Algal and Cyanobacteria Symbioses

The following sections are included:

• Contents
• Foreword
• Acknowledgments
• About the Editors
• List of Contributors

Symbioses with one photosynthetically competent partner can have "added value" in terms of the metabolic capacity of the symbiosis relative to that of the individual partners, though there may also be other reciprocal rewards. All photosynthetic eukaryotes depend on symbiosis. Symbioses, in the past, gained chloroplasts by genetic integration of inhabitant cyanobacteria in primary endosymbiosis. Some subsequent lineages genetically integrated eukaryotic photosynthetic inhabitants in secondary and tertiary endosymbioses. There are three main categories of symbioses involving a photosynthetic partner without genetic integration of the inhabitant. One involves inhabitant photosynthetic cyanobacteria or eukaryotes with a great variety of non-photosynthetic eukaryotic exhabitants and a significant range of inhabitants, though with three predominant higher taxa (cyanobacteria, Dinophyta, Trebouxiophyceae). The second category involves diazotrophs, which in free-living organisms is restricted to Archaea and Bacteria. Diazotrophic symbioses with photosynthetic eukaryotic exhabitants involve cyanobacteria, proteobacteria, and Frankia as the inhabitant Bacteria. The third main category involves mycorrhizal fungal inhabitants that aid the supply of nutrients other than light, CO₂, and N₂ to the photosynthetic exhabitants. Many inhabitants in symbioses lack obvious sexuality. This lack leaves the organisms prone to Mullers’ Ratchet, i.e., the build-up of deleterious mutations in the absence of recombination. Free-living clonal organisms have a variety of means to deal with deleterious mutations. These are less available to many symbiotic inhabitants, especially those intracellular inhabitants that are strictly vertically transmitted and, of course, organelles. Despite this, there are evidences of recombination and genome repair in inhabitants and in organelles.

The evolution of oxygenic photosynthesis in the ancestors of presentday cyanobacteria was a pivotal event in the history of life on our planet. It contributed to a transformation of the earth’s atmosphere and paved the way for the evolution of more complex life forms. More than a billion years ago, a specific lineage of heterotrophic eukaryotes took up cyanobacteria as endosymbionts; over time these endosymbionts became permanent fixtures inside the cell and evolved into the plastids (chloroplasts) that now reside in algal and plant cells. Plastids have also spread among unrelated eukaryotic lineages by so-called "secondary" endosymbiotic events, greatly expanding algal biodiversity. In this chapter, we review recent advances in our understanding of the origin and evolution of plastids in eukaryotes. Comparative genomics has made it possible to gain insight into the biochemical and cell biological events that accompanied the conversion of the primordial cyanobacterial endosymbiont into a photosynthetic organelle, as well as the movement of plastids across the eukaryotic tree of life.

Trentepohliaceae are a distinct lineage of green algae frequently found in association with lichen-forming fungi. We review the current knowledge about the phenotypic diversity of this group of algae, which is complemented by currently available molecular data. These data do not confirm the traditional classification of the genera based on morphological inferences. We also present more specific data on the association patterns of epiphyllous representatives in tropical rain forests. Leaf-colonizing Porina species associate specifically with Phycopeltis algae of characteristic growth types. The analysis of partial rbcL sequences of the algal partners suggests that morphologically similar growth types are genetically highly diverse and may represent different species. The fungal species may switch among morphologically similar but genetically distinct algal lineages. This strategy could promote rapid colonization of the ephemeral leaf habitats. Bark-inhabiting lichens of temperate habitats seem to represent a few known morphological species of Trentepohliales, whereas tropical bark lineages seem to be more diverse. Even though Trentepohliales are pending, a modern revision before precise figures can be given, their diversity appears to be much higher than thought before.

The algal genus Trebouxia comprises non-motile coccoid green algae with characteristic pyrenoids and a lobate chloroplasts. Strains of Trebouxia are present in approximately one half of all lichens. Therefore, it has become one of the best studied phycobionts with respect to its diversity in lichen symbioses. Studies have focused on phylogenetic relationships of the lichenized strains and on the specificity of occurrence with mycobiont (= lichen) species, and on its ecological responses. Here we present a review of the body of literature gathered mainly during the past 10 years on the genus Trebouxia, reporting the most recent results of molecular and morphological analyses. We also revise the most up to date methodological approaches to study the diversity and the morphological variability of Trebouxia photobionts in lichen thalli and in axenic culture.

