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Diatoms represent one of the most successful groups of marine phytoplankton and are major contributors to ocean biogeochemical cycling. They have colonized marine, freshwater and ice environments and inhabit all regions of the World's oceans, from poles to tropics. Their success is underpinned by a remarkable ability to regulate their growth and metabolism during nutrient limitation and to respond rapidly when nutrients are available. This requires precise regulation of membrane transport and nutrient acquisition mechanisms, integration of nutrient sensing mechanisms and coordination of different transport pathways. This review outlines transport mechanisms involved in acquisition of key nutrients (N, C, P, Si, Fe) by marine diatoms, illustrating their complexity, sophistication and multiple levels of control.

Diatoms are a group of microalgae that are important primary producers in a range of open ocean, freshwater, and intertidal environments. The latter can experience substantial long- and short-term variability in temperature, from seasonal variations to rapid temperature shifts caused by tidal immersion and emersion. As temperature is a major determinant in the distribution of diatom species, their temperature sensory and response mechanisms likely have important roles in their ecological success. We examined the mechanisms diatoms use to sense rapid changes in temperature, such as those experienced in the intertidal zone. We found that the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana exhibit a transient cytosolic Ca2+ ([Ca2+]cyt) elevation in response to rapid cooling, similar to those observed in plant and animal cells. However, [Ca2+]cyt elevations were not observed in response to rapid warming. The kinetics and magnitude of cold-induced [Ca2+]cyt elevations corresponded with the rate of temperature decrease. We did not find a role for the [Ca2+]cyt elevations in enhancing cold tolerance but showed that cold shock induces a Ca2+-dependent K+ efflux and reduces mortality of P. tricornutum during a simultaneous hypo-osmotic shock. As intertidal diatom species may routinely encounter simultaneous cold and hypo-osmotic shocks during tidal cycles, we propose that cold-induced Ca2+ signaling interacts with osmotic signaling pathways to aid in the regulation of cell volume. Our findings provide insight into the nature of temperature perception in diatoms and highlight that cross-talk between signaling pathways may play an important role in their cellular responses to multiple simultaneous stressors.

The evolution of Na+-selective four-domain voltage-gated channels (4D-Navs) in animals allowed rapid Na+-dependent electrical excitability, and enabled the development of sophisticated systems for rapid and long-range signalling. Whilst bacteria encode single-domain Na+-selective voltage-gated channels (BacNav), they typically exhibit much slower kinetics than 4D-Navs, and are not thought to have crossed the prokaryote-eukaryote boundary. As such, the capacity for rapid Na+-selective signalling is considered to be confined to certain animal taxa, and absent from photosynthetic eukaryotes. Certainly, in land plants, such as the Venus Flytrap where fast electrical excitability has been described, this is most likely based on fast anion channels. Here, we report a unique class of eukaryotic Na+-selective single-domain channels (EukCatBs) that are present primarily in haptophyte algae, including the ecologically important calcifying coccolithophores. The EukCatB channels exhibit very rapid voltage-dependent activation and inactivation kinetics, and sensitivity to the highly selective 4D-Nav blocker tetrodotoxin. The results demonstrate that the capacity for rapid Na+-based signalling in eukaryotes is not restricted to animals or to the presence of 4D-Navs. The EukCatB channels therefore represent an independent evolution of fast Na+-based electrical signalling in eukaryotes that likely contribute to sophisticated cellular control mechanisms operating on very short time scales in unicellular algae.One Sentence SummaryThe capacity for rapid Na+-based signalling has evolved in ecologically important coccolithophore species via a novel class of voltage-gated Na+ channels, EukCatBs.

Significance Coccolithophore calcification is a major ocean biogeochemical process. While this process is likely to be sensitive to acidification-driven changes in ocean carbonate chemistry, incomplete understanding of the underlying mechanisms and constraints is a major bottleneck in predicting ocean acidification effects on calcification. We report severe disruption of pH homeostasis linked to a loss of H + channel function in the coccolithophore Coccolithus braarudii acclimated to seawater pH values that are likely to be encountered currently in localized regions and more widely in future oceans. This disruption leads to specific defects in coccolith morphology. These findings provide mechanistic insight into how calcification in different coccolithophores is affected by changes in seawater carbonate chemistry.

