OK, well really this post is about the algae they eat. I’ve dedicated enough space to an animal on this blog. Regardless of whether or not (and it’s decidedly not) the slugs need the chloroplasts to be photosynthetically active, the kleptoplasts are remarkably stable. At least as far as the light reactions go based on long term chlorophyll fluorescence measurements, they are capable of photosynthetic electron transfer after months of residing within the digestive tract of the slug.
How remarkable is that? Remember last time I mentioned that it took hundreds of genes coming from the nuclear genome of a photosynthetic eukaryote to keep their chloroplasts functional? It’s no small feat.
Remember how I told you that photosynthesis researchers had to be fast and cold (and often dark) when it comes to biochemical preparations of chloroplasts and thylakoid membranes? I’ll tell you isolated chloroplasts kept on ice in the dark won’t remain that active for a week much less at ambient ocean temperatures.
I’m sure the slugs still have some yet-to-be-discovered secrets of chloroplast maintenance, but the first one is knowing where to steal their chloroplasts. Acetabularia acetabulum is the food of choice of the chloroplast-thieving sacoglossan slug Elysia timida. Researchers sequenced the chloroplast genome of Acetabularia acetabulum and identified clues as to the longevity of the stolen chloroplasts- the ftsH and tufA genes.
Here’s why that’s important: As far as stability goes, the weak link in the photosynthetic electron transfer chain is the Photosystem II (PSII) enzyme. As part of its normal activity, the core D1 subunit is irreversibly damaged. The damaged protein must be removed and replaced with a newly synthesized version. In photosynthetic organisms, an elaborate PSII damage-repair cycle keeps the light reactions functional. One of the auxiliary factors required for this process is FtsH, a protease involved in the removal of the damaged D1 protein. Normally, the ftsH gene is encoded within the nuclear genome of photosynthetic eukaryotes. Translation elongation factor Tu (encoded by tufA) is essential for the translation of chloroplast proteins. Since the PSII damage-repair cycle and the longevity of chloroplasts require a significant amount of new protein synthesis, the translation elongation factor Tu would be limiting over a period of months if the chloroplasts couldn’t make this protein on their own.
In the case of Acetabularia acetabulum, ftsH and tufA are encoded within the chloroplast genome. Similar trends were found in aquatic algae that were food sources for the sacoglossan slugs. This means the chloroplasts can make this critical factor for themselves, giving them a higher level of autonomy for maintaining their photosynthetic machinery. Elysia timida wouldn’t be so lucky if it tried stealing chloroplasts from the alga Chlamydomonas. Likewise for any plant chloroplast.
Additional experiments are necessary to directly evaluate the effect these chloroplast genes have on the long term retention of active kleptoplasts in the sea slugs. I’m sure there’s more to the story of how the stolen chloroplasts are maintained, but the ability to independently make these two essential proteins is a good start.
Reference (open access)