Answer: Animal? Plant? How do you know?

Mixotrophs for the win...  

Elysia chlorotica, a sea slug that incorporates chloroplasts into its body.
P/C Patrick J. Krug, Creative Commons CC BY-NC 3.0 license, via Wikimedia Commons

Remember the Elysia cholorotica (the big green slug above, aka "Emerald green sea slug")?   

It's technically a mixotroph, but it's pretty big (up to 60 mm in length, but typically between 20 mm to 30 mm).  As you know, they feed on algae, extracting chloroplasts and then merging them into its body, making it able to both eat food AND use sunlight to photosynthesize for energy.  

We also know about other slugs (also called nudibranchs) that will steal the nematocysts of cnidarians (such as sea anemones, hydroids, jellyfish, corals, siphonophores, etc.). The slugs use them for defense, with the nematocysts stinging anything that tries to eat the nudibranch.  The nematocysts are stored in protruding cerata (those spine-looking things) on their backs.  And at the tip of each branch of the cerata, the nematocysts are stored in a small sac, the cnidosac. 

Here's a spectacular photo of an aeolid nudibranch taken by SRS friend Randall Spangler.  

The cerata on the back of this Aeolidia papillosa end with cnidosacs.  Like Elysia, they incorporate dinoflagellates that provides some photosynthesis capability.  P/C Randall Spangler. 

But the term mixotroph usually refers to really small critters--plankton--that are microscopic--much smaller than slugs. The size table from the Wikipedia article about plankton gives this as the breakdown for different kinds of small plankton.  

We're primarily interested in the smallish single-celled plankton--and only those that are carnivorous... if that means anything at the single-cell level.  

This led to a couple of questions about the microscopic mixotrophic plankton. To wit,   

1. Do mixotrophic plankton ONLY steal chloroplasts, or are their other kinds of organelles that they rip out of the prey?  If so, what else gets stolen? 

Started my search with a "fill in the blank" style search: 

      [ mixotrophs “steal the *”] 

And find this article on mixotrophs in Science Daily. 

“Traditionally mixotrophs are considered as curiosities or irrelevant. ... for the first time, [they have] mapped the presence of these "body-snatching" mixotrophs which shows that they are present in oceans all over the world.

These "body-snatchers" come in two forms. One type steal the photosynthetic factories from their prey during digestion and use these chloroplasts to make food as do plants. Other mixotrophs enslave whole photosynthetic prey communities within their body, drifting in the oceans like microscopic green houses. Marine researchers have been mislabelling 50% of these organisms as "animals" when they are actually eating and photosynthesizing within the one cell. The good body-snatchers support fisheries, while the bad ones can be highly toxic.” 

Fascinating.  Clearly, some mixotrophs steal the chloroplasts and incorporate them into their own bodies.  That's not too much of a surprise.  Unlike the Elysia, it's happening at the microscopic level, but it's the same idea.  Steal a chlorplast and live a microscopic life of luxury.  

In addition, some single cell mixotrophs ingest the whole prey organism and keep them intact, but within their body.  Plankton using this "swallow them whole" strategy are called endosymbiotic specialist nonconstitutive mixotrophs.  (You can use that phrase at your next party as you devour your very tasty living oyster.) 

Example: Noctiluca scintillans eats Pedinomonas notilucae, harboring them like a little garden of plants to generate important nutrients. 

But in my reading, I seem to remember some mixotrophs that ingested more than just the chloroplasts. 

On a lark, I turned to both Bard and ChatGPT for their opinions, asking both 

     [ do some mixotrophs steal organelles
       other than chloroplasts? ] 

Bing's version of ChatGPT told me that: 

"Yes, some mixotrophs can steal organelles other than chloroplasts. According to a Wikipedia article on mixotrophs, there are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and those that acquire them through kleptoplasty or through symbiotic associations with prey or enslavement of their organelles." 

That's fine, but it's basically what we already knew.  