Coccolithophores are major producers of ocean biogenic calcite, but this process is predicted to be negatively affected by future ocean acidification scenarios. Since coccolithophores calcify intracellularly, the mechanisms through which changes in seawater carbonate chemistry affect calcification remain unclear. Here we show that voltage-gated H+ channels in the plasma membrane of Coccolithus braarudii serve to regulate pH and maintain calcification under normal conditions but have greatly reduced activity in cells acclimated to low pH. This disrupts intracellular pH homeostasis and impairs the ability of C. braarudii to remove H+ generated by the calcification process, leading to specific coccolith malformations. These coccolith malformations can be reproduced by pharmacological inhibition of H+ channels. Heavily calcified coccolithophore species such as C. braarudii, which make the major contribution to carbonate export to the deep ocean, have a large intracellular H+ load and are likely to be most vulnerable to future decreases in ocean pH.

Diatoms are a group of microalgae that are important primary producers in a range of open ocean, freshwater and intertidal environments. The latter can experience significant long- and short-term variability in temperature, from seasonal variations to rapid temperature shifts caused by tidal immersion and emersion. As temperature is a major determinant in the distribution of diatom species, their temperature sensory and response mechanisms likely have important roles in their ecological success. We have examined the mechanisms diatoms use to sense rapid changes in temperature, such as those experienced in the intertidal zone. We find that the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana exhibit a transient cytosolic Ca2+ ([Ca2+]cyt) elevation in response to rapid cooling, similar to those observed in plant and animal cells. However, [Ca2+]cyt elevations were not observed in response to rapid warming. The kinetics and magnitude of cold-induced [Ca2+]cyt elevations correlate with the rate of temperature decrease. We do not find a role for the [Ca2+]cyt elevations in enhancing cold tolerance, but show that cold shock induces a Ca2+-dependent K+ efflux and reduces mortality of P. tricornutum during a simultaneous hypo-osmotic shock. As inter-tidal diatom species may routinely encounter simultaneous cold and hypo-osmotic shocks during tidal cycles, we propose that cold-induced Ca2+ signalling interacts with osmotic signalling pathways to aid in the regulation of cell volume. Our findings provide insight into the nature of temperature perception in diatoms and highlight that cross-talk between signalling pathways may play an important role in their cellular responses to multiple simultaneous stressors.

Coccolithophores produce the bulk of ocean biogenic calcium carbonate but this process is predicted to be negatively affected by future ocean acidification scenarios. Since coccolithophores calcify intracellularly, the mechanisms through which changes in seawater carbonate chemistry affect calcification remain unclear. Here we show that voltage-gated H+ channels in the plasma membrane of Coccolithus braarudii serve to regulate pH and maintain calcification under normal conditions, but have greatly reduced activity in cells acclimated to low pH. This disrupts intracellular pH homeostasis and impairs the ability of C. braarudii to remove H+ generated by the calcification process, leading to specific coccolith malformations. These coccolith malformations can be reproduced by pharmacological inhibition of H+ channels. Heavily-calcified coccolithophore species such as C. braarudii, which make the major contribution to carbonate export to the deep ocean, have a large intracellular H+ load and are likely to be most vulnerable to future decreases in ocean pH.

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.

Calcium-dependent signalling mechanisms play a major role in the perception of environmental stimuli by algae and are central to all forms of algal motility. Genomic studies have revealed a surprising diversity of mechanisms through which Ca2+ signals may be generated and perceived by algal cells, with significant differences in the classes of Ca2+ channels, Ca2+ efflux proteins and Ca2+ sensors between lineages. In the green alga Chlamydomonas, significant progress has been made in identifying the specific role of signalling proteins in flagella function and in regulating photosynthesis. Characterisation of Ca2+-dependent signalling processes in other algal lineages is providing insight into how these mechanistic differences may have evolved and will influence the ability of different algae to sense and respond to their environment.

Diatoms are globally important phytoplankton that dominate coastal and polar-ice assemblages. These environments exhibit substantial changes in salinity over dynamic spatiotemporal regimes. Rapid sensory systems are vital to mitigate the harmful consequences of osmotic stress. Population-based analyses have suggested that Ca 2+ signalling is involved in diatom osmotic sensing. However, mechanistic insight of the role of osmotic Ca 2+ signalling is limited. Here, we show that Phaeodactylum Ca 2+ elevations are essential for surviving hypo-osmotic shock. Moreover, employing novel single-cell imaging techniques we have characterised real-time Ca 2+ signalling responses in single diatom cells to environmental osmotic perturbations. We observe that intracellular spatiotemporal patterns of osmotic-induced Ca 2+ elevations encode vital information regarding the nature of the osmotic stimulus. Localised Ca 2+ signals evoked by mild or gradual hypo-osmotic shocks are propagated globally from the apical cell tips, enabling fine-tuned cell volume regulation across the whole cell. Finally, we demonstrate that diatoms adopt Ca 2+-independent and dependent mechanisms for osmoregulation. We find that efflux of organic osmolytes occurs in a Ca 2+-independent manner, but this response is insufficient to mitigate cell damage during hypo-osmotic shock. By comparison, Ca 2+-dependent signalling is necessary to prevent cell bursting via precise coordination of K + transport, and therefore is likely to underpin survival in dynamic osmotic environments.

