiting and co-limiting nutrients can be produced in situ by a subset of microbial community members, suggesting that interactions within communities can underpin co-limitation. To address this, we propose a new category of nutrient co-limitation, community interaction co-limitation (CIC). During CIC, one part of the community is limited by one nutrient, which results in the insufficient production or transformation of a biologically produced nutrient that is required by another part of the community, often primary producers. Using cobalamin (vitamin B₁₂) and nitrogen fixation as our models, we outline three different ways CIC can arise based on current literature and discuss CIC’s role in biogeochemical cycles. Accounting for the inherent and complex roles microbial community interactions play in generating this type of co-limitation requires an expanded toolset – beyond the traditional approaches used to identify and study other types of co-limitation. We propose incorporating processes and theories well-known in microbial ecology and evolution to provide meaningful insight into the controls of community-based feedback loops and mechanisms that give rise to CIC in the environment. Finally, we highlight the data gaps that limit our understanding of CIC mechanisms and suggest methods to overcome these and further identify causes and consequences of CIC. By providing this framework for understanding and identifying CIC, we enable systematic examination of the impacts this co-limitation can have on current and future marine biogeochemical processes.

A comprehensive review on the use of algal-bacterial systems for wastewater treatment with emphasis on nutrient and micropollutant removal

The scarcity of water resources and environmental pollution have highlighted the need for sustainable wastewater treatment. Existing conventional treatment systems are energy-intensive and not always able to meet stringent disposal standards. Recently, algal-bacterial systems have emerged as environmentally friendly sustainable processes for wastewater treatment and resource recovery. The algal-bacterial systems work on the principle of the symbiotic relationship between algae and bacteria. This paper comprehensively discusses the most recent studies on algal-bacterial systems for wastewater treatment, factors affecting the treatment, and aspects of resource recovery from the biomass. The algal-bacterial interaction includes cell-to-cell communication, substrate exchange, and horizontal gene transfer. The quorum sensing (QS) molecules and their effects on algal-bacterial interactions are briefly discussed. The effect of the factors such as pH, temperature, C/N/P ratio, light intensity, and external aeration on the algal-bacterial systems have been discussed. An overview of the modeling aspects of algal-bacterial systems has been provided. The algal-bacterial systems have the potential for removing micropollutants because of the diverse possible interactions between algae-bacteria. The removal mechanisms of micropollutants - sorption, biodegradation, and photodegradation, have been reviewed. The harvesting methods and resource recovery aspects have been presented. The major challenges associated with algal-bacterial systems for real scale implementation and future perspectives have been discussed. Integrating wastewater treatment with the algal biorefinery concept reduces the overall waste component in a wastewater treatment system by converting the biomass into a useful product, resulting in a sustainable system that contributes to the circular bioeconomy.

Induction of vitamin B12 to purify biogas slurry and upgrade biogas using co-culture of microalgae and fungi

Different gradient concentrations of vitamin B12 (0, 10, 100, 1,000 ng L^-1 ) were used in the symbiosis system (Chlorella vulgaris-Ganoderma lucidum or Chlorella vulgaris-Pleurotus ostreatus) to assess their effect on simultaneous purification of biogas and removal of nutrients in biogas slurry using co-culture of microalgae and fungi. When B12 was added to the symbiosis system, biomass growth, intracellular carbonic anhydrase activity (CA), chlorophyll a content (CHL-a), photosynthetic characteristics of the two cultivation system, and removal efficiency of nutrients in biogas slurry and CO₂ in biogas were significantly higher than those in the control group. The optimal concentration of B12 was determined to be 100 ng L^-1 considering the removal efficiency of nutrients and CO₂ . Maximum mean chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and CO₂ removal efficiencies were 75.98 ± 6.26%, 78.46 ± 6.21%, 80.21 ± 6.83% and 61.08 ± 5.21% in Chlorella vulgaris-Ganoderma lucidum, respectively. This study showed the potential of microalgae and fungi symbiosis system with B12 addition for nutrient removal and biogas upgrading. PRACTITIONER POINTS: Vitamin B12 had positive effects on algal-fungal pellets growth. The optimal vitamin B12 concentration was 100 ng L^-1 . The highest CO₂ remove rate was 61.08% by G. lucidum/C. vulgaris pellets. Vitamin B12 significantly improved photosynthetic performance of pellets.

How Do Quorum-Sensing Signals Mediate Algae-Bacteria Interactions?