Bard told me that: 

"Yes, some mixotrophs steal organelles other than chloroplasts. For example, the green hydra (Hydra viridissima) is a freshwater polyp that can steal chloroplasts from the algae it consumes. The chloroplasts are incorporated into the hydra's cells and allow it to photosynthesize. The hydra can also steal other organelles from algae, such as mitochondria and nuclei.

Another example of a mixotroph that steals organelles is the coral Pocillopora damicornis. This coral can steal chloroplasts from the algae that live in its tissues. The chloroplasts provide the coral with energy, which it uses to grow and reproduce.

The ability to steal organelles is a rare adaptation, but it is found in a variety of organisms. This ability allows these organisms to get the best of both worlds: they can photosynthesize and produce their own food, but they can also consume other organisms for nutrients.

Here are some other examples of mixotrophs that steal organelles:

The dinoflagellate Gonyaulax tamarensis can steal mitochondria from other dinoflagellates.

The ciliate Paramecium bursaria can steal chloroplasts from the green algae Euglena gracilis.

The fungus Pythium insidiosum can steal mitochondria from the plant cells it infects."

Again, we knew most of this.  But Bard also told me that some mixotrophs also steal mitochondria.  And that's a different organelle than the chloroplasts.  Interesting.  Are there others beyond mitochondria?  

So I changed my prompt to be: 

     [ do mixotrophic plankton only steal chloroplasts or are there other organelles that get ripped out of their prey?  If so, what else gets stolen? ] 

Bing/ChatGPT told me that: 

According to an article in Scientific American, Mesodinium is a mixotroph that sucks photosynthesizing organs from another organism called Dinophysis. Mesodinium takes in the organelles responsible for photosynthesis whole. Within minutes the pale Mesodinium starts to turn darker red as it fills its body with the stolen parts—chloroplasts and nucleosomes—which remain intact and functional. Mesodinium is not able to take in and use carbon dioxide as true photosynthesizers are, so it relies on its victims’ chloroplasts to accomplish that task

Bing also gave me a link to the Scientific American article with the information.  Nice job. 

The same prompt to Bard told me basically the same information, but without the details and certainly with the link to a useful article.  It did, however, mention that some mixotrophs also steal vacuoles.  

That doesn't make a lot of sense, so I checked it by reading a few papers on mixotrophs with vacuoles, and found that the mixotrophs often ingest their prey into THEIR vacuoles, not that they ingest the vacuole from the prey.  That's an important distinction that Bard got just slightly wrong. The vacuoles are where the stolen organisms are kept in the klepto-organism!    

Curious about the role of nucleosome theft in Mesodinium, I did a search for: 

     [ mesodinium mixotrophy nucleosomes ] 

and found a number of articles about how Mesodinium DOES in fact steal nucleosomes.  But why?  

In their Nature (2007) paper "Retention of transcriptionally active cryptophyte nuclei", Johnson, et al. show that the nucleosomes are "transcriptionally active,"  (that is, they're busy working within the body of the mixotroph) making substances that the chloroplasts that were also stolen just happen to need.  

Isn't that handy?  The mixotroph steals the chloroplasts AND the nucleosomes that also contain the instructions for the care and feeding of the chloroplasts.  

Alas, the nuclei only seem to last for about a month in the body of mixotroph, so they need keep eating in order to keep things running.  

So we've figured out that mixotrophs not only steal chloroplasts, but also mitochondria and nucleosomes.  

The dual presence of mitochondria and functional chloroplasts within these mixotrophic cells that have been stolen from another species of plankton suggests a high degree of biochemical similarity.  I didn't expect that degree of interoperability.  This is likely the key to their functional presence and essential endosymbiotic activity for over 2.5 billion years.

Totally amazing.  

2. How long has the concept of mixotrophic plankton been around?  Is it possible that this is relatively new understanding of planktonic ecology?  

The obvious approach here is to check out Google Ngrams for a brief history of the use of the terms that start with "mixotroph-."  

Here's that chart: 

It looks like the first uses of "mixotrophic" starts in 1900, but then really picks up in the 1970-2000, falls a bit, then really takes off around 2010.  