Diatoms are a diverse and globally important phytoplankton group, responsible for an estimated 20% of carbon fixation on Earth. They frequently form spatially extensive phytoplankton blooms, responding rapidly to increased availability of nutrients, including phosphorus (P) and nitrogen (N). Although it is well established that diatoms are common first responders to nutrient influxes in aquatic ecosystems, little is known of the sensory mechanisms that they employ for nutrient perception. Here, we show that P-limited diatoms use a Ca

The evolution of Na

Diatoms are a diverse and globally important phytoplankton group, responsible for an estimated 20% of carbon fixation on Earth. They frequently form spatially extensive phytoplankton blooms, responding rapidly to increased availability of nutrients including phosphorus and nitrogen. Although it is well established that diatoms are common first-responders to nutrient influxes in aquatic ecosystems, little is known of the sensory mechanisms that they employ for nutrient perception. Here we show that diatoms use a novel and highly-sensitive Ca2+-dependent signalling pathway, not previously described in eukaryotes, to sense and respond to the critical macronutrient phosphorus. We demonstrate that phosphorus-Ca2+ signalling is essential for regulating diatom recovery from phosphorus limitation, by controlling rapid and substantial increases in nitrogen assimilation. Phosphorus-Ca2+ signalling thus mediates fundamental cross-talk between the vital nutrients P and N to maximise resource competition, and likely governs the success of diatoms as major bloom formers in regions of pulsed nutrient supply. Importantly, our study demonstrates that distinct mechanisms for nutrient sensing have evolved in photosynthetic eukaryotes.

Diatoms are globally important phytoplankton that dominate coastal and polar-ice assemblages. These environments exhibit substantial changes in salinity over dynamic spatiotemporal regimes. Rapid sensory systems are vital to mitigate the harmful consequences of osmotic stress. Population-based analyses have suggested that Ca

Diatoms are a diverse and globally important phytoplankton group, responsible for an estimated 20% of carbon fixation on Earth. They frequently form spatially extensive phytoplankton blooms, responding rapidly to increased availability of nutrients, including phosphorus (P) and nitrogen (N). Although it is well established that diatoms are common first responders to nutrient influxes in aquatic ecosystems, little is known of the sensory mechanisms that they employ for nutrient perception. Here, we show that P-limited diatoms use a Ca2+-dependent signaling pathway, not previously described in eukaryotes, to sense and respond to the critical macronutrient P. We demonstrate that P-Ca2+ signaling is conserved between a representative pennate (Phaeodactylum tricornutum) and centric (Thalassiosira pseudonana) diatom. Moreover, this pathway is ecologically relevant, being sensitive to sub-micromolar concentrations of inorganic phosphate and a range of environmentally abundant P forms. Notably, we show that diatom recovery from P limitation requires rapid and substantial increases in N assimilation and demonstrate that this process is dependent on P-Ca2+ signaling. P-Ca2+ signaling thus governs the capacity of diatoms to rapidly sense and respond to P resupply, mediating fundamental cross-talk between the vital nutrients P and N and maximizing diatom resource competition in regions of pulsed nutrient supply.

The corrinoid B12 is synthesised 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-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 disrupted C1 metabolism, caused an accumulation of starch and triacylglycerides and a decrease in photosynthetic pigments, proteins and free amino acids. B12 deprivation also caused a substantial increase in reactive oxygen species (ROS), which preceded rapid cell death. Surprisingly, 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 B12 deprivation, association with B12-producers can ensure long-term survival whilst also providing the environment to evolve mechanisms to better tolerate B12 limitation.

