Thursday, 23 October 2014

Four Solar-Powered Animals

Photosynthesis, a process used by plants and some bacteria to convert light from the sun into chemical energy which can be later released to fuel their activities. Animals on the other hand, have to consume other organisms in order to cover their energy needs. But every rule has an exception.

In recent years, researchers have discovered a small number of animals that much like plants have found a way to directly harness and feed off the Sun’s energy.

1# Oriental Hornet (Vespa orientalis)
Typically, wasps and hornets are most active during the early morning when they do the majority of their daily activities. However, this is not the case with the oriental hornet that is most active during the middle of the day. This social insect nests underground and the workers correlate their digging activity with the intensity of sunlight. It turns out there is actually a good reason why these insects love intense sunlight.

Oriental Hornet turns light into electricity
Oriental Hornet
Photo By MattiPaavola (Own work)
[CC-BY-SA-3.0], via Wikimedia Commons

The species has an outer layer (cuticle) that allows it to harvest solar energy. The yellow parts of the body (in the head and abdomen) contain a pigment called Xanthopterin. Xanthopterin works as a light harvesting molecule, transforming light into electrical energy. Currently, it is assumed that part of this energy is transformed in a photo-biochemical process which aids the species with energy demanding activities, like flying and digging. The harvested energy appears to also provide enough energy to carry out some metabolic functions, as researchers have found that most of the metabolic activity occurs in the yellow pigment layer.

As for the brown tissues, although incapable of directly harnessing the sun's energy, they still play an important role in the whole process. Structural analysis has found that they are full of grooves that capture light by channeling rays into the tissues and breaking them apart into smaller rays. Essentially, the brown areas act as a light trap, only 1% of the light that strikes is reflected away.

2# Eastern Emerald Elysia (Elysia chlorotica)
Elysia chlorotica is a medium-sized green sea slug of the Plakobranchidae family. Elysia chlorotica is a partially solar-powered slug that sequesters and retains active chloroplasts from the Vaucheria litorea algae it eats. During the feeding process, it first punctures the algal cell wall with its radula. The slug then holds the algae firmly in its mouth and,sucks out the contents. Instead of digesting the entire cell it retains the algal chloroplasts, by storing them within its own cells throughout its digestive system.

Elysia chlorotica, a sea slug with chloroplasts
Elysia chlorotica Photo
By EOL Learning and Education Group
[CC-BY-2.0], via Wikimedia Commons
The incorporation of chloroplasts within the cells of the slug allows it to capture energy directly from light, like most plants do, through photosynthesis. In periods where algae is not readily available as a food supply, the species may be able to survive for months on the sugars produced through the photosynthesis done by the incorporated chloroplasts.

Although E. chlorotica slugs are unable to synthesize their own chloroplasts, the ability to maintain the chloroplasts acquired from Vaucheria litorea in a functional state indicates that Elysia chlorotica must possess photosynthesis-supporting genes within its own nuclear genome, most likely acquired through horizontal gene transfer*.

3# Pea Aphid (Acyrthosiphon pisum)
Pea aphids are notable for being the only so-far known animals to synthesize a carotenoid, Torulene. Carotenoids are pigments produced by plants, fungi and other microorganisms and play an important role in photosynthesis. A 2010 study on pea aphids found that they have gained the ability to synthesize torulene through horizontal gene transfer from fungi. Two years later, new research revealed that this carotenoid may be behind a photosynthetic-like ability.

Pea aphid, a bug that can photosynthesize
Pea aphids extracting sap from the stem and leaves of garden peas
Photo by Shipher Wu and Gee-way Lin (aphid provision), National Taiwan University
[CC-BY-2.5], via Wikimedia Commons

The authors of the latest study examined three different types of the species: green aphids, which have the highest levels of carotenoids, orange aphids which produce intermediate levels of carotenoids and white aphids, which have little to no carotenoids. When researchers measured their ATP* levels, they found that the green aphids produced significantly more ATP than white aphids. What's more interesting is that the orange ones produced more ATP when exposed to sunlight than when moved into the dark. The researchers also crushed the orange aphids and purified their carotenoids to show that these extracts could absorb light and create energy.

The findings strongly suggest that the little critters can trap light and convert it into cellular energy. According to Maria Capovilla, co-author of the study, this ability could function as an emergency energy source that helps aphids survive their treks from plant to plant.

