Microscopic friends help anemones tolerate the heat

Sea anemones contain millions of microscopic organisms that provide them with nutrients critical to their survival. Unfortunately, small increases in temperature can trigger a reaction called bleaching in some anemones, causing them to eject the microbes from their tissue and subsequently starve. But other anemones are more resilient to high heat. For example, anemones that live in tidepools commonly experience temperature fluctuations of more than 15°C each day, which begs the question – how are these animals able to maintain a stable relationship with their microbes in such harsh conditions, while others will eject their microbes when the water warms by only 1–2°C? Maria Ruggeri and colleagues from the University of Southern California, USA, studied how a tidepool anemone (Anthopleura elegantissima) maintains its relationship with microbes in extreme heat.First, the team measured temperature on rocks where the anemones live and found that anemones living in deeper water experienced lower temperatures with smaller daily swings than those living closer to the shore. The team collected anemones from the deeper and shallower rocky pools and brought them back to the lab, where they placed the anemones for a week in tanks of 18°C water, reflecting nearby ocean temperature when the study took place. The researchers then warmed the water to 28°C for 10 days for some of the animals – simulating a heatwave – while leaving the other half of the animals at a cool 18°C.To assess the impact of temperature on the anemones and their microbes, the researchers measured the microbes’ ability to absorb light necessary for making nutrients and measured the amount of microbe DNA found in the anemones’ tentacles. This allowed the team to estimate how helpful the microbes were to the anemones and the number of microbes present to see if they were getting ejected. The team predicted that anemones from the shallower, more stressful environment would be better at holding on to their microbes under high heat compared to the anemones from the deeper, cooler environment. While the number of microbes living in all the anemones decreased at 28°C, as expected, anemones that were used to more extreme conditions maintained more microbes that were better able to produce nutrients than anemones from the cooler environment. Clearly, something about the anemones from warmer tidepools was helping them thrive in high heat and the researchers were determined to find out what it was.The team wondered if the more resilient anemones had special genetic adaptations that helped them tolerate the warmer water or if the animals contained more heat-tolerant species of microbes. However, when the researchers performed a genetic analysis, they discovered that several of the animals were clones of one another, even though they were found at different water depths, meaning that genetic differences between the animals could not explain their responses to warming. The researchers also performed DNA analysis to identify the types of microbes living in the anemones and found that all anemones harbored similar species of microbes. Together, these findings indicated it was the environmental conditions the anemones experienced in their home tidepools that determined how they fared in the heatwave. It seems that growing up in a harsher environment primed anemones and their microbes to withstand extreme heat.This study highlights the remarkable ability of these anemones to maintain their partnership with microbes even in the most inhospitable conditions. As our oceans continue to warm due to the ongoing climate crisis, studies like this provide valuable insights into the mechanisms that limit more vulnerable species that rely on microbes to survive. By understanding what allows some animals to thrive while others struggle, we can better prepare for the environmental changes ahead.