The corrinoid B

Rapid Na + /Ca 2+ -based action potentials govern essential cellular functions in eukaryotes, from the motile responses of unicellular protists, such as Paramecium [1, 2], to complex animal neuromuscular activity [3]. A key innovation underpinning this fundamental signaling process has been the evolution of four-domain voltage-gated Na + /Ca 2+ channels (4D-Ca v s/Na v s). These channels are widely distributed across eukaryote diversity [4], albeit several eukaryotes, including land plants and fungi, have lost voltage-sensitive 4D-Ca v /Na v s [5–7]. Because these lineages appear to lack rapid Na + /Ca 2+ -based action potentials, 4D-Ca v /Na v s are generally considered necessary for fast Na + /Ca 2+ -based signaling [7]. However, the cellular mechanisms underpinning the membrane physiology of many eukaryotes remain unexamined. Eukaryotic phytoplankton critically influence our climate as major primary producers. Several taxa, including the globally abundant diatoms, exhibit membrane excitability [8–10]. We previously demonstrated that certain diatom genomes encode 4D-Ca v /Na v s [4] but also proteins of unknown function, resembling prokaryote single-domain, voltage-gated Na + channels (BacNa v s) [4]. Here, we show that single-domain channels are actually broadly distributed across major eukaryote phytoplankton lineages and represent three novel classes of single-domain channels, which we refer collectively to as EukCats. Functional characterization of diatom EukCatAs indicates that they are voltage-gated Na + - and Ca 2+ -permeable channels, with rapid kinetics resembling metazoan 4D-Ca v s/Na v s. In Phaeodactylum tricornutum, which lacks 4D-Ca v /Na v s, EukCatAs underpin voltage-activated Ca 2+ signaling important for membrane excitability, and mutants exhibit impaired motility. EukCatAs therefore provide alternative mechanisms for rapid Na + /Ca 2+ signaling in eukaryotes and may functionally replace 4D-Ca v s/Na v s in pennate diatoms. Marine phytoplankton thus possess unique signaling mechanisms that may be key to environmental sensing in the oceans. Diatoms exhibit fast animal-like action potentials, but many species lack 4D-Ca v /Na v channels that underpin membrane excitability in animals. Diatoms do encode novel 1D voltage-gated channels (EukCatAs). Helliwell, Chrachri et al. show that EukCatAs are fast Na + and Ca 2+ channels that provide alternative mechanisms for rapid signaling in eukaryotes.

The ocean is home to a huge diversity of life that perform a wide range of functions key to earth systems. Marine algae communities contribute a significant proportion of net carbon fixation of the globe as well as forming the basis of the food webs for every trophic level above their own. Despite their importance, many of the taxa that make up these microalgal communities are relatively unknown and understudied, due to their recalcitrance to lab culturing. This concept is known as microbial dark matter and applies in particular to many non-photosynthetic lineages which have been long overlooked by the research community. To investigate whether dependencies of these taxa on certain B vitamins may play a role in this unculturability, data sets produced form the Tara oceans expedition will be analysed. The Stramenopile lineage will be analysed in greater detail for their metabolic potential to synthesise the B vitamins, and/or whether they may dependent on an external source. Both Single-Amplified Genomes (SAG) and metatrascriptomic data sets will be analysed to determine the complement of genes present in different Stramenopile lineages. This will then be correlated with the global distribution data attainable from analysing Tara data sets. By this means, we hope to understand more about the metabolic contribution of this taxonomic group to the community, how this supports the community as a whole and whether dependence on B vitamins is a contributing factor to unculturability.

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 (B(1)) and cobalamin (B(12))(.) Here we demonstrate that the B(12) and B(1) 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 (B(3)), biotin (B(7)), and p-aminobenzoic acid (a precursor for folate, B(9)), 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 B(12), 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 B(12) 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.

Summary The unicellular green alga Lobomonas rostrata requires an external supply of vitamin B12 (cobalamin) for growth, which it can obtain in stable laboratory cultures from the soil bacterium Mesorhizobium loti in exchange for photosynthate. We investigated changes in protein expression in the alga that allow it to engage in this mutualism.We used quantitative isobaric tagging (iTRAQ) proteomics to determine the L. rostrata proteome grown axenically with B12 supplementation or in coculture with M. loti. Data are available via ProteomeXchange (PXD005046).Using the related Chlamydomonas reinhardtii as a reference genome, 588 algal proteins could be identified. Enzymes of amino acid biosynthesis were higher in coculture than in axenic culture, and this was reflected in increased amounts of total cellular protein and several free amino acids. A number of heat shock proteins were also elevated. Conversely, photosynthetic proteins and those of chloroplast protein synthesis were significantly lower in L. rostrata cells in coculture. These observations were confirmed by measurement of electron transfer rates in cells grown under the two conditions.The results indicate that, despite the stability of the mutualism, L. rostrata experiences stress in coculture with M. loti, and must adjust its metabolism accordingly.

