Imagine every salad you ate gave your skin a grassy green color, allowing you to recharge in the sun. This is effectively what sea anemones and corals do. They host algae in their tentacles that pay rent by providing food. But this algae–anemone teamwork is fragile, as even small increases in temperature trigger anemones to expel the algae. Without their algal allies, the anemones become ‘bleached’, threatening their survival. This divorce is bad for the newly separated algae and anemones, but also for the fish and invertebrates that need anemones for food and shelter. As oceans warm, bleaching events are increasing, so it's urgent we study this marine mutualism. But it's hard to untangle such an intertwined connection. Scientists often study metabolism by sampling tissues, but in this case, the anemone tissues would be ‘contaminated’ with algae. How else can you distinguish what each species is doing? Madeleine van Oppen and her team from the University of Melbourne, Australia, tackled this question with a clever new technique, shedding light on just how algae and anemone unite metabolisms to thrive as a team.
The researchers first anesthetized the anemones (Exaiptasia diaphana) so they wouldn't sting during sampling – after all, anemones are related to stinging jellyfish. Then, the team took slices of the anemones, scanning each location for biological compounds. This created a sort of ‘metabolic map’, displaying the metabolism of each body part. Van Oppen and colleagues made maps for three groups of anemones: one with a species of algae commonly found in this anemone, one with an algal species not normally found in these anemones, and one without algae. Anemones were noticeably bigger when they hosted algae, but it mattered which algae they were hosting. Anemones with non-native algae had fewer fatty acids, which are important energy sources in animals. This matched results from a previous study showing that anemones had weaker immune systems and fewer fatty acids if they were cultured with the wrong algae. So algae altered anemone metabolism. But did all body parts undergo similar changes?
As it turns out, the metabolic maps really highlighted differences among body parts. To van Oppen and colleagues, two types of chemicals stood out: one in the tentacles, and another in the gut. The first, betaine, is made by algae and was in the tentacles of all anemones with algae. This was unsurprising – algal chemicals were in algae. But these betaines had different structures than what you'd normally find, suggesting that the anemones had changed the algal metabolism. Second, in the gut, there were high levels of ceramides. These waxy chemicals are used in hand creams because they stiffen connections between skin cells, like the cement in a brick wall. Neither anemone nor algae produce much of this compound alone, but production ramped up when they were partnered together. This means that algae coax their hosts into producing a pathway, like a waxy red carpet, to guide in more algal coworkers and keep the partnership strong.
But above all, the metabolic maps highlighted just how little we really know about marine invertebrates. Only a third of the chemicals the researchers found in the anemones could be identified. This means we only understand a fraction of how these animals digest their food, even though they form the foundation of one of Earth's most diverse ecosystems. Yet, this information is becoming increasingly urgent. Recently, global temperatures broke world records 4 days in a row, both on land and in the ocean. If we come to understand anemones better, we may save their algal tenants from eviction, and preserve the homes of countless marine animals as they cope with Earth's rapidly warming oceans.