Cyanolichens are obligate symbioses between fungi and cyanobacteria. They occur in many types of environments ranging from Arctic tundra and semi-deserts to tropical rainforests. Possibly even a majority of their global species diversity has not yet been described. Symbiotic cyanobacteria provide both photosynthate and fixed nitrogen to the fungal host and the relative importance of these functions differs in different cyanolichens. The cyanobiont can either be the sole photosynthetic partner or a secondary symbiont in addition to a primary green algal photobiont. In addition, the cyanolichen thallus may incorporate a plethora of other microorganisms. The fungal symbionts in cyanolichens are almost exclusively ascomycetes. Nostoc is by far the most commonly encountered cyanobacterial genus. While the cyanobacterial symbionts are presently not readily identifiable to species, molecular methods work well on the generic level and offer practical means for identifying symbiotic cyanobacterial genotypes. The present diversity of lichen cyanobionts may partly reflect the evolutionary effects of their lichen-symbiotic way of life and dispersal.

The trebouxiophyte algal genera Coccomyxa Schmidle and Elliptochloris Tschermak-Woess are worldwide distributed and occur free-living and in symbioses colonizing terrestrial and aquatic habitats. The genus Coccomyxa displays a remarkable versatility in its ecological range and choice of host species: it associates with asco- and basidiomycetes in obligate or optional symbiosis and with plants and aquatic invertebrates in mutualistic or parasitic associations. Its closest relative Elliptochloris seems to be less frequent, but similarly variable in its symbiotic associations. Besides the comprehensive characterization of these versatile genera, this chapter reviews their occurrence in diverse symbiotic associations and analyses the ecological and biogeographical pattern of phylogenetically unrelated lichen species sharing Coccomyxa as photobiont. Further, it summarizes data on the physiological plasticity of Coccomyxa, which became a model organism because its whole genome sequence has been published recently.

The symbiosis between the eukaryotic green alga Oophila amblystomatis and embryos of the spotted salamander, Ambystoma maculatum, is unique among vertebrates. This association occurs in many vernal pools and emergent wetlands throughout Eastern North America. Exclusion experiments have shown the association to be a mutualism, with material benefits of oxygen production and possibly nutrient transfer to the embryos, ammonia production, or a refugium within the egg capsules for the algae. Unlike other vertebrate–algal associations, this symbiosis also includes the algal symbionts entering host tissues and cells. No other similar vertebrate endosymbiosis has been reported. To date, little is known about how the association with Oophila becomes established, or the mechanisms of cellular entry. Fortunately, several aspects of both host and symbiont biology, along with substantial research on closely related species to both partners, makes this association uniquely poised to address these lingering questions. Our current working hypothesis is that the Oophila endosymbiosis is initiated by induced phagocytosis rather than an “aggressive” invasion driven by motility of the alga. We review recent research on this association and present novel data on comparative algal–amphibian associations, cell culture, and flow cytometry. These approaches will extend the experimental tractability of this association to include modern comparative transcriptomics, and potentially transgenics, to further characterize the only naturally occurring vertebrate–algal endosymbiosis currently known.

The occurrence of symbiotic relationships between single celled algae and animals has resulted in a range of host adaptations to maximize the efficiency of the relationship. However, the intimate mutual interdependency of many symbiotic relationships means that any short-term fitness benefits to the hosts may come at a long-term cost, one of which is that host distributions are limited to environments suitable for symbiont photosynthesis. Despite the apparent benefits to hosts of symbiosis with algae, these limitations, for example, among corals, have resulted in corresponding limitations in host capacity to mitigate the effects of anthropogenic climate change. In contrast, due to the interactions between symbiont and host and the attendant unique alterations to host physiology and morphology, some symbiotic relationships may not be significantly impacted, or may even benefit from by climate change. In this chapter, we explore how symbiotic relationships with algae have altered ecology, morphology, and physiology of aquatic host species, then discuss how these relationships may offer resilience to climate change.