Quorum sensing (QS) describes a process by which bacteria can sense the local cell density of their own species, thus enabling them to coordinate gene expression and physiological processes on a community-wide scale. Small molecules called autoinducers or QS signals, which act as intraspecies signals, mediate quorum sensing. As our knowledge of QS has progressed, so too has our understanding of the structural diversity of QS signals, along with the diversity of bacteria conducting QS and the range of ecosystems in which QS takes place. It is now also clear that QS signals are more than just intraspecies signals. QS signals mediate interactions between species of prokaryotes, and between prokaryotes and eukaryotes. In recent years, our understanding of QS signals as mediators of algae-bacteria interactions has advanced such that we are beginning to develop a mechanistic understanding of their effects. This review will summarize the recent efforts to understand how different classes of QS signals contribute to the interactions between planktonic microalgae and bacteria in our oceans, primarily N-acyl-homoserine lactones, their degradation products of tetramic acids, and 2-alkyl-4-quinolones. In particular, this review will discuss the ways in which QS signals alter microalgae growth and metabolism, namely as direct effectors of photosynthesis, regulators of the cell cycle, and as modulators of other algicidal mechanisms. Furthermore, the contribution of QS signals to nutrient acquisition is discussed, and finally, how microalgae can modulate these small molecules to dampen their effects.

Quantitative proteomic comparison of salt stress in Chlamydomonas reinhardtii and the snow alga Chlamydomonas nivalis reveals mechanisms for salt-triggered fatty acid accumulation via reallocation of carbon resources

BACKGROUND

Chlamydomonas reinhardtii is a model green alga strain for molecular studies; its fully sequenced genome has enabled omic-based analyses that have been applied to better understand its metabolic responses to stress. Here, we characterised physiological and proteomic changes between a low-starch C. reinhardtii strain and the snow alga Chlamydomonas nivalis, to reveal insights into their contrasting responses to salinity stress.

RESULTS

Each strain was grown in conditions tailored to their growth requirements to encourage maximal fatty acid (as a proxy measure of lipid) production, with internal controls to allow comparison points. In 0.2 M NaCl, C. nivalis accumulates carbohydrates up to 10.4% DCW at 80 h, and fatty acids up to 52.0% dry cell weight (DCW) over 12 days, however, C. reinhardtii does not show fatty acid accumulation over time, and shows limited carbohydrate accumulation up to 5.5% DCW. Analysis of the C. nivalis fatty acid profiles showed that salt stress improved the biofuel qualities over time. Photosynthesis and respiration rates are reduced in C. reinhardtii relative to C. nivalis in response to 0.2 M NaCl. De novo sequencing and homology matching was used in conjunction with iTRAQ-based quantitative analysis to identify and relatively quantify proteomic alterations in cells exposed to salt stress. There were abundance differences in proteins associated with stress, photosynthesis, carbohydrate and lipid metabolism proteins. In terms of lipid synthesis, salt stress induced an increase in dihydrolipoyl dehydrogenase in C. nivalis (1.1-fold change), whilst levels in C. reinhardtii remained unaffected; this enzyme is involved in acetyl CoA production and has been linked to TAG accumulation in microalgae. In salt-stressed C. nivalis there were decreases in the abundance of UDP-sulfoquinovose (- 1.77-fold change), which is involved in sulfoquinovosyl diacylglycerol metabolism, and in citrate synthase (- 2.7-fold change), also involved in the TCA cycle. Decreases in these enzymes have been shown to lead to increased TAG production as fatty acid biosynthesis is favoured. Data are available via ProteomeXchange with identifier PXD018148.

CONCLUSIONS

These differences in protein abundance have given greater understanding of the mechanism by which salt stress promotes fatty acid accumulation in the un-sequenced microalga C. nivalis as it switches to a non-growth state, whereas C. reinhardtii does not have this response.

Toward the Enhancement of Microalgal Metabolite Production through Microalgae-Bacteria Consortia

Engineered mutualistic consortia of microalgae and bacteria may be a means of assembling a novel combination of metabolic capabilities with potential biotechnological advantages. Microalgae are promising organisms for the sustainable production of metabolites of commercial interest, such as lipids, carbohydrates, pigments, and proteins. Several studies reveal that microalgae growth and cellular storage of these metabolites can be enhanced significantly by co-cultivation with growth-promoting bacteria. This review summarizes the state of the art of microalgae-bacteria consortia for the production of microalgal metabolites. We discuss the current knowledge on microalgae-bacteria mutualism and the mechanisms of bacteria to enhance microalgae metabolism. Furthermore, the potential routes for a microalgae-bacteria biorefinery are outlined in an attempt to overcome the economic failures and negative energy balances of the existing production processes.

