Predicting substrate exchange in marine diatom-heterocystous cyanobacteria symbioses

Cyclic electron flow and Photosystem II-less photosynthesis

Oxygenic photosynthesis is characterised by the cooperation of two photo-driven complexes, Photosystem II (PSII) and Photosystem I (PSI), sequentially linked through a series of redox-coupled intermediates. Divergent evolution has resulted in photosystems exhibiting complementary redox potentials, spanning the range necessary to oxidise water and reduce CO2 within a single system. Catalysing nature's most oxidising reaction to extract electrons from water is a highly specialised task that limits PSII's metabolic function. In contrast, potential electron donors in PSI span a range of redox potentials, enabling it to accept electrons from various metabolic processes. This metabolic flexibility of PSI underpins the capacity of photosynthetic organisms to balance energy supply with metabolic demands, which is key for adaptation to environmental changes. Here, we review the phenomenon of 'PSII-less photosynthesis' where PSI functions independently of PSII by operating cyclic electron flow using electrons derived from non-photochemical reactions. PSII-less photosynthesis enables supercharged ATP production and is employed, for example, by cyanobacteria's heterocysts to host nitrogen fixation and by bundle sheath cells of C4 plants to boost CO2 assimilation. We discuss the energetic benefits of this arrangement and the prospects of utilising it to improve the productivity and stress resilience of photosynthetic organisms.

Rhizobia-diatom symbiosis fixes missing nitrogen in the ocean

Nitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3-5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, 'Candidatus Tectiglobus diatomicola', which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia-legume symbioses on land6. Our results show that the rhizobia-diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Metatranscriptomics reveals diversity of symbiotic interaction and mechanisms of carbon exchange in the marine cyanolichen Lichina pygmaea

Lichens are exemplar symbioses based upon carbon exchange between photobionts and their mycobiont hosts. Historically considered a two-way relationship, some lichen symbioses have been shown to contain multiple photobiont partners; however, the way in which these photobiont communities react to environmental change is poorly understood. Lichina pygmaea is a marine cyanolichen that inhabits rocky seashores where it is submerged in seawater during every tidal cycle. Recent work has indicated that L. pygmaea has a complex photobiont community including the cyanobionts Rivularia and Pleurocapsa. We performed rRNA-based metabarcoding and mRNA metatranscriptomics of the L. pygmaea holobiont at high and low tide to investigate community response to immersion in seawater. Carbon exchange in L. pygmaea is a dynamic process, influenced by both tidal cycle and the biology of the individual symbiotic components. The mycobiont and two cyanobiont partners exhibit distinct transcriptional responses to seawater hydration. Sugar-based compatible solutes produced by Rivularia and Pleurocapsa in response to seawater are a potential source of carbon to the mycobiont. We propose that extracellular processing of photobiont-derived polysaccharides is a fundamental step in carbon acquisition by L. pygmaea and is analogous to uptake of plant-derived carbon in ectomycorrhizal symbioses.

Microalgal and Nitrogen-Fixing Bacterial Consortia: From Interaction to Biotechnological Potential

Microalgae are used in various biotechnological processes, such as biofuel production due to their high biomass yields, agriculture as biofertilizers, production of high-value-added products, decontamination of wastewater, or as biological models for carbon sequestration. The number of these biotechnological applications is increasing, and as such, any advances that contribute to reducing costs and increasing economic profitability can have a significant impact. Nitrogen fixing organisms, often called diazotroph, also have great biotechnological potential, mainly in agriculture as an alternative to chemical fertilizers. Microbial consortia typically perform more complex tasks than monocultures and can execute functions that are challenging or even impossible for individual strains or species. Interestingly, microalgae and diazotrophic organisms are capable to embrace different types of symbiotic associations. Certain corals and lichens exhibit this symbiotic relationship in nature, which enhances their fitness. However, this relationship can also be artificially created in laboratory conditions with the objective of enhancing some of the biotechnological processes that each organism carries out independently. As a result, the utilization of microalgae and diazotrophic organisms in consortia is garnering significant interest as a potential alternative for reducing production costs and increasing yields of microalgae biomass, as well as for producing derived products and serving biotechnological purposes. This review makes an effort to examine the associations of microalgae and diazotrophic organisms, with the aim of highlighting the potential of these associations in improving various biotechnological processes.