According to the commonly accepted view, transformations of endosymbionts into cellular organelles are extremely rare evolutionary events. The thecate amoeba Paulinella chromatophora provides a new challenge to this view. This amoeba contains two cyanobacteriumderived bodies, called chromatophores, which preserve several prokaryotic features such as the peptidoglycan wall. Phylogenetic analyses of host and chromatophore genes demonstrate clearly that Paulinella chromatophores evolved independently of primary plastids (i.e., those of glaucophytes, red algae, and green plants), and from a distinct cyanobacterial group. The ancestor of chromatophores was probably an α-cyanobacterium resembling species of the genus Synechococcus. Originally, it was hotly debated whether Paulinella chromatophores are endosymbionts or, rather, organelles; however, currently available data verify that they have already reached the status of cellular organelles. The chromatophore genome is about three times smaller than that of its closest free-living Synechococcus relative, and over time at least 32 cyanobacterial genes have been transferred to the host nuclear genome via endosymbiotic gene transfer (EGT). Some of these genes encode signal peptide-like domains, which indicate that their protein products are targeted to the chromatophores through the endomembrane system. Endomembrane system-mediated transport has been confirmed experimentally, but the existence of other import routes, such as direct import from the cytosol, cannot be excluded. At least some nuclear-encoded proteins of Paulinellachromatophores must cross the peptidoglycan wall during their import. This transport step seems to restrict the size and electrical charge of targeted proteins, and it may be assisted by chaperones that reside in the intermembrane space. It is possible that the inner membrane of chromatophores has a Tic-like translocon resembling that of primary plastids. The genus Paulinella contains several heterotrophic species, one of which (P. ovalis) feeds on Synechococcus cyanobacteria. Thus, P. ovalis is a good model for the study of chromatophore acquisition, especially as its genome has cyanobacterium-derived genes. This chapter also includes a discussion of the homology of chromatophore envelope membranes, and whether the Paulinella chromatophore is a good model for the more ancient evolution of import machinery in primary plastids. Further, several other bacterial endosymbioses are discussed in the context of the presumed exceptional rarity of the transformation of endosymbionts into cellular organelles.

Azolla is a small-leafed floating or semi-aquatic fern with overlapping leaves that float or have submersed lobes. The chlorophyllous dorsal lobe contains an ovoidal cavity with filamentous nitrogenfixing cyanobacteria, usually assigned to Anabaena azollae, as well as several genera of bacteria. The vegetative cells of the cyanobacteria conserve RuBisCO during the symbiosis development and are probably able to fix CO₂ during the different stages of the association, maintaining their autotrophic condition. The leaf cavity behaves as both the physiological and dynamic interface unit of this symbiotic association, where the main metabolic and energetic flows occur. In this sense, it can be considered to be a natural microcosm. This unique symbiosis is sustained throughout the fern’s life cycle, where the cyanobiont and bacteria are always present and transferred from one generation to the next, either in the dorsal leaf cavities or in the megasporocarps, indicating the obligatory nature of the symbiosis. This also suggests a phylogenetic coevolution of the symbionts and the host, so that the association represents a good example of hereditary symbiosis. All these facts leads to the idea that the Azolla–Anabaena–bacteria symbiotic system works as a whole and can be considered to be a superorganism in both biological and ecological terms.

Two kinds of symbiosis are recognized in fresh water cnidarian hydra. The first is a stable symbiosis between Chlorella species and green hydra. The second is an unstable and probably now occurring symbiosis between Chlorococcum species and brown hydra. The chlorellae harboring green hydra has long been used as one of the best systems for the study of symbiosis between green algae and animals. An extensive knowledge has been accumulated in various field of research on the chlorella/green hydra symbiosis. On the other hand, there have been only a few studies on the chlorococcum/brown hydra symbiosis, because the chlorococci harboring brownhydra represent an unstable symbiosis and has been found only from Japan. The symbiotic chlorococci are often lost from hosts under the culturing condition in laboratories. However, intriguing data were reported in some pioneer works on the symbiosis of chlorococcum/brown hydra, and we found and confirmed horizontal transmission of the symbiotic chlorococci from the symbiotic brown hydra polyps to non-symbiotic polyps. These suggest that the chlorococcum/brown hydra symbiosis might be useful for the research on the origin and establishment process of symbiosis between green algae and animals. Moreover, within the green hydra (viridissima-group), genetic distances between strains are relatively large compared the case of brown hydra (vulgaris- and oligactis-groups), and the symbioses in green hydra are also diversified among themselves in many aspects. Therefore, comparison between diversified green hydra symbioses will also serve as useful materials for the study of the origin and the establishment of symbiosis. Taking together further coordinated investigations on both the two symbiotic system between green algae and hydra will provide an advance in the study of this field.

