Algae or cyanobacteria are the first atmospheric oxygen producers which also caused a great oxidation event in the era between 2.3 and 2.4 billion years ago. That led to the massive oxidative death of anaerobic bacterial species and even microalgae. Along with changes in the earth’s course around the sun, the atmospheric changes that happened on earth due to an oxidation event contributed to the first Ice Age around 2.3 billion years ago. After the first ice age life on earth started reshaping and gave rise to new eukaryotic cell forms. 

Microalgae being one of the most primitive and photoautotrophic life forms on the earth, evolved and partnered with many other living entities in symbiotic relationships. As a primary producer, they were the food of the first protozoan species that formed the post-first ice age era. Since then, microalgae have formed multiple associations in marine and terrestrial habitats. This article will reveal some of nature’s unusual and exceptional secrets of algae and their associations. Most of these symbiotic relations are examples of the type of commensalism, mutualism, and even parasitism.    

As photo-symbionts (and/or endosymbionts) they form associations with cnidarians, sponges, molluscs, protists (i.e., lichens), and corals, etc. Nitrogen-fixing cyanobacterial species form an association with plants. Some very uncommon relations with microalgae also involve their relationship with vertebrates, which have been revealed in recent years. In this context, we will see some important examples of the symbiotic relationship between algae and other organisms.    

A. Corals (Scleractinia) and Dinoflagellate algae:

Coral reef ecosystems are the best place to observe various associations between different life forms and one of them is Symbiodinium (zooxanthellae). Symbiodinium is the relationship between corals and endosymbiotic Dinoflagellate algae. In order to support coral growth and calcification and provide the necessary nutrients for these diverse and fruitful ecosystems, symbiodinium converts sunlight and carbon dioxide into organic carbon and oxygen. Thus, light has a crucial role in controlling the coral holobiont’s productivity, physiology, and ecology. Symbiodinium has to safely capture sunlight for photosynthesis and expel extra energy to avoid oxidative stress, just like all oxygenic photoautotrophs.

Oxidative stress by environmental stressors like climate change causes coral reefs to bleach and break down coral-algal symbiosis. Large-scale coral bleaching events have increased in frequency and prevalence recently, jeopardizing coral reefs. There is an additional level of diversity in the coral–algal symbiosis because individual corals can host multiple types of Symbiodinium on various temporal and spatial scales.

B. Anemones (Anthopleura elegantissima) and Dinoflagellate algae:

Sea anemones typically feed on mussels, shrimp, squid, and other prey. But if this food isn’t available, they obtain sugar from the Dinoflagellates, and Dinoflagellates receive nitrogen, nutrients, shelter, and consistent exposure to sunlight from the host anemone.

Not all cnidarians that support algae can alter their carbon source. In most cases, such hosts cease to die due to their obligatory association with the symbiont. But in the case of Anthopleura sp. they have heterotrophy with symbionts where they are able to change their nutrient source depending upon the environmental conditions. Which is the same as the case of freshwater hydra. During predatory feeding, hydra manages to reduce the symbiont algal density and during starved conditions, it increases algal density to generate an alternative energy source.

The cost of this symbiotic relationship is that sometimes oxygen stress increased by the symbionts can damage the host cells. Anthopleura sp. can exocytose and egest algal cells to control their densities, but the mechanism behind this phenomenon is not completely understood.

In marine environments as move beyond coral reefs and their attached anemones, we can find host-specific relations between Dinoflagellates and other cnidarians species.

C. Jellyfish (Scyphozoan Cotylorhiza tuberculata) and Dinoflagellates:

Despite the richness of this sort of mutualism, jellyfish and other symbiotic cnidarians remain unexplored. In the 1800 century, scientists found yellow cells inside the tissues of sea animals such as Jellyfish. To this in the year 1882, biologist Sir Patrick Geddes of Edinburgh University proffered a new genus, Philozoon from the Greek phileo, meaning ‘to love as a friend,’ and zoon, meaning ‘animal’, but Philozoon genus name was officially never used. Recently, LaJeunesse et al.2022, supported the postulation made by Sir Patrick Geddes, that the relationship between sea animals and algae was truly symbiotic and not parasitic.

Cotylorhiza tuberculata (Rhizostomae, Scyphozoa) is a Mediterranean jellyfish that hosts an endosymbiotic Dinoflagellate from the Symbiodiniaceae family. In this species, the endosymbiotic relationship begins during the polyp stage of the jellyfish’s early life cycle. Eventually, symbionts are incorporated into their endodermal cells (Via lysosomes), and many of the symbionts containing cells develop into mesogleal amoebocytes. The overpopulated algal cells inside the amoebocytes build up close to the endoderm. Symbiotic Dinoflagellates play a very essential role in the nourishment of jellyfish and spread throughout the gastrovascular system of adult C. tuberculate.