4# Spotted Salamander (Ambystoma maculatum)
Finally we have the Spotted Salamander, an animal that has long been suspected to be in a symbiotic relationship with photosynthetic algae. Back in the distant 1888, biologist Henry Orr first reported that the species' eggs often contain a single-celled green algae called Oophila amblystomatis.

Spotted Salamander egg-mass, algae clearly visible
Spotted Salamander egg-mass with algae visible inside the eggs
Photo By Fredlyfish4 (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

Today we know that the eggs are routinely colonized within a matter of hours. During this stage, the embryos release waste material, which the algae uses for food. In return the algae photosynthesizes and release oxygen for the developing embryos. In general, embryos that have more algae have a higher survival ratio and develop faster than the ones with few or none. But all this is old news..

In 2011, a study examining the species' eggs found that some of the algae was present within the embryos themselves, and in some cases invaded embryonic cells and tissues. This suggested that the embryos weren't just receiving oxygen but glucose too. In simple words, the algae inside their body generates fuel for the salamanders during the embryonic stage.

Spotted Salamanders have a symbiotic relationship with photosynthetic algae
Spotted Salamander
Photo By Jared C. Benedict (French Wikipedia)
[GFDL], via Wikimedia Commons

Two years later, another study shed new light into this symbiotic relationship. The researchers incubated salamander eggs in water containing a radioactive carbon isotope, as a result the algae produced radioactive glucose. The embryos that developed in that water were also radioactive. But the ones incubated in darkness (where photosynthesis is impossible) where not.

In short, it appears that embryos are capable of alga-powered photosynthesis. Still, many questions remain. Does the algae invade the eggs, or is it passed down from the parents? Are there any other algae species involved? How important is the presence of algae during adulthood? Are there any other amphibians with similar symbioses out there?

Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction. Also termed lateral gene transfer (LGT), it contrasts with vertical transfer, the transmission of genes from the parental generation to offspring via sexual or asexual reproduction
** Adenosine Triphosphate (ATP) are energy-bearing molecules found in all living cells. The energy in ATP is obtained from the breakdown of foods. Often dubbed as "molecular unit of currency" of intracellular energy transfer.

Mujer CV, Andrews DL, Manhart JR, Pierce SK, & Rumpho ME (1996). Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica. Proceedings of the National Academy of Sciences of the United States of America, 93 (22), 12333-8 PMID: 8901581
- Rumpho ME, Worful JM, Lee J, Kannan K, Tyler MS, Bhattacharya D, Moustafa A, & Manhart JR (2008). Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy of Sciences of the United States of America, 105 (46), 17867-71 PMID: 19004808
- Plotkin M, Hod I, Zaban A, Boden SA, Bagnall DM, Galushko D, & Bergman DJ (2010). Solar energy harvesting in the epicuticle of the oriental hornet (Vespa orientalis). Die Naturwissenschaften, 97 (12), 1067-76 PMID: 21052618
- Moran, N., & Jarvik, T. (2010). Lateral Transfer of Genes from Fungi Underlies Carotenoid Production in Aphids Science, 328 (5978), 624-627 DOI: 10.1126/science.1187113
- Valmalette, J., Dombrovsky, A., Brat, P., Mertz, C., Capovilla, M., & Robichon, A. (2012). Light- induced electron transfer and ATP synthesis in a carotene synthesizing insect Scientific Reports, 2 DOI: 10.1038/srep00579
- Gilbert, P. (1942). Observations on the Eggs of Ambystoma Maculatum with Especial Reference to the Green Algae Found Within the Egg Envelopes Ecology, 23 (2) DOI: 10.2307/1931088
- Gilbert, P. (1944). The Alga-Egg Relationship in Ambystoma Maculatum, A Case of Symbiosis Ecology, 25 (3) DOI: 10.2307/1931284
- Kerney, R., Kim, E., Hangarter, R., Heiss, A., Bishop, C., & Hall, B. (2011). Intracellular invasion of green algae in a salamander host Proceedings of the National Academy of Sciences, 108 (16), 6497-6502 DOI: 10.1073/pnas.1018259108
- Graham, E., Fay, S., Davey, A., & Sanders, R. (2014). Intracapsular algae provide fixed carbon to developing embryos of the salamander Ambystoma maculatum Journal of Experimental Biology, 217 (16), 2983-2983 DOI: 10.1242/jeb.111732

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