Half plant, half predator, all weird.

I was planning on brushing up my knowledge of chloroplasts today, as next week I'm starting a plantsci course for my options lectures, but I got sidetracked by Captain Skellet alerting me to Hatena. I've heard of several organisms containing proto-plasmids; symbiotic chloroplasts which haven't completely been endosymbiosed, but Hatena was a new one so I went to look it up. And I'm very glad I did, because it's pretty amazing.

Hatena, taken from the reference. Green blob is the symbiont. Scale bar is 10um

Quick background: chloroplasts are little membrane enclosed vesicles in plants which carry out photosynthesis. Current theory for how they developed is that they were once free-living bacterial type organisms (cyanobacteria) which were engulfed by a larger cell and over time lost their own identity and became little photosynthesising factories inside the larger cell. (I've got another post on it here for anyone particularly interested in the subject.)

Hatena arenicola doesn't have a chloroplast, but it does have a symbiotic relationship with another organism; nephroselmis. The nephroselmis is always found in the same place in the Hatena, and carries out photosynthesis to provide energy for both of them. Unlike regular chloroplasts, nephroselmis has it's own proper nucleus and even it's own mitochondria although most of the internal cellular organisation and any kind of motile apparatus (such as flagella) has been lost.

The weirdest thing about these two organisms though, is their replication cycles. When Hatena replicates, the nephroselmis doesn't, and as a result only one of the offspring gets the photosynthesising symbiont. The other organisms remains colourless and develops a complex feeding apparatus at the apex of the cell, presumably as it can no longer rely on the symbiont for food. This wierd 'half plant, half predator' lifecycle is shown below. (Picture taken from the reference, scale bar 10um):


That's just weird. Seriously odd. The Hatena is able to move seemingly freely between being a predator consuming other cells for food, and being a plant-like organism, once it settles down with it's symbiotic partner. The grey non-symbiont organisms can be induced to take up free-moving nephroselmis and (in the words of the paper) "tentitavely" maintain a symbiotic relationship with them.

The paper suggests that Hatena cycles between these two modes of living, depending on circumstance. Thus the 'predator' grey cell shown above will continue eating fellow cells until it consumes a nephroselmis, at which point it degrades its complex feeding apparatus, accepts energy from the symbiont until it's ready to divide. One of the daughter cells will then go through the whole cycle again while the other remains as a non-predating plant. The authors freely admit that there is little evidence for much of these stages, but it seems a reasonable way to explain what is going on.

As this is clearly a very early stage in symbiotic capture it has important implications for the endosymbiotic theory of chloroplast evolution. Along with various other 'intermediate' symbionts (such as Karenia mikimotoi and Lepidodinium viride) the Hatena helps to show how chloroplasts might have first formed in the cellular ancestor of plants. Hatena and its symbiont have already acquired an intimate structural association, only the coordination of their cell cycles would be required to turn the nephroselmis into an internally replicating plastid.

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OKAMOTO, N., & INOUYE, I. (2006). Hatena arenicola gen. et sp. nov., a Katablepharid Undergoing Probable Plastid Acquisition Protist, 157 (4), 401-419 DOI: 10.1016/j.protis.2006.05.011