Cytologia Focus
Natural and Artificial Photosymbiosis in Vertebrates
2022 Volume 87 Issue 2 Pages 69-72
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2022 Volume 87 Issue 2 Pages 69-72
Photosymbiosis occurs in symbiotic relationships between heterotrophs and photosymbiotic organisms, where a photosynthetic symbiont provides photosynthate to a host. While almost all known host animals are invertebrates, experiments have determined that artificial symbiotic relationships can be established between vertebrates and phototrophs. The ability to generate photosymbiotic relationships in vertebrates has important applications for medical treatments and commercial resource production.
Diverse phototrophic algae support the Earth’s ecosystems (Hosokawa and Kawano 2020, Kuroiwa et al. 2020, 2021, Miyagishima and Fujiwara 2020, Yoshida et al. 2020, Takeshita et al. 2021). Among them, some algae live symbiotically in heterotrophic hosts where they undertake photosynthesis (Decelle et al. 2019, Uwizeye et al. 2021). This unique symbiosis is called photosymbiosis. Photosymbiosis between phototrophs and animals occurs among many species. The symbiont lives within the animal host’s body, photosynthesizes, and in doing so supplies photosynthate to the host. In return the host supplies the symbiont with a living space and nutrients such as ammonium. Many algal species are symbionts, including taxa in the Chlorophyta (green algae), Dinophyta (dinoflagellates), and in a few cases, Bacillariophyta (diatoms) and Rhodophyta (red algae) (Venn et al. 2008, Melo Clavijo et al. 2018). Additionally, Cyanobacteria exist in photosymbiotic relationships with animal hosts (Venn et al. 2008, Melo Clavijo et al. 2018). Most hosts are invertebrates, including Porifera (sponges), Cnidaria (corals, hydras, jellyfish), Platyhelminthes (flatworms), Mollusca (bivalves, sea slugs, snails), and Tunicata (ascidians) (Venn et al. 2008, Melo Clavijo et al. 2018). The very few known vertebrate hosts include several amphibian species (salamanders, frogs) (Melo Clavijo et al. 2018).
Yellow-spotted salamander Ambystoma maculatum is the only well-studied vertebrate to have established a symbiotic relationship with a green alga Chlorococcum amblystomatis (Fig. 1A). This symbiosis is apparent as a green coloration (the symbiont alga) within the salamander’s egg capsule; some of these algae also enter the embryo at an early stage of host development (Kerney et al. 2011). Algae occur throughout the embryonic tissues of this species, and some even reside within the host’s cells. During host development, alga outside the embryo, within the egg capsule, photosynthesize, and the fixed carbon and oxygen are used in embryonic development (Tattersall and Spiegelaar 2008, Graham et al. 2014, Small et al. 2014); the host reciprocates by providing nitrogen compounds to the symbiont (Small et al. 2014). While this suggests that a mutual relationship exists, other results indicate this relationship is more complicated: algae within embryonic tissue cease to photosynthesize (Small et al. 2014), and the metabolism of intracellular alga might change to fermentation (Burns et al. 2017), indicating that the host’s cytoplasm may be unsuitable for the symbiont’s normal growth. Competition between the alga and salamander for available carbogen might also exist (Burns et al. 2017), and photosynthetic oxygen might have a deleterious effect on embryonic development (Small and Bishop 2020). Because algae occur only during a host’s developmental stages (Kerney et al. 2011) this symbiosis might be incomplete. While more study on this alga-salamander relationship is needed, the biological characteristics of these species make it difficult for research to proceed easily.
Salamander genome information remains incomplete, in part because of its size (∼31 Gb) (Burns et al. 2017, Licht and Lowcock 1991), which renders whole-genome sequencing difficult. The molecular classification of the symbiont algae is also obscure (Kerney et al. 2019). Thus, as a robust tool for uncovering the molecular mechanism, transgenic experiments are not presently feasible. Analyses are also hampered by the need to collect specimens from the wild, because techniques for their culture are not well established.
Other vertebrates assumed to have symbiotic relationships with algae include the northwestern (A. gracile), Jefferson (A. jeffersonianum), tiger (A. tigrinum), and Japanese black (Hynobius nigrescens) salamanders (Kerney 2011, Muto et al. 2017), and wood (Lithobates sylvaticus) and red-legged (Rana aurora) frogs (Gilbert 1942, Kerney 2011, Muto et al. 2017, Melo Clavijo et al. 2018, Kerney et al. 2019). In each case, algae have been observed within egg capsules, but no further in-depth research has yet been conducted to determine if they truly have a symbiotic relationship. Excepting these amphibians, photosymbiosis in vertebrates (fishes, reptiles, birds, and mammals) is unknown.
