Are there carnivorous plants in the sea?

Whether because of Audrey from Frank Oz’s musical Little Shop of Horrors, or because of The day of the Triffids of John Wyndham’s, or even because of the little venus fly trap plants sold in the flower shops, everybody knows what carnivorous or mixotrophic plants are. You may even have tried to grow one at home, probably with little success. This is because they are plants adapted to very particular environments, usually characterized by acidic soils and very poor in nutrients, high humidity, and a very precise temperature range. But if you keep all this in mind, it’s not that hard to keep a few species at home; I have over twenty different ones on my balcony. 

Due precisely to the environments in which these plants live, they have had to find evolutionary ways that allow them to grow taking advantage of what they had at their disposal. And that’s why these plants extract the nutrients they don’t find in the soil from insects and other critters that they attract and capture with modified leaves in the form of traps of different kinds. However, in environments that deviate minimally from their peculiar requirements they die or are quickly excluded by faster-growing competitors. In fact, we find carnivorous plants in very few places on earth, while non-carnivorous plants, the strictly photosynthetic ones, are everywhere. 

Mesodinium rubrum. Drawing Albert Calbet

But what about the sea? Well, there are a lot of mixotrophic plants in the sea that eat other organisms. What happens is that they are unicellular and invisible without a microscope and that is why they are not so well known. Apart from diatoms (algae with a siliceous skeleton) and very few representatives of other groups, the other planktonic algae can feed on live prey. Can you imagine that almost every plant on Earth was carnivorous? There would be no insects left! At sea, however, the range of prey they have is quite wide. Most constituent mixotrophs (plant-eating organisms capable of eating prey) eat other algae and do so either to obtain inorganic nutrients, such as nitrogen or phosphorus, to replenish the stock of chloroplasts, as a carbon source, or simply to eliminate competition for resources. Many marine mixotrophs, despite being vegetarian, do not despise a good animal prey, whether unicellular or even pluricellular, which immobilize and kill with the help of venom-bathed spears or releasing toxins into the water. In the marine ecosystem, we also find another type of mixotroph that does not exist on earth, apart from some science fiction films and comics, such as “The thing from another world” or “The swamp thing”. They are animals with plant characteristics (non-constitutive mixotrophs). These characteristics are acquired by capturing plant prey to incorporate their chloroplasts (or whole algae) and thus be able to do photosynthesis. Can you imagine a rabbit green as lettuce? There are, in fact, a few marine multicellular animals that have this ability as well; some corals, sponges, worms, or bivalves catch symbiont algae. Even the sea green slug Elysia chloroticacan synthesize a rudimentary chlorophyll. But the kings of non-constitutive mixotrophy in the sea are unicellular. Ciliates, dinoflagellates, foraminifera, radiolarians, etc., are some of the groups capable of capturing and enslaving whole algae or their chloroplasts. And not only that, but there are also those that, in addition to chloroplasts, incorporate the nucleus of the taken cell into their cytoplasm to help them in the duplication process. 

Mixotrophic Acantharida.  Lithoptera sp. Drawing Albert Calbet

An incredibly complex process in a single cell. Some reach impressive levels of specialization, preying only on one species of prey, or even only on one species of mixotroph that has previously captured the chloroplasts from a given prey, such as the dinoflagellates of the genus Dinophysis, which feeds on the ciliate Mesodinium rubrum, which in turn eats and captures chloroplasts of a given group of algae. The reason for this plasticity and ease of incorporation of foreign organelles from many marine protists is probably due to the evolution of the eukaryotic cell in the sea. It is believed that the origin of algae began with a cell without phototrophic capacity that was able to capture and retain an autotrophic bacterium (the first chloroplast). This occurred ca. 1500-2000 million years ago, at the beginning of life on earth. As you can see, despite being a widespread phenomenon, we are still far from understanding all the factors involved in mixotrophy in the sea, because we find that each species is a world, or even that each strain of the same species behaves differently. We are indeed still not entirely sure how relevant mixotrophs are in marine trophic food webs, because having the ability to use a particular metabolic pathway does not necessarily imply its use. 

Process of capture and ingestion of an alga (Rhodomonas salina) by a mixotrophic dinoflagellate (Karlodinium veneficum). Albert Calbet

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