Diatoms are a diverse lineage of unicellular algae that have been flexible in their associations with other organisms. This review examines the relationships in which diatoms have been formed by endosymbiosis and the situations where they harbor and serve as symbionts. Diatoms possess a red algal plastid by secondary endosymbiosis, but some genomic data suggest diatoms may have harbored an ancient green algal plastid prior to the red algal plastid. Evidence on both sides of this debate is presented and indicates that additional data will be necessary to fully untangle the history of diatom formation. Some diatoms also harbor cyanobacteria as endosymbionts. In freshwaters, members of a single order (Rhopalodiales) inherit and use specific cyanobacterial endosymbionts to fix nitrogen. Culture studies and phylogenies indicate a close relationship between host and symbiont that suggests the symbionts are on their way to become organelles, making diatoms the first eukaryotes to fix nitrogen. In marine environments, the relationships between host and symbionts is more fluid, with lineages of diatoms that are not closely related being host to distantly related cyanobacteria. Diatoms can also be endosymbionts with different levels of integration in foraminifera and dinoflagellates. The diatom’s relationship with foraminifera indicates low host–symbiont specificity, with foraminifera hosting many diatom species, while the host–symbiont relationship with dinoflagellates is more integrated with the diatoms unable to live freely or build cell walls outside the host.

Symbiotic relationships between heterotrophic and photosynthetic organisms are found in all lineages of the eukaryotic tree, forming the basis for key biological innovations. Testate amoebae, a polyphyletic assemblage of primarily phagotrophic protists, include some species that are known to permanently harbor green algal symbionts, thus relaying on both phagocytosis and on by-products of the photosynthesis for food and energy (mixotrophy). Although the mixotrophic testate amoebae (MTA) belong to taxonomically unrelated clades (Amoebozoa, Rhizaria, and Heterokonta), they all share identical symbionts that are closely related to Chlorella variabilis (trebouxiophytes). This strategy has proven highly successful in the oligotrophic peatlands of the Northern Hemisphere, and occurred possibly simultaneously with the establishment of the first true Sphagnum peatlands. Since then, MTA have settled all favorable microniches and dominate often by their numbers testate amoebae diversity. They may be responsible for the fixation of a significant part of atmospheric carbon in peatlands, which in turn are known to stock an important part of the world’s soil carbon pool.

Hermatypic zooxanthellae–coral symbiosis allows coral reefs to exist and flourish in oligotrophic water. The translocation of carbon and energy, as well as nitrogen cycling between the partners, is the basis of this symbiosis. Transmission mode of zooxanthellae to the host, and recognition and specification between the host and the zooxanthellae, were shown to be dependent on the host and zooxanthellate genotype and on environmental conditions. Local stress, as well as global thermal stress and acidification, leads to bleaching and death of the corals. Their survival in the future is doubtful.

A major determinant of coral reef resistance and resilience is the intracellular symbiosis between scleractinian (reef-building) corals and dinoflagellates in the genus Symbiodinium. The inherent complexity of coral–Symbiodinium interactions however, presents a significant challenge to understanding and predicting reef dynamics. Research focuses on the dynamics of coral–algal symbioses from the molecular to the ecosystem levels and new methods, including next generation sequencing (NGS), real-time polymerase chain reaction (PCR), mathematical and computational analyses, and metabolomics, are all providing novel insight into the mechanisms that initiate, sustain and disrupt coral–Symbiodinium symbioses. As these approaches continue to be developed and synthesized, our understanding of complex coral–Symbiodinium interactions is becoming progressively more comprehensive. This chapter focuses on recent progress in the field and highlights novel approaches to embracing complexity in coral–algal symbioses.