Of course, this is only from the Books corpus, so to get a better sense of the use of the word in the scientific literature, I turn to Scholar.Google.com and filter by date, here I'm filtering for papers published up to 1940. 

Note that there are about 34 results pre-1940.  

But ALSO note that several of those (results 3, 4, 5) are all actually from the 2000s.  (I checked: they actually are from the 2000s and are not pre-1940. Metadata problems.)  

Luckily, those date errors don't change the overall story.  Mixotroph-* came into common usage around 1950, with the other forms of the word (mixotrophy, mixotrophic) really becoming common after 1990.  Since I had completed my graduate training by 1984, it's very likely I never heard the word before running across it in that Scientific American article.  In the grand sweep of scientific history, it's a relatively recent word.  

SearchResearch Lessons 

1. Remember the fill-in-the-blank search method.  (That is, the * search.)  It can come in handy when you're looking for possible completions.  

2. LLMs can be handy tools for finding concepts and terms to further your own searching.  In all of the LLM examples here, there were problems with the text they generated.  HOWEVER, they also gave me some great ideas about what to search for next.  Be sure to check out their claims--don't just accept what an LLM tells you at face value.  (Example: Bard was deeply confused about the role of vacuoles in mixotrophy.)  

3. Bing shows citations for claims made--that's a really nice feature.  I sure wish that Bard would do the same.  (Oddly enough, for a few short weeks, Bard included citations.  It has since stopped doing that.  Don't know why, but I can guess that it was hallucinating too many references. Pity.)  

P.S.  A couple of people pointed out that they're not biologists and found reading the papers about this fairly esoteric topic to be heavy going.  One nice feature of the LLMs is that they are pretty decent at summarization.  Here's one such summary of an abstract from the  Johnson paper of above.  Here's the prompt, and Bard's nice summary... 

Dan:  I need a simplified summary of this article.  Can you give this to me at a 9th grade level? It is well documented that organelles can be retained and used by predatory organisms, but in most cases such sequestrations are limited to plastids of algal prey. Furthermore, sequestrations of prey organelles are typically highly ephemeral as a result of the inability of the organelle to remain functional in the absence of numerous nuclear-encoded genes involved in its regulation, division and function. The marine photosynthetic ciliate Myrionecta rubra (Lohmann 1908) Jankowski 1976 (the same as Mesodinium rubrum) is known to possess organelles of cryptophyte origin, which has led to debate concerning their status as permanent symbiotic or temporary sequestered fixtures. Recently, M. rubra has been shown to steal plastids (that is, chloroplasts) from the cryptomonad, Geminigera cryophila, and prey nuclei were observed to accumulate after feeding. Here we show that cryptophyte nuclei in M. rubra are retained for up to 30 days, are transcriptionally active and service plastids derived from multiple cryptophyte cells. Expression of a cryptophyte nuclear-encoded gene involved in plastid function declined in M. rubra as the sequestered nuclei disappeared from the population. Cytokinesis, plastid performance and their replication are dependent on recurrent stealing of cryptophyte nuclei. Karyoklepty (from Greek karydi, kernel; kleftis, thief) represents a previously unknown evolutionary strategy for acquiring biochemical potential.

Bard: 

Sure, here is a simplified summary of the article at a 9th grade level:

* Some organisms can steal organelles from other organisms and use them for their own benefit. This is called organelle sequestration.

* In most cases, organelle sequestration is short-lived because the organelles can't function without the genes from the organism that they were stolen from.

* The marine organism Myrionecta rubra can steal plastids (chloroplasts) from the cryptomonad Geminigera cryophila.

* M. rubra can keep the cryptophyte nuclei for up to 30 days. These nuclei are still active and help to keep the plastids functioning.

* Without the cryptophyte nuclei, the plastids in M. rubra stop working. This means that M. rubra needs to keep stealing nuclei from cryptomonads in order to stay alive.

* This is a new way for organisms to get the nutrients they need. It's called karyoklepty, which means "nucleus theft."

I hope this summary is helpful! Let me know if you have any other questions.

That was nice, Bard.  

For you humans... Keep searching!