by Matthew Norton
There are so many wonders in the underwater world, diverse habitats spanning miles of seabed, ocean giants that inspire both intrigue and fear and the weird and wonderful collection of marine invertebrates. How easy it is to forget that it is the billions of microscopic organisms that hold it all together, and zooxanthellae are an informative example of this.
Zooxanthellae are photosynthetic dinoflagellates, one of many groups of single celled organism which retain many of the fundamental cellular features of plants and animals. You may know of zooxanthellae from their mutually beneficial (symbiotic) relationship with tropical, reef building corals. They live deep within the tissues of their coral host, supporting most of their energetic requirements in the form of organic compounds, such as glucose, produced during photosynthesis. In return the zooxanthellae are protected by the coral’s calcified skeleton and provided with the essential nutrients they cannot produce themselves.
This relationship is not limited to just corals, zooxanthellae support hundreds of marine invertebrates including sea anemones, jellyfish, giant clams and boring sponges. They can also be acquired by some species of sea slug following the consumption of zooxanthellae bearing prey, but this is a temporary arrangement. These hosts clearly value the contribution of their microscopic partners as they have evolved ways to provide zooxanthellae with what they need.
They can provide nutrients to their symbionts simply by continuing to catch food, usually by filtering out microscopic prey floating in the water. This may have the added benefit of keeping the host alive when the zooxanthellae is struggling to photosynthesise, but only in the short term. To provide their symbionts with light giants clams expand their mantle, the soft structure that secretes their shell and holds their zooxanthellae, and develop lens-like structures, called ocelli, to focus light toward the zooxanthellae. Other host species also need to protect their zooxanthellae from intense sunlight, such as host anemones, who contract their bodies in response to both low and intense sunlight, expanding only in moderate sunlight.
Unfortunately other environmental changes are not so easy to overcome, such as extreme changes in temperature. The first stages of heat induced coral bleaching, which is becoming more common due to global warming, is damage to the zooxanthellae’s photosynthetic machinery. In this situation there is little that an individual zooxanthellae, or its host, can do in response. However, in successive generations it is possible for the zooxanthellae species, or strain, to evolve greater heat resistance. The host may also be able to retain heat resistant zooxanthellae, which in theory could help them to hang on as the climate changes.
Zooxanthellae are just one small part of the billions, if not trillions, of micro-organisms that keep the underwater world turning, biologically speaking. Free-floating organisms, including bacteria, phytoplankton and even zooxanthellae in between hosts, have great importance by supporting most ocean food chains and recycling a lot of the nutrients locked up in dead animals. However, zooxanthellae provide their individual host species with the photosynthesis derived energy to perform some truly remarkable feats.
From a human perspective
I have already touched on the effect that human-induced climate change has on the coral-zooxanthellae symbiotic relationship. However, while the issue of coral bleaching is a serious one it has also been covered extensively. Instead I summarise how zooxanthellae, through supporting their host species, can affect us in other ways, and hopefully this will inspire appreciation for the impacts that climate change might have on both zooxanthellae and host.
By keeping coral reefs alive, and their bright colours intact, zooxanthellae bring in an estimated $36billion in tourism revenue. Some of these corals may also be valuable aquarium species, although some species are harder to keep in captivity than others, as are Beghia nudibranchs for the purpose of removing Aiptasia sea anemones, a common pest species in aquariums.
Other zooxanthellae supported species provide us with food, such as giant clams which are delicacies in France, the Pacific Islands and some parts of Asia. Also, certain chemical compounds can be extracted from these animals, which we can use in medicine. For example the “fried egg” jellyfish produce compounds that could be used to treat cancer and the sea anemone Anthopleura elegantissima produces a peptide (a protein-like molecule) which could treat a particular genetic defect that can affect the heart.
Sadly, not all interactions with humans are positive. In particular jellyfish blooms, which in some circumstances are perpetrated by zooxanthellae supported species, such as the fried egg jellyfish. These blooms can, and have, spread to fish farms, causing mass mortality among the fish and huge costs to the farmers, and clog the pipes of coastal industries, including nuclear power plants. Worse still, after the jellyfish die the decomposition of their bodies in such numbers sucks all of the oxygen out of the water, causing the formation and expansion of oxygen starved ‘dead zones’.
The key message is that zooxanthellae have strong, if subtle, links with humanity due to their symbiotic relationships with an impressive range of host species. This goes to show that just because some marine organisms are microscopic does not mean they are any less deserving of attention than the larger and more charismatic creatures.
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