Effects of exogenous vitamin B12 on nutrient removal and protein expression of algal-bacterial consortium

Chlorella vulgaris and Bacillus licheniformis consortium was added to synthetic wastewater with exogenous vitamin B₁₂. In the presence of 100 ng/L vitamin B₁₂, removal efficiencies of TN, NH₃-N, PO₄^3-P, and COD were 80.1%, 76.8%, 87.9%, and 76.7%, respectively. The functional groups on the cell surface of the consortium, including -NH, -CH₃, C=O, C=C, and P-O-C, increased with 100 ng/L vitamin B₁₂. These functional groups improved the biological adsorption of the consortium; however, higher concentrations of vitamin B₁₂ resulted in an occlusion of the functional groups. Furthermore, there were 5 significantly enriched protein pathways, namely carbon fixation in photosynthetic organisms; amino acid metabolic pathways; the pathway of one carbon pool by folate; nitrogen metabolism; and photosynthesis. Most proteins in these pathways were upregulated, which enhanced carbon fixation and photosynthesis in the algal cells. Simultaneously, B₁₂ promoted significant upregulation of proteins associated with the quorum-sensing pathway, which promoted the interaction between algae and bacteria.

Adapting the algal microbiome for growth on domestic landfill leachate

We aimed to improve algal growth rate on leachate by optimising the algal microbiome. An algal-bacterial consortium was enriched from landfill leachate and subjected to 24 months of adaptive laboratory evolution, increasing the growth rate of the dominant algal strain, Chlorella vulgaris, almost three-fold to 0.2 d^-1. A dramatic reduction in nitrate production suggested a shift in biological utilisation of ammoniacal-N, supported by molecular 16S rRNA taxonomic analyses, where Nitrosomonas numbers were not detected in the adapted consortium. A PICRUSt approach predicted metagenomic functional content and revealed a high number of sequences belonging to bioremediation pathways, including degradation of aromatic compounds, benzoate and naphthalene, as well as pathways known to be involved in algal-bacterial symbiosis. This study enhances our understanding of beneficial mechanisms in algal-bacterial associations in complex effluents, and ultimately enables the bottom-up design of optimised algal microbiomes for exploitation within industry.

Metabolic Insights Into Infochemicals Induced Colony Formation and Flocculation in Scenedesmus subspicatus Unraveled by Quantitative Proteomics

Microalgae can respond to natural cues from crustacean grazers, such as Daphnia, by forming colonies and aggregations called flocs. Combining microalgal biology, physiological ecology, and quantitative proteomics, we identified how infochemicals from Daphnia trigger physiological and cellular level changes in the microalga Scenedesmus subspicatus, underpinning colony formation and flocculation. We discovered that flocculation occurs at an energy-demanding ‘alarm’ phase, with an important role proposed in cysteine synthesis. Flocculation appeared to be initially stimulated by the production of an extracellular matrix where polysaccharides and fatty acids were present, and later sustained at an ‘acclimation’ stage through mitogen-activated protein kinase (MAPK) signaling cascades. Colony formation required investment into fatty acid metabolism, likely linked to separation of membranes during cell division. Higher energy demands were required at the alarm phase, which subsequently decreased at the acclimation stage, thus suggesting a trade-off between colony formation and flocculation. From an ecological and evolutionary perspective, our findings represent an improved understanding of the effect of infochemicals on microalgae-grazers interactions, and how they can therefore potentially impact on the structure of aquatic communities. Moreover, the mechanisms revealed are of interest in algal biotechnology, for exploitation in low-cost, sustainable microalgal biomass harvesting.

Responses of a Newly Evolved Auxotroph of Chlamydomonas to B12 Deprivation

The corrinoid B₁₂ is synthesized only by prokaryotes yet is widely required by eukaryotes as an enzyme cofactor. Microalgae have evolved B₁₂ dependence on multiple occasions, and we previously demonstrated that experimental evolution of the non-B₁₂-requiring alga Chlamydomonas reinhardtii in media supplemented with B₁₂ generated a B₁₂-dependent mutant (hereafter metE7). This clone provides a unique opportunity to study the physiology of a nascent B₁₂ auxotroph. Our analyses demonstrate that B₁₂ deprivation of metE7 disrupts C1 metabolism, causes an accumulation of starch and triacylglycerides, and leads to a decrease in photosynthetic pigments, proteins, and free amino acids. B₁₂ deprivation also caused a substantial increase in reactive oxygen species, which preceded rapid cell death. Survival could be improved without compromising growth by simultaneously depriving the cells of nitrogen, suggesting a type of cross protection. Significantly, we found further improvements in survival under B₁₂ limitation and an increase in B₁₂ use efficiency after metE7 underwent a further period of experimental evolution, this time in coculture with a B₁₂-producing bacterium. Therefore, although an early B₁₂-dependent alga would likely be poorly adapted to coping with B₁₂ deprivation, association with B₁₂-producers can ensure long-term survival whilst also providing a suitable environment for evolving mechanisms to tolerate B₁₂ limitation better.