Heterologous expression of genes from a cyanobacterial endosymbiont highlights substrate exchanges with its diatom host

A few genera of diatoms are widespread and thrive in low-nutrient waters of the open ocean due to their close association with N₂-fixing, filamentous heterocyst-forming cyanobacteria. In one of these symbioses, the symbiont, Richelia euintracellularis, has penetrated the cell envelope of the host, Hemiaulus hauckii, and lives inside the host cytoplasm. How the partners interact, including how the symbiont sustains high rates of N₂ fixation, is unstudied. Since R. euintracellularis has evaded isolation, heterologous expression of genes in model laboratory organisms was performed to identify the function of proteins from the endosymbiont. Gene complementation of a cyanobacterial invertase mutant and expression of the protein in Escherichia coli showed that R. euintracellularis HH01 possesses a neutral invertase that splits sucrose producing glucose and fructose. Several solute-binding proteins (SBPs) of ABC transporters encoded in the genome of R. euintracellularis HH01 were expressed in E. coli, and their substrates were characterized. The selected SBPs directly linked the host as the source of several substrates, e.g. sugars (sucrose and galactose), amino acids (glutamate and phenylalanine), and a polyamine (spermidine), to support the cyanobacterial symbiont. Finally, transcripts of genes encoding the invertase and SBPs were consistently detected in wild populations of H. hauckii collected from multiple stations and depths in the western tropical North Atlantic. Our results support the idea that the diatom host provides the endosymbiotic cyanobacterium with organic carbon to fuel N₂ fixation. This knowledge is key to understanding the physiology of the globally significant H. hauckii-R. euintracellularis symbiosis.

Genetic and physiological insights into the diazotrophic activity of a non-cyanobacterial marine diazotroph

Nitrogen (N₂ ) fixation, or diazotrophy, supports a large part of primary production in oceans. Culture-independent approaches highlighted the presence in abundance of marine non-cyanobacterial diazotrophs (NCD), but their ecophysiology remains elusive, mostly because of the low number of isolated NCD and because of the lack of available genetic tools for these isolates. Here, a dual genetic and functional approach allowed unveiling the ecophysiology of a marine NCD affiliated to the species Vibrio diazotrophicus. Physiological characterization of the first marine NCD mutant obtained so far was performed using a soft-gellan assay, demonstrating that a ΔnifH mutant is not able to grow in nitrogen-free media. Furthermore, we demonstrated that V. diazotrophicus produces a thick biofilm under diazotrophic conditions, suggesting biofilm production as an adaptive response of this NCD to cope with the inhibition of nitrogen fixation by molecular oxygen. Finally, the genomic signature of V. diazotrophicus is essentially absent from metagenomic data of Tara Ocean expeditions, despite having been isolated from various marine environments. We think that the genetically tractable V. diazotrophicus strain used in this study may serve as an ideal model to study the ecophysiology of these overlooked procaryotic group.

Low-Ammonium Environment Increases the Nutrient Exchange between Diatom-Diazotroph Association Cells and Facilitates Photosynthesis and N2 Fixation-a Mechanistic Modeling Analysis

Diatom-diazotroph associations (DDAs) are one of the most important symbiotic dinitrogen (N2) fixing groups in the oligotrophic ocean. Despite their capability to fix N2, ammonium (NH4+) remains a key nitrogen (N) source for DDAs, and the effect of NH4+ on their metabolism remains elusive. Here, we developed a coarse-grained, cellular model of the DDA with NH4+ uptake and quantified how the level of extracellular NH4+ influences metabolism and nutrient exchange within the symbiosis. The model shows that, under a fixed growth rate, an increased NH4+ concentration may lower the required level of N2 fixation and photosynthesis, and decrease carbon (C) and N exchange. A low-NH4+ environment leads to more C and N in nutrient exchange and more fixed N2 to support a higher growth rate. With higher growth rates, nutrient exchange and metabolism increased. Our study shows a strong effect of NH4+ on metabolic processes within DDAs, and thus highlights the importance of in situ measurement of NH4+ concentrations.

Non-Proteinogenic Amino Acid β-N-Methylamino-L-Alanine (BMAA): Bioactivity and Ecological Significance

Research interest in a non-protein amino acid β-N-methylamino-L-alanine (BMAA) arose due to the discovery of a connection between exposure to BMAA and the occurrence of neurodegenerative diseases. Previous reviews on this topic either considered BMAA as a risk factor for neurodegenerative diseases or focused on the problems of detecting BMAA in various environmental samples. Our review is devoted to a wide range of fundamental biological problems related to BMAA, including the molecular mechanisms of biological activity of BMAA and the complex relationships between producers of BMAA and the environment in various natural ecosystems. At the beginning, we briefly recall the most important facts about the producers of BMAA (cyanobacteria, microalgae, and bacteria), the pathways of BMAA biosynthesis, and reliable methods of identification of BMAA. The main distinctive feature of our review is a detailed examination of the molecular mechanisms underlying the toxicity of BMAA to living cells. A brand new aspect, not previously discussed in any reviews, is the effect of BMAA on cyanobacterial cells. These recent studies, conducted using transcriptomics and proteomics, revealed potent regulatory effects of BMAA on the basic metabolism and cell development of these ancient photoautotrophic prokaryotes. Exogenous BMAA strongly influences cell differentiation and primary metabolic processes in cyanobacteria, such as nitrogen fixation, photosynthesis, carbon fixation, and various biosynthetic processes involving 2-oxoglutarate and glutamate. Cyanobacteria were found to be more sensitive to exogenous BMAA under nitrogen-limited growth conditions. We suggest a hypothesis that this toxic diaminoacid can be used by phytoplankton organisms as a possible allelopathic tool for controlling the population of cyanobacterial cells during a period of intense competition for nitrogen and other resources in various ecosystems.

