May 26

Organism of the week #14 – Turtles all the way down

Cassiopea, which looks rather like a spelling mistake (but isn’t), also looks a bit like an anemone (but isn’t):

Cassiopea sp. [CC-BY-SA-3.0 Steve Cook]

Cassiopea ‘upside-down jellyfish’ in the aquarium beneath the Palm House at Kew Gardens

It’s actually a jellyfish, but one that spends most of its time living upside-down (relative to its relatives). Unlike its close relatives, it gets much of its energy from sunbathing rather than from fishing.

Inside the cells of the jellyfish live tiny photosynthetic algae called dinoflagellates. These algae produce food for the jellyfish by photosynthesis, and presumably benefit from the protection afforded by the jellyfish’s stings.

Beneficial relationships like this between two different organisms are called ‘mutualistic symbioses‘. Mutualistic symbiosis is very common in biology. Many large organisms act as hosts for smaller organisms, and relationships based on photosynthesis are some of the commonest:

Ramalina farinacea [CC-BY-SA-3.0 Steve Cook]

Ramalina farinacea, like all lichens is an intimate mutualistic symbiosis between a fungus and an photosynthetic alga. Like the upside-down jellyfish, the algae here (arguably) benefit from the fungus’s protection, and the fungus benefits from the sugars made by the algae.

The marvellous thing about the jellyfish/dinoflagellate symbiosis is that the dinoflagellate itself is the product of a mutualistic symbiosis. Inside the cell of a dinoflagellate there is a sub-structure that actually performs the photosynthesis. This structure itself is the remains of a different kind of alga that has taken up residence inside the dinoflagellate.

Although they are not closely related to dinoflagellates, the cells of brown algae have a very similar set-up. Brown algae are perhaps more familiar to you than dinoflagellates, as they include most of the larger seaweeds like wracks, kelps and rockweeds:

Fucus vesiculosus [CC-BY-SA-3.0 Steve Cook]

Fucus vesiculosus or bladder-wrack is a common brown alga found around the UK coast

The photosynthetic structures of dinoflagellates and brown algae are (usually) the remains of red algal cells. In the case of the brown algae, these were taken up in the dim and distant past and are now permanent residents. The many species of dinoflagellate are flightier, and appear to have gained, lost, and re-acquired the photosynthetic helpers by gobbling up other algae on many occasions during their evolutionary history.

Kryptoperidinium [CC-By-SA-3.0 Steve Cook]

The dinoflagellate Kryptoperidinium has not just one, but two different kinds of photosynthetic structures (‘plastids’). The yellow thing the the left is the remains of a red alga, and acts as an eye-spot. The much larger plastid to the right contains its own nucleus (grey wibbly thing) mitochondria (pink things) and internal plastids (yellow). This plastid is the remains of another algal cell, in this case a diatom , which itself has a structure similar to the brown algal cell shown in the next diagram

But it doesn’t even stop there! If you peel open a red algal cell, you’ll find that the photosynthetic structures are again restricted to small sub-compartments of the cell. These structures look very similar to certain sorts of photosynthetic bacteria called cyanobacteria, and indeed that is precisely what they are: the dwindled remains of photosynthetic bacteria captured by the ancestors of red algae about a billion years ago.

This ancestor of the red algae also happens to be the ancestor of all green algae and land plants too, which means inside every butterwort and cock-of-the-gods there are also trillions of these bacterial wraiths, photosynthesising away.

Endosymbiotic matryoshka [CC-BY-SA-3.0 Steve Cook]

Symbiotic matryoshka

So an upside-down jellyfish is like a Russian matryoshka doll.

  1. The jellyfish contains dinoflagellate algae, which make food for the jellyfish.
  2. But the dinoflagellate contains the remains of a red alga, which is the source of this food.
  3. But even the red alga is only able to make food because it contains the remains of the real player here…
  4. …which is the remains of a photosynthetic bacterium.

The upside-down jellyfish is a four-level deep symbiosis of bacterial cells living within red-algal cells living within dinoflagellate cells living within jellyfish cells. In comparison to this, living upside down is the least of its marvels.

If anyone knows of a five-deep endosymbiosis I’d be delighted to hear about it…

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