Porphyra umbilicalis (laver) belongs to an ancient group of red algae (Bangiophyceae), is harvested for human food, and thrives in the harsh conditions of the upper intertidal zone. Here we present the 87.7-Mbp haploid Porphyra genome (65.8% G + C content, 13,125 gene loci) and elucidate traits that inform our understanding of the biology of red algae as one of the few multicellular eukaryotic lineages. Novel features of the Porphyra genome shared by other red algae relate to the cytoskeleton, calcium signaling, the cell cycle, and stress-Tolerance mechanisms including photoprotection. Cytoskeletal motor proteins in Porphyra are restricted to a small set of kinesins that appear to be the only universal cytoskeletal motors within the red algae. Dynein motors are absent, and most red algae, including Porphyra, lack myosin. This surprisingly minimal cytoskeleton offers a potential explanation for why red algal cells and multicellular structures are more limited in size than in most multicellular lineages. Additional discoveries further relating to the stress tolerance of bangiophytes include ancestral enzymes for sulfation of the hydrophilic galactan-rich cell wall, evidence for mannan synthesis that originated before the divergence of green and red algae, and a high capacity for nutrient uptake. Our analyses provide a comprehensive understanding of the red algae, which are both commercially important and have played a major role in the evolution of other algal groups through secondary endosymbioses.

Fossil evidence shows that red algae (Rhodophyta) are one of the most ancient multicellular lineages. Their ecological, evolutionary, and commercial importance notwithstanding, few red algal nuclear genomes have been sequenced. Our analyses of the Porphyra umbilicalis genome provide insights into how this macrophyte thrives in the stressful intertidal zone and into the basis for its nutritional value as human food. Many of the novel traits (e.g., cytoskeletal organization, calcium signaling pathways) we find encoded in the Porphyra genome are extended to other red algal genomes, and our unexpected findings offer a potential explanation for why the red algae are constrained to small stature relative to other multicellular lineages.

Contents 62 I. 62 II. 63 III. 63 IV. 66 V. 66 VI. 67 67 References 67 SUMMARY: B vitamins play essential roles in central metabolism. These organic water-soluble molecules act as, or as part of, coenzymes within the cell. Unlike land plants, many eukaryotic algae are auxotrophic for certain B vitamins. Recent progress in algal genetic resources and environmental chemistry have promoted a renewal of interest in the role of vitamins in governing phytoplankton dynamics, and illuminated amazing versatility in phytoplankton vitamin metabolism. Accumulating evidence demonstrates metabolic complexity in the production and bioavailability of different vitamin forms, coupled with specialized acquisition strategies to salvage and remodel vitamin precursors. Here, I describe recent advances and discuss how they redefine our view of the way in which vitamins are cycled in aquatic ecosystems and their importance in structuring phytoplankton communities.

Global demand for macroalgal and microalgal foods is growing, and algae are increasingly being consumed for functional benefits beyond the traditional considerations of nutrition and health. There is substantial evidence for the health benefits of algal-derived food products, but there remain considerable challenges in quantifying these benefits, as well as possible adverse effects. First, there is a limited understanding of nutritional composition across algal species, geographical regions, and seasons, all of which can substantially affect their dietary value. The second issue is quantifying which fractions of algal foods are bioavailable to humans, and which factors influence how food constituents are released, ranging from food preparation through genetic differentiation in the gut microbiome. Third is understanding how algal nutritional and functional constituents interact in human metabolism. Superimposed considerations are the effects of harvesting, storage, and food processing techniques that can dramatically influence the potential nutritive value of algal-derived foods. We highlight this rapidly advancing area of algal science with a particular focus on the key research required to assess better the health benefits of an alga or algal product. There are rich opportunities for phycologists in this emerging field, requiring exciting new experimental and collaborative approaches. Electronic supplementary material The online version of this article (doi:10.1007/s10811-016-0974-5) contains supplementary material, which is available to authorized users.

