Small Friends Help Sea Anemones Survive the Heat

Microbiota mediated plasticity promotes thermal adaptation in the sea anemone Nematostella vectensis

Laura Baldassarre, Hua Ying, Adam M. Reitzel, Sören Franzenburg, Sebastian Fraune

Summarized by Blair Stuhlmuller

What data were used? Researchers used cloned Nematostella vectensis, a sea anemone found in estuaries and brackish water environments of the US and UK. N. vectensis hosts many helpful small friends, or symbiotic microbiota. In other words, microscopic organisms that live on the host anemone and help it deal with environmental stressors like temperature changes. These symbionts can be passed onto the offspring from the parent anemone or be acquired from the environment during development. The symbiote assemblage can also change during an anemone’s lifetime in response to changing environmental conditions. The researchers looked at the composition of the microbial communities, the genetics of the host anemone and mortality rates at different temperatures.

Methods: First, in order to control for genetic diversity between individuals, the researchers created clones from a single female polyp (anemone). These individuals were divided into different test groups based on temperature–low (15℃) temperature, medium (20℃)  temperature and high (25℃) temperature–that were studied over the course of three years. Each test group had 5 cultures of 50 cloned anemones.

Results:  After 40 weeks and after 132 weeks, the polyps were exposed to high heat stress (6 hours at 40 ℃) and mortality was measured. In both tests, all of the polyps in the low temperature group died. The high temperature group had the highest survival rate after 132 weeks. Polyps in the high temperature group experienced a lower mortality rate overall, but were also 3 times smaller, and asexually reproduced 7 times more rapidly than those in the low temperature group. These results show that long-term temperature differences have a great impact on heat tolerance, organism size, and reproduction rates.

Next, changes in the microbial symbiont communities were measured through 16S rRNA sequencing (or the process of reading the small section of ribosomal RNA molecules that is in charge of turning the genetic code into actual functioning cell parts) at the 40, 84 and 132 week intervals. The results showed that both the temperature and exposure duration to said temperature had a significant effect on the microbial community composition. Three distinct microbial communities were found for each temperature test group and these communities stabilized within the first two years. 


A bar graph showing the survival rate of each temperature group after experiencing heat stress. After 40 weeks, the survival rate of the group acclimated at 15℃ is 0, the second group, acclimated at 20℃ has a survival rate of 70% and the third group, acclimated at 25℃, has a survival rate of 30%. After 132 weeks, both the 15℃ acclimated group and the 20℃ acclimated group experienced a 0% survival rate. Only the last group, acclimated at 25℃, remained with a survival rate of nearly 100%.
Figure a shows the survival rate of each temperature group (AT is acclimated temperature) to heat stress. Heat stress experiments were conducted at 40 weeks of acclamation (woa) and 132 weeks.

Third, all the active genes (or genes that are making mRNA) were analyzed in order to see if any changes occurred. One polyp from each culture was selected. The polyp’s mRNA was extracted and sequenced or read. Gene expression, or what genes are actively determining an organism’s features and functions, can be influenced by outside factors and can cause changes to an organism’s phenotypes, or physical characteristics, within its lifetime. While the actual DNA sequence is not changed, certain genes can be turned on or off that can then help or hurt the organism. In this study, polyps in the high temperature group experienced a significantly increased expression of genes involved with immunity, metabolism, outer skin cell production and other positive changes. 

Lastly, researchers wanted to determine if the microbial community and thus changes in gene expression were transferable and could increase the heat tolerance of new individuals and future generations of anemones. Thus they transplanted the temperature adapted microbial communities/symbionts to new, non temperature adapted polyps which were cloned from the same female as the experiment population. Then the heat tolerance of the new polyps were tested. Survival rates of the polyps with transplanted microbial communities depended on the source of the transplanted microbial community. Polyps with microbes from the high temperature group had an 80% survival rate, a significantly higher rate compared to the 33% of the polyps with the low temperature microbes. This shows that microbial transplants could prove to be a quick and effective way to help certain organisms cope with environmental changes. 

The researchers also tested if both the gene expression and microbial communities could be naturally transferred from one generation to the next. rRNA sequencing revealed that large parts of the parent microbial community were successfully transplanted to the offspring. The offspring were then subjected to high heat stress. The offspring from the high temperature group showed a significantly higher survival rate compared to the offspring from both the low and medium temperature groups.

Why is this study important? Members of the Cnidarian phylum like corals and sea anemones are under threat due to rapid climate change. Warming water temperatures are causing coral bleaching and other harmful effects. Since coral and many anemones are mostly sessile, or non moving, when mature, they only have two options–adapt or die. And with climate changing so quickly in recent decades, one might expect extinction to be the more likely option for many species. Adaptation is typically limited by random mutations and natural selection, neither of which happens overnight. However, this study shows how adaptation can happen within just one generation. 

Sessile animals that host a range of symbiotic microbiota exposed to high water temperatures can adapt and become more heat stress resistant. Microbes tend to have much faster generation times and can thus evolve more quickly than their hosts. These microbes can then influence the gene expression of their host by turning on or off certain genes further helping the host to survive and adapt during its lifetime. Most excitingly, these changes in gene expression and microbial communities can be passed to the next generation. This study also helps pave the way towards assisted evolution and potentially huge successes in coral conservation. Heat tolerant microbial communities could potentially be selected for in the lab and then transplanted to wild populations. This would allow scientists and conservation groups to improve the fitness of wild populations quickly and effectively help counter the effects of climate change. 

The big picture: Climate change is a looming threat especially for those living in the oceans. As ocean temperatures rise, many marine species will likely migrate towards the poles in order to remain in their desired temperature ranges. However, sessile or mostly non-moving marine organisms like sponges, coral and sea anemones will have a harder time doing that as many are only mobile during their planktonic larvae stages. This study gives a glimpse of hope that these animals will be better able to adapt and survive than previously expected. This study specifically shows that animals exposed to high temperatures, like N. vectensis, can quickly become more heat stress resistant as the symbiotic microbiota shift and adapt. Most importantly these high heat tolerances can be passed to other organisms and future generations. These lab results are mirrored by long term observational studies that show wild populations becoming less heat sensitive than past generations. Overall, this has huge positive conservation implications for coral reefs and other sessile marine communities as climate rapidly changes.

Citation: Baldassarre, L., Ying, H., Reitzel, A.M. et al. Microbiota mediated plasticity promotes thermal adaptation in the sea anemone Nematostella vectensis. Nature Communications 13, 3804 (2022).

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