As symbionts play a significant role in jellyfish nutrition, the host may exhibit some behavioral and morphological modifications to keep their photosynthetic partners functioning under optimal lighting conditions. To ensure illumination and maximize photosynthesis, Zooxanthellate jellyfish carry out intricate horizontal and vertical migrations or circadian-regulated tissue contractions. In their medusa stage, Zooxanthellate jellyfish get the majority of their nutritional energy from the symbiont’s photosynthesis. The host gives the symbiont nitrogen and phosphorus in exchange. (Enrique-Navarro, A. et al. 2022).

D. Sponges and Algae:

Many sponges co-evolved with others species, forming obligatory associations with other organisms, ranging from microorganisms to macroalgae. Endosymbiont green algae live close to the surface of some sponges, for example, breadcrumb sponges (Halichondria panicea. The alga is therefore shielded from predators, while the sponge is given oxygen and carbohydrates, which in some species can account for 50 to 80% of sponge growth (Olson, J. B., & Kellogg, C. A. 2010). Many of the macroalgae investigated are found in mesophotic habitats, in association with sponges that include the Halimeda spp., Lobophora variegata, Amphiroa spp., Caulerpa spp., and Dictyota spp. The sponge was also found to be associated with dinoflagellates. It is now known that freshwater sponges can also be found in association with yellow-green algae, cryptophytes, dinoflagellates, and diatoms.

The example of the mutualistic association between the sponge Haliclona caerulea and the calcareous red macroalga Jania adherens is observed on shallow rocky regions of Mazatlán Bay (eastern tropical Pacific, Mexico) (Ávila, E., Carballo, J. L., & Cruz-Barraza, J. A. 2007). In this association, it is found that algae also contribute to the inorganic structure (27%) of the sponge growth specifically under high wave exposure. When experimental studies were carried out on the sponge Haliclona caerulea in association with macroalga Jania adherens, it is observed that in shallow water the wave force impacts greatly the structural properties of the sponge. Here, algal contribution significantly reduces the energy costs of spicule (branches) production in sponges. With increasing depth the increase in the Si: CaCO₃ ratio in the sponge structure is observed which implies that the mutualistic relationship between sponge and algae reduces with the depth (Carballo, J. L., et al. 2006).

E. Lichens and Algae:

An association of a fungus (mycobiont) and a photosynthetic (photobiont) resulting in a stable vegetative body having a specific structure is called as a Lichens. It is estimated that around 6-8% of the land surface is covered by lichens with about 20,000 unknown species. In this association, fungi provide water and minerals to the alga, while the algae perform photosynthesis and supply food in the form of sugars to the fungi. Lichens act as pollution indicators as they do not grow in highly polluted environments.

Ascomycota and a few Basidiomycota phylum of kingdom Fungi are found to majorly forms Lichens. As they never occurred separately in nature they might have evolved as a symbiont with one or rarely two species of cyanobacteria as their photobiont. The exception would be a common green alga Trentepohlia is an example that can grow on its own or be lichenized. Lichens also share some specific habitats and even structural morphologies with some algal species (aerophytes) and grow on a tree trunk, rock, etc.

Lichens are miniature ecosystems of fungi, algae, or cyanobacteria which interacts with other microorganisms to evolve as an even more complex composite organism. Due to their long life and slow growth rate they have become an important tool to date the events by lichenometry. The schematic cross-section of foliose lichen explains various parts in its structure (a) the cortex tightly woven out from fungal hyphae (b) photobiont green algae (c) the Medulla with loosely packed hyphae (d) a tightly woven lower cortex (e) Anchoring hyphae called rhizines where the fungus attaches to the substrate.

Example: In India, a Lichen commonly called black stone flower (Parmotrema perlatum) is used as a spice in traditional cuisine. Usually, the dried flowers are tasteless and odorless but heating with oil produces a special earthy fragrance and smoky flavor which enhances the taste of the food.

F. Plants and Algae:

The cyanobacterial in association with other plant species fixes atmospheric nitrogen and makes it available to the host plant. They also provide fixed carbon to the non-photosynthetic host in the form of sugar. The major plant hosts for cyanobacteria are bryophytes, cycads, the angiosperm Gunnera, the water-fern Azolla, and fungi (to form lichens) (Adams, D. G., & Duggan, P. S. 2008).

1. Bryophytes – Nostoc Association

Nostoc spp. by means of its specialized motile filament called hormogonia avails entry into the host system. They can enter into the roots, stems leaves in plants, and thallus of bryophytes such as liverworts and hornworts. After chemoattraction and hormogonia entry of nostoc in the host’s symbiotic cavity, the host inhibits further hormogonia formation. This begins with heterocyst development and dinitrogen fixation. Furthermore, the host suppresses the CO₂ fixation rate of the Nostoc and induced more and more dinitrogen fixation for enhanced plant growth (Adams, D. G., & Duggan, P. S. 2008, loc. cit.).

2. Azolla and Anabaena azollae Association Another example of nitrogen-fixing cyanobacterial association with plants is of water fern Azolla’s symbiosis with a cyanobacterium Anabaena azolla. Anabaena colonizes in the base cavities of Azolla fronds. Cyanobacterial heterocyst fixes a sizable amount of nitrogen there. For 1000 years they have been utilized as a source of nitrogen-enriching fertilizers in Southeast Asian wetland paddies. Azolla “blooms” that can fix up to 600 Kg N per hectare per year commonly blanket rice paddies.