Mechanisms and Impact of Symbiotic Phosphate Acquisition

Phosphorous is important for life but often limiting for plants. The symbiotic pathway of phosphate uptake via arbuscular mycorrhizal fungi (AMF) is evolutionarily ancient and today occurs in natural and agricultural ecosystems alike. Plants capable of this symbiosis can obtain up to all of the phosphate from symbiotic fungi, and this offers potential means to develop crops less dependent on unsustainable P fertilizers. Here, we review the mechanisms and insights gleaned from the fine-tuned signal exchanges that orchestrate the intimate mutualistic symbiosis between plants and AMF. As the currency of trade, nutrients have signaling functions beyond being the nutritional goal of mutualism. We propose that such signaling roles and metabolic reprogramming may represent commitments for a mutualistic symbiosis that act across the stages of symbiosis development.

Model Microbial Consortia as Tools for Understanding Complex Microbial Communities

A major biological challenge in the postgenomic era has been untangling the composition and functions of microbes that inhabit complex communities or microbiomes. Multi-omics and modern bioinformatics have provided the tools to assay molecules across different cellular and community scales; however, mechanistic knowledge over microbial interactions often remains elusive. This is due to the immense diversity and the essentially undiminished volume of not-yet-cultured microbes. Simplified model communities hold some promise in enabling researchers to manage complexity so that they can mechanistically understand the emergent properties of microbial community interactions. In this review, we surveyed several approaches that have effectively used tractable model consortia to elucidate the complex behavior of microbial communities. We go further to provide some perspectives on the limitations and new opportunities with these approaches and highlight where these efforts are likely to lead as advances are made in molecular ecology and systems biology.

Cross-exchange of B-vitamins underpins a mutualistic interaction between Ostreococcus tauri and Dinoroseobacter shibae

Ostreococcus tauri, a picoeukaryotic alga that contributes significantly to primary production in oligotrophic waters, has a highly streamlined genome, lacking the genetic capacity to grow without the vitamins thiamine (B₁) and cobalamin (B₁₂)_. Here we demonstrate that the B₁₂ and B₁ auxotrophy of O. tauri can be alleviated by co-culturing with a heterotrophic bacterial partner Dinoroseobacter shibae, a member of the Rhodobacteraceae family of alpha-proteobacteria, genera of which are frequently found associated with marine algae. D. shibae lacks the complete pathway to synthesise three other B-vitamins: niacin (B₃), biotin (B₇), and p-aminobenzoic acid (a precursor for folate, B₉), and the alga is in turn able to satisfy the reciprocal vitamin requirements of its bacterial partner in a stable long-term co-culture. Bioinformatics searches of 197 representative marine bacteria with sequenced genomes identified just nine species that had a similar combination of traits (ability to make vitamin B₁₂, but missing one or more genes for niacin and biotin biosynthesis enzymes), all of which were from the Rhodobacteraceae. Further analysis of 70 species from this family revealed the majority encoded the B₁₂ pathway, but only half were able to make niacin, and fewer than 13% biotin. These characteristics may have either contributed to or resulted from the tendency of members of this lineage to adopt lifestyles in close association with algae. This study provides a nuanced view of bacterial-phytoplankton interactions, emphasising the complexity of the sources, sinks and dynamic cycling between marine microbes of these important organic micronutrients.

Human Intrinsic Factor Expression for Bioavailable Vitamin B12 Enrichment in Microalgae

Dietary supplements and functional foods are becoming increasingly popular complements to regular diets. A recurring ingredient is the essential cofactor vitamin B₁₂ (B₁₂). Microalgae are making their way into the dietary supplement and functional food market but do not produce B₁₂, and their B₁₂ content is very variable. In this study, the suitability of using the human B₁₂-binding protein intrinsic factor (IF) to enrich bioavailable B₁₂ using microalgae was tested. The IF protein was successfully expressed from the nuclear genome of the model microalga Chlamydomonasreinhardtii and the addition of an N-terminal ARS2 signal peptide resulted in efficient IF secretion to the medium. Co-abundance of B₁₂ and the secreted IF suggests the algal produced IF protein is functional and B₁₂-binding. Utilizing IF expression could be an efficient tool to generate B₁₂-enriched microalgae in a controlled manner that is suitable for vegetarians and, potentially, more bioavailable for humans.