There may be few vertebrate hosts with symbionts because certain anatomical features such as: a thick non-transparent tissue which prevents sunlight from reaching a symbiont (Alvarez et al. 2015, Yang et al. 2022); a high energy demand which might not benefit from a symbiont’s photosynthetic supply (Cowen 1988, Yang et al. 2022); sexual reproduction, which might complicate transfer of symbionts from one generation to another, whereas in asexually reproducing invertebrate taxa the host divides and the symbiont community is inherited within a body (Cowen 1988, Yang et al. 2022); and the vertebrate-specific adaptive immune system might prove to challenge formation of symbiotic relationships (Yang et al. 2022). None of these hypotheses has been sufficiently investigated.
The objectives of attempts made to generate an “artificial symbiotic relationship” between vertebrates and phototrophs have been to ascertain if vertebrates can build symbiotic relationships with phototrophs, and to develop applications by adding a photosynthetic capacity to vertebrates. The green alga Chlamydomonas reinhardtii and cyanobacterium Synechococcus elongatus have been injected into zebrafish Danio rerio embryos, and embryonic hatching then monitored (Agapakis et al. 2011, Alvarez et al. 2015) (Fig. 1B). Innate immune responses have been also investigated by monitoring leukocytes or macrophages in injected larva, with few immune reactions against algal cells detected (Alvarez et al. 2015, Schenck et al. 2015). These results demonstrate that a fish embryo can tolerate phototroph invasion. Investigations of phototroph viability have also revealed cyanobacterial autofluorescence within fish fry to persist for at least 12 days post-injection (Agapakis et al. 2011), with algae extracted from hatched larvae subsequently growing on medium plate (Alvarez et al. 2015). This both reveals the high adaptability of phototrophs in a fish body and indicates that a phototroph–fish coexistent system is feasible. However, another experiment involving the injection of embryos with the Gram-negative bacterium Escherichia coli caused immediate embryo death (Agapakis et al. 2011), suggesting that a species-specific affinity might exist between a host and its symbiont.
The green alga C. reinhardtii and cyanobacterium Synechocystis sp. PCC6803 have also been injected into the vascular system of Xenopus laevis tadpoles to determine if a vertebrate could utilize photosynthetic oxygen from a phototroph (Özugur et al. 2021) (Fig. 1C). Both phototrophs produced oxygen during light illumination of the tadpole, and accelerated the host’s neural activity, demonstrating that a vertebrate host can benefit from photosynthetic oxygen.
An overall conclusion of these studies is that an artificial symbiotic relationship between a vertebrate and a phototroph can be generated in which the host vertebrate benefits from a symbiont. Further investigation is needed to ascertain the feasibility of vertebrate–algal symbiosis, and address vertebrate host limitations in natural photosymbiosis. Because the photosymbiosis phenomenon is analogous to the first stage of transformation from animal cells to plant cells, understanding it may provide clues as to how chloroplasts have formed, and the origin of kleptoplasty (Aoki and Matsunaga 2021, Maeda et al. 2021).
In addition to purely scientific research implications, the challenge to generate artificial photosymbiosis has important applied uses. The most accessible photosynthetic product is oxygen, with applications for utilizing this oxygen including medical treatments of hypoxia and for wound healing (Hopfner et al. 2014, Schenck et al. 2015, Chávez et al. 2016, 2020, Zhong et al. 2021). Carbohydrates and other photosynthates could also benefit a vertebrate host in a manner similar to the synthetic Calvin–Benson–Bassham cycle of unicellular organisms (Antonovsky et al. 2016, Gassler et al. 2020, Kawanishi and Matsunaga 2021). Should the host be a cultivated animal, then feeding costs could be greatly reduced because a host could partially, or even entirely be converted from a heterotrophic condition to that of an autotroph. Genome editing of phototrophs would likely expand the usefulness of the symbiotic system because many kinds of biochemical compounds that are useful for a host (or human) could be produced by a genetically modified symbiont (Alvarez et al. 2015, Chávez et al. 2016, 2020, Özugur et al. 2021). Accordingly, artificial photosymbiosis is a hot topic in biology, and could well initiate a new era of biological research.
This research was supported by grants from MXT/JSPS KAKENHI (20H05911), JST-CREST Grant Number JPMJCR20S6 and JST-OPERA Program Grant Number JPMJOP1832 to SM. We thank Steve O’Shea, PhD, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
