Hudspith, M., de Goeij, J.M., Streekstra, M. et al. Harnessing solar power: photoautotrophy supplements the diet of a low-light dwelling sponge. ISME J 16, 2076–2086 (2022). https://doi.org/10.1038/s41396-022-01254-3
Most people agree that there is a very clear distinction between animals and plants. In our early science classes, we were taught that animals eat other things to get food (“heterotrophy”), while plants make their own via photosynthesis (“autotrophy”).
But then what do you call a carnivorous plant? Or a sea sponge that can photosynthesize?
It turns out the line between heterotrophy and autotrophy is more blurred than anyone would expect. It’s so blurry, in fact, that there is a new term to describe the many plants, animals, and microbes that can do both: “mixotrophy”.
Plant? Animal? Mixotroph.
Under the sea mixotrophy rules supreme, but we still don’t fully understand how much of the animal kingdom can participate. While some microscopic plankton can be both autotrophic and heterotrophic on their own (like a carnivorous plant), animals like corals and sponges have to rely on symbiotic algae which perform photosynthesis for them. This partnership seems to benefit both parties – the animal gets easy meals, and the algae lives in a protected environment inside the animal’s body. In a given community of sponges, 30-70% of them have photosynthetic algae living inside their tissues, so the type of environment and the species present make a big difference.
In shallow, well-lit environments, sponges can get up to 80% of their food requirement from their symbiotic partners (about as much as corals). However space is limited in well-lit areas of the shallow ocean, as corals often out-compete sponges. Thus, the majority of sponges inhabit poorly lit habitats such as the dim crevices underneath rocks or in the shade of a coral reef. Deprived of light, these sponges can’t rely on their microbial partners to help them out as much as those who bask in the sun, yet researchers have found photosynthetic bacteria living within their tissues.
Marine scientists want to know how these low-light sponges maintain this symbiotic relationship. How much nutrition can they get from their partners? What’s in it for the algae?
The answer to these questions is important for understanding how shallow-water ecosystems are structured in places like the Caribbean, where once-dominant coral reefs are now under threat due to bleaching.
An international team of researchers sought to measure how much low-light sponges rely on their bacterial symbionts through studying a common Caribbean sponge called Chondrilla caribensis.
Like all sponges, Chondrilla caribensis is a filter feeder. It anchors itself to the seafloor and filters out microscopic plankton and other particles with special feeding cells. C. caribensis prefers shallow seas, but it hangs out in low-light areas on vertical surfaces or shady rock overhangs. In the past, researchers believed that low-light species like C. caribensis got most of their nutrition through filter-feeding and their microbiome received little attention until recently.
Despite its preference for low-light, C. caribensis hosts a diverse microbiome, including a common photosynthetic symbiont called cyanobacteria. This paper found that the cyanobacteria living inside the sponges are photosynthesizing enough to provide their host with a significant amount of food despite living in deeper, darker water. About half of the sponge’s needs are met by symbiotic bacteria. This is much lower than sponges in high light environments, who get 130% of their carbon from photosynthetic bacteria (enough to build up a surplus), but it’s still quite impressive. Imagine how low our grocery bills would be if 50% of our nutrients came from photosynthesis!
Fuel-efficiency for future generations
The ability to harness solar power through cyanobacteria reduces C. caribensis’ need to filter feed. This could help protect the sponge from periods of low food availability, which will worsen with warming water temperatures. Because filter feeders like sponges and corals can’t move, they are completely dependent on underwater currents delivering their food like a conveyor belt sushi restaurant. When that current-driven conveyor belt slows due to hotter temperatures, immobile filter feeders can experience mass die-offs. Events like these have occurred in other shallow, warm-water ecosystems like the Mediterranean, and are expected to be more frequent with climate change.
We have a lot more to learn about how sponges adjust the balance of photosynthesis and filter-feeding to environmental changes. Powered by cyanobacteria like tiny solar batteries, low-light sponges are able to thrive in a diverse range of depths and light intensities, so they could be well-equipped to tackle changing ocean conditions. The energy they conserve by feeding less can be saved for reproduction and growth, passing down the tradition of mixotrophy to the next generation.
Cover photo: Twilight Zone Expedition Team 2007, NOAA-OE, NOAA Photo Library via Flickr (CC BY 2.0)
I am a masters student at the University of Rhode Island, studying the ecology of the harmful algal bloom genus, Pseudo-nitzschia. My work is interdisciplinary and revolves around coastal health, water quality, and public engagement with science. In my free time I enjoy doing crafts, writing, cooking, and exploring the outdoors on my feet or a pair of skis.