Microorganisms are important players in marine ecosystems and often form close associations with larger eukaryotic organisms including seaweeds (or macroalgae). Recent technological advances in the study of molecular microbial ecology (e.g., next generation DNA sequencing and omic technologies) have now led to a greater understanding of the microbial diversity associated with these macroalgal hosts. It is now clear that macroalgae are home to a diverse community of microorganisms, with the majority of studies focusing on bacterial diversity highlighting that these communities display both temporal and spatial variation yet remain distinct from the surrounding seawater. Symbiotic interactions between marine microorganisms and macroalgae can have both positive (e.g., providing nutrients and morphogenic cues or protection from biofouling) and negative (e.g., disease and decay) outcomes for the host, with the microbial partners benefiting from the direct supply of organic carbon and oxygen and from the protective environment provided by the host. This chapter gives an overview of the microorganisms typically associated with macroalgae with a focus on the bacterial symbionts. Details of how bacteria successfully colonize macroalgal hosts are discussed with specific examples of the functional role of microbial epiphytes in macroalgal health and function highlighted from a “holobiont” perspective.

Ascophyllum nodosum provides the ultimate host for a symbiotic community of diverse eukaryotes comprising two brown algae (Ascophyllum nodosum, Elachista fucicola), two red algae (Vertebrata lanosa, Choreocolax polysiphoniae), a fungus (Mycophycias ascophylli), an insect (Halocladius variabilis), and a diatom (Navicula endophytica). The interactions of these symbionts vary from apparent mutualists (i.e., Ascophyllum–Mycophycias, Elachista–Halocladius) to obligate or facultative parasites (Choreocolax–Vertebrata, Elachista–Ascophyllum), with several interactions being either commensal (Ascophyllum–Vertebrata), or as yet uncharacterized (Navicula–Ascophyllum, Mycophycias– Vertebrata). The facultative nature of some of the associations suggests that the community may still be evolving host specificity. In addition, the components of the symbiotic community vary geographically and ecologically. In this chapter, we review the interactions of these species from ecological, physiological, and ultrastructural perspectives.

Lichen symbioses show outstanding resistance to extreme environmental conditions and are, therefore, used as model systems in astrobiological research. Lichens from harsh environments such as the Antarctic continent are of special interest. Although the symbiotic state provides a variety of adaptations to the extremes of radiation, temperature, and water supply, some aposymbiotically cultured lichen photobionts show considerable autonomous stress tolerance. Although lichens are valuable model organisms of recent astrobiological studies, isolated photobionts have not yet been subjected to space exposure experiments. Nonetheless, the performance of these algae under astrobiologically relevant stress conditions reveals substantial adaptations to harsh environments. The underlying physiological mechanisms behind photobiont and lichen stress adaptation are still being investigated and combining different stressors, such as temperature, radiation, and water restriction provide fundamental insights into the stress physiology of lichen photobionts.

Cyanobacteria enter into various symbiotic interactions with a wide spectrum of organisms distantly related to the tree of life. The level of proximity varies immensely in these interactions. As many other prokaryotes, the majority of cyanobacterial strains possess synthetic machineries of employing non-ribozomal peptide synthetases, and polyketide synthetases, which can be combined to produce a large diversity of chemical structures from low-molecular alkaloids up to large peptides. Some of the resulting compounds, as for example, well-known heptatotoxic peptides microcystins, were intensively studied in the past decades for their adverse effect on many organisms, including human. Recent studies show that despite their adverse effect on many organisms, some secondary metabolites are produced frequently by cyanobacteria in symbiotic interactions. This fact is raising important questions concerning the possible role of cyanobacterial metabolites in symbioses. Moreover, as the link between production of neurotoxic β-methylamino-L-alanine (BMAA) by symbiotic cyanobacteria and human neurodegenerative diseases has been proposed in the case of Guam population in Micronesia, symbiotic cyanobacteria seems to be important also from a toxicological point of view. This chapter is reviewing some currently known cases of symbiotic interaction where cyanobacterial secondary metabolites are produced within the association—microcystin and nodularin production in lichen and higher plants, production of various peptides in marine sponges, and production of neurotoxic BMAA by symbionts of Cycas and other higher plants.

The following sections are included:

• General Introduction
• Symbiosis as a Concept of Planetary Life
• A Short List Demonstrating Importance and Ubiquity of Algal Symbioses
• Acknowledgments
• References

The following sections is included:

• Index