Adaptation to an Intracellular Lifestyle by a Nitrogen-Fixing, Heterocyst-Forming Cyanobacterial Endosymbiont of a Diatom

The symbiosis between the diatom Hemiaulus hauckii and the heterocyst-forming cyanobacterium Richelia intracellularis makes an important contribution to new production in the world's oceans, but its study is limited by short-term survival in the laboratory. In this symbiosis, R. intracellularis fixes atmospheric dinitrogen in the heterocyst and provides H. hauckii with fixed nitrogen. Here, we conducted an electron microscopy study of H. hauckii and found that the filaments of the R. intracellularis symbiont, typically composed of one terminal heterocyst and three or four vegetative cells, are located in the diatom's cytoplasm not enclosed by a host membrane. A second prokaryotic cell was also detected in the cytoplasm of H. hauckii, but observations were infrequent. The heterocysts of R. intracellularis differ from those of free-living heterocyst-forming cyanobacteria in that the specific components of the heterocyst envelope seem to be located in the periplasmic space instead of outside the outer membrane. This specialized arrangement of the heterocyst envelope and a possible association of the cyanobacterium with oxygen-respiring mitochondria may be important for protection of the nitrogen-fixing enzyme, nitrogenase, from photosynthetically produced oxygen. The cell envelope of the vegetative cells of R. intracellularis contained numerous membrane vesicles that resemble the outer-inner membrane vesicles of Gram-negative bacteria. These vesicles can export cytoplasmic material from the bacterial cell and, therefore, may represent a vehicle for transfer of fixed nitrogen from R. intracellularis to the diatom's cytoplasm. The specific morphological features of R. intracellularis described here, together with its known streamlined genome, likely represent specific adaptations of this cyanobacterium to an intracellular lifestyle.

The rate and fate of N2 and C fixation by marine diatom-diazotroph symbioses

N2 fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N2 fixing cyanobacteria (Richelia intracellularis, hereafter Richelia) form symbiosis with a few genera of diatoms. High rates of N2 fixation and carbon (C) fixation have been measured in the presence of diatom-Richelia symbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N2 fixation. Here, both the N2 and C fixation in wild diatom-Richelia populations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N2 fixation, suggesting that despite the symbionts ability to fix their own C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78-91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N2 and C fixation, and only N2 fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d-1). Several eco-physiological parameters necessary for incorporating symbiotic N2 fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided.

The symbiotic 'all-rounders': Partnerships between marine animals and chemosynthetic nitrogen-fixing bacteria

Nitrogen fixation is a widespread metabolic trait in certain types of microorganisms called diazotrophs. Bioavailable nitrogen is limited in various habitats on land and in the sea, and accordingly, a range of plant, animal, and single-celled eukaryotes have evolved symbioses with diverse diazotrophic bacteria, with enormous economic and ecological benefits. Until recently, all known nitrogen-fixing symbionts were heterotrophs such as nodulating rhizobia, or photoautotrophs such as cyanobacteria. In 2016, the first chemoautotrophic nitrogen-fixing symbionts were discovered in a common family of marine clams, the Lucinidae. Chemosynthetic nitrogen-fixing symbionts use the chemical energy stored in reduced sulfur compounds to power carbon and nitrogen fixation, making them metabolic 'all-rounders' with multiple functions in the symbiosis. This distinguishes them from heterotrophic symbionts that require a source of carbon from their host, and their chemosynthetic metabolism distinguishes them from photoautotrophic symbionts that produce oxygen, a potent inhibitor of nitrogenase. In this review, we consider evolutionary aspects of this discovery, by comparing strategies that have evolved for hosting intracellular nitrogen-fixing symbionts in plants and animals. The symbiosis between lucinid clams and chemosynthetic nitrogen-fixing bacteria also has important ecological impacts, as they form a nested symbiosis with endangered marine seagrasses. Notably, nitrogen fixation by lucinid symbionts may help support seagrass health by providing a source of nitrogen in seagrass habitats. These discoveries were enabled by new techniques for understanding the activity of microbial populations in natural environments. However, an animal (or plant) host represents a diverse landscape of microbial niches due to its structural, chemical, immune and behavioural properties. In future, methods that resolve microbial activity at the single cell level will provide radical new insights into the regulation of nitrogen fixation in chemosynthetic symbionts, shedding new light on the evolution of nitrogen-fixing symbioses in contrasting hosts and environments.

Quantitative models of nitrogen-fixing organisms

Nitrogen-fixing organisms are of importance to the environment, providing bioavailable nitrogen to the biosphere. Quantitative models have been used to complement the laboratory experiments and in situ measurements, where such evaluations are difficult or costly. Here, we review the current state of the quantitative modeling of nitrogen-fixing organisms and ways to enhance the bridge between theoretical and empirical studies.