G. Hydrozoans:

Another example of Cnidaria is Hydrozoa which are small predatory colonial animals misunderstood as plants and are found in benthic strata (rock and pilings). They have stem pedicles and flower-like heads with mouths and tentacles, the polyps designed for feeding and initial digestion. Some of the polyp colonies are designed for reproduction. The hydrocaulus acts as a root to anchor the colony to the substrate and distribute leftover nutrition to the rest of the colony. Many of the colonies obtain their nutrients from symbiotic algae.

H. Spotted Salamanders and Algae:

The Spotted Salamander (Amblystoma maculatum) species is found across eastern North America. They rise from the soil usually on the first warm and humid night of the spring and travel towards the breeding pool. Females lay a couple or more masses of gelatinous capsules each containing up to 250 fertilized eggs. The egg laid down places are shallow in water and water there contains a very low level of oxygen. And there the secrete of spotted salamander and their symbiont microalgae is concealed.

It is found that egg gelatinous capsules contain green algal growth in them along with the embryo. This algal strain is identified as Chlorococcum amblystomatis, synonym Oophila amblystomatis, commonly known as chlamydomonad algae or salamander algae. This symbiotic algae in the egg capsule produce oxygen with photosynthesis and supply that oxygen to developing embryos. In return, they receive ammonia-rich waste from the embryo to fulfill their nitrogen requirements.

In the year 2010, the assumption that algae reside only in the egg capsule was slacked when researchers found algal cells inside the embryonic cells in early developmental stages. Which is the first of its kind discovery where algae cells are found inhabiting the cells of invertebrates during specific stages of embryo development. The exact mechanism of how and when algae invade embryos is not yet clearly understood.  

Furthermore, in the year 2017, John Burns and colleagues found that a suppressed protein named NF-kappa-b in embryos reduces immunity response. This facilitates the embryos to grow algae inside them (Burns, J. A., et al. 2017).

Conclusion:

The above-given examples suggest that algae can have a symbiotic relationship with smaller unicellular organisms to multicellular vertebrates. And in most relations algae serves as the best partner to nurture its host. In symbiotic relationships, very distinctive partner plays a key role in each other’s survival. From the beginning of life on earth, natural events have been altering the course of species’ development and survival. However, due to anthropogenic changes and environmental pollution by human interventions many such relations are now ceasing to exist. It has also risked and even vanished many of the species and their associations that were not even discovered. However, life’s struggle for sustenance leads to breaching the boundaries and making an ambiguous and unimaginable alliance, and nature keeps evolving the life forms.

References:

Garrido, A. G., Machado, L. F., Zilberberg, C., & Leite, D. C. D. A. (2021). Insights into ‘Symbiodiniaceae phycosphere’in a coral holobiont. Symbiosis, 83(1), 25-39.

Bedgood, S. A., Mastroni, S. E., & Bracken, M. E. (2020). Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species. Proceedings of the Royal Society B, 287(1940), 20201860.

LaJeunesse, T. C., Wiedenmann, J., Casado-Amezúa, P., D’ambra, I., Turnham, K. E., Nitschke, M. R., … & Suggett, D. J. (2022). Revival of Philozoon Geddes for host-specialized Dinoflagellates,‘zooxanthellae’, in animals from coastal temperate zones of northern and southern hemispheres. European Journal of Phycology, 57(2), 166-180.

Enrique-Navarro, A., Huertas, E., Flander-Putrle, V., Bartual Magro, A., Navarro, G., Ruiz, J., … & Prieto, L. (2022). Living Inside a Jellyfish: The Symbiosis Case Study of Host-Specialized Dinoflagellates,” Zooxanthellae“, and the Scyphozoan Cotylorhiza tuberculata.

Olson, J. B., & Kellogg, C. A. (2010). Microbial ecology of corals, sponges, and algae in mesophotic coral environments. FEMS microbiology ecology, 73(1), 17-30

Carballo, J. L., Avila, E., Enríquez, S., & Camacho, L. (2006). Phenotypic plasticity in a mutualistic association between the sponge Haliclona caerulea and the calcareous macroalga Jania adherens induced by transplanting experiments. I: morphological responses of the sponge. Marine Biology, 148(3), 467-478.

Ávila, E., Carballo, J. L., & Cruz-Barraza, J. A. (2007). Symbiotic relationships between sponges and other organisms from the Sea of Cortes (Mexican Pacific coast): same problems, same solutions. Innovation and Sustainability, 1, 147-156.

Adams, D. G., & Duggan, P. S. (2008). Cyanobacteria–bryophyte symbioses. Journal of experimental botany, 59(5), 1047-1058. Burns, J. A., Zhang, H., Hill, E., Kim, E., & Kerney, R. (2017). Transcriptome analysis illuminates the nature of the intracellular interaction in a vertebrate-algal symbiosis. Elife, 6, e22054.