Vitamin B(12) (cobalamin) is a dietary requirement for humans because it is an essential cofactor for two enzymes, methylmalonyl-CoA mutase and methionine synthase (METH). Land plants and fungi neither synthesize or require cobalamin because they do not contain methylmalonyl-CoA mutase, and have an alternative B(12)-independent methionine synthase (METE). Within the algal kingdom, approximately half of all microalgal species need the vitamin as a growth supplement, but there is no phylogenetic relationship between these species, suggesting that the auxotrophy arose multiple times through evolution. We set out to determine the underlying cellular mechanisms for this observation by investigating elements of B(12) metabolism in the sequenced genomes of 15 different algal species, with representatives of the red, green, and brown algae, diatoms, and coccolithophores, including both macro- and microalgae, and from marine and freshwater environments. From this analysis, together with growth assays, we found a strong correlation between the absence of a functional METE gene and B(12) auxotrophy. The presence of a METE unitary pseudogene in the B(12)-dependent green algae Volvox carteri and Gonium pectorale, relatives of the B(12)-independent Chlamydomonas reinhardtii, suggest that B(12) dependence evolved recently in these lineages. In both C. reinhardtii and the diatom Phaeodactylum tricornutum, growth in the presence of cobalamin leads to repression of METE transcription, providing a mechanism for gene loss. Thus varying environmental conditions are likely to have been the reason for the multiple independent origins of B(12) auxotrophy in these organisms. Because the ultimate source of cobalamin is from prokaryotes, the selective loss of METE in different algal lineages will have had important physiological and ecological consequences for these organisms in terms of their dependence on bacteria.

Riboswitches are regulatory elements in messenger RNA to which specific ligands can bind directly in the absence of proteins. Ligand binding alters the mRNA secondary structure, thereby affecting expression of the encoded protein. Riboswitches are widespread in prokaryotes, with over 20 different effector ligands known, including amino acids, cofactors, and Mg(2+) ions, and gene expression is generally regulated by affecting translation or termination of transcription. In plants, fungi, and microalgae, riboswitches have been found, but only those that bind thiamine pyrophosphate. These eukaryotic riboswitches operate by causing alternative splicing of the transcript. Here, we review the current status of riboswitch research with specific emphasis on microalgae. We discuss new riboswitch discoveries and insights into the underlying mechanism of action, and how next generation sequencing technology provides the motivation and opportunity to improve our understanding of these rare but important regulatory elements. We also highlight the potential of microalgal riboswitches as a tool for synthetic biology and industrial biotechnology.

Photosynthetic microalgae play a vital role in primary productivity and biogeochemical cycling in both marine and freshwater systems across the globe. However, the growth of these cosmopolitan organisms depends on the bioavailability of nutrients such as vitamins. Approximately one-half of all microalgal species requires vitamin B12 as a growth supplement. The major determinant of algal B12 requirements is defined by the isoform of methionine synthase possessed by an alga, such that the presence of the B12-independent methionine synthase (METE) enables growth without this vitamin. Moreover, the widespread but phylogenetically unrelated distribution of B12 auxotrophy across the algal lineages suggests that the METE gene has been lost multiple times in evolution. Given that METE expression is repressed by the presence of B12, prolonged repression by a reliable source of the vitamin could lead to the accumulation of mutations and eventually gene loss. Here, we probe METE gene regulation by B12 and methionine/folate cycle metabolites in both marine and freshwater microalgal species. In addition, we identify a B12-responsive element of Chlamydomonas reinhardtii METE using a reporter gene approach. We show that complete repression of the reporter occurs via a region spanning −574 to −90 bp upstream of the METE start codon. A proteomics study reveals that two other genes (S-Adenosylhomocysteine hydrolase and Serine hydroxymethyltransferase2) involved in the methionine-folate cycle are also repressed by B12 in C. reinhardtii. The strong repressible nature and high sensitivity of the B12-responsive element has promising biotechnological applications as a cost-effective regulatory gene expression tool.

The advent of modern genomics has provided an unparalleled opportunity to consider the gene complement of an organism, and scrutinize metabolic pathways that are no longer active. This approach has led to an increasing number of reports of vitamin-associated pathway deterioration, with many indicating that independent gene-loss events of one or a few key genes have led to vitamin auxotrophy. Nonfunctional unitary pseudogenes belonging to these pathways are found in several species, demonstrating that these are recent evolutionary processes. Here, we examine the commonalities in the cellular roles and metabolism of vitamins that might have led to these losses. The complex pattern of vitamin auxotrophy across the eukaryotic tree of life is intimately connected with the interdependence between organisms. The importance of this process in terms of shaping communities on the one hand, and facilitating symbioses between organisms on the other, is only just beginning to be recognized.