Algae: The Powerhouse of the Anemone

Did you know that Sea anemones have algae symbionts? Me neither. As soon as I caught wind of it, I knew I had to dig deeper. And so I did. These squishy sea flowers have tiny single celled algae that live inside their tissues. But what are they doing there?  Just hanging out? Apparently not. In exchange of a house and home these tiny algae produce food for the anemone when needed through photosynthesis. 

Algae: A type of aquatic plant group that includes seaweed and can be tiny single celled organisms.

Symbiont: An organism that has a close relationship with another and usually is the smaller one of the two.

A recent study by Bedgood, Mastroni & Bracken (2020), found that there was a correlation between the density of the algae symbionts and the amount of prey that Giant Green anemones (A. xanthogrammica) feed on.  Meaning that the amount of symbiont algae in the anemone depended on how much food the anemone caught and then ingested. When an anemone is eating lots of prey items like mussels or small fish there is no need for the algae’s photosynthesis, but when there is a lull in the catches (during low tide perhaps) the algae steps in and helps the anemone out!  

Bedgood et al., (2020), also proposed a feedback mechanism that the anemones employ to control the density of symbiont present in their body. This proposed mechanism uses ammonium. This is a waste product of the anemone breaking down their prey items. Ammonium is used as a sort of control for the amount of fats and lipids the algae symbionts give to the anemone. The more prey the anemone catches, the more ammonium it produces as a waste product which, in turn, signals the algae symbionts to send less sugars and lipids the anemone’s way.  This mechanism goes the opposite way when prey is scarce: less ammonium means more sugars and lipids produced by the symbionts for the anemone to eat. Check out the diagram below from Bedgood et al. 2020 that illustrates this mechanism!

This is incredibly cool, because this is an example of an organism switching between two different energy sources.  Anemones are simultaneously heterotrophs AND autotrophs, which means they are able to switch between eating other organisms and photosynthesis. Most organisms can only do one mechanism of obtaining energy including us humans! We are heterotrophs, this means we need to eat other organisms to gain energy to survive. But could you imagine if nothing was in your fridge so you went out into the sun to do photosynthesis instead? That’s what anemones do!

Heterotroph: an organism that consumes other organisms to make the energy it needs to live. 

Autotroph: an organism that uses sunlight to make the energy it needs to live.

Next we have the Aggregating Anemone (A. elegantissima), a small greenish anemone that is often found in clusters in the mid to low intertidal.  This is usually the anemone found while tidepooling on PNW beaches, as it is an incredibly hardy species, surviving temperature fluctuations up to 20 C on the daily (Bingham, Freytes, Emery, Dimond, & Muller-Parker, 2011). 

These anemones are no exception to the symbiont trend, and have been known to have two main species in their tissues: Elliptochloris marina, and Symbiodinium muscatinei (Dimond, Bingham, Muller-Parker, Wuesthoff, & Francisa, 2011). E. marina is a green algae and Symbiodinium is a brown algae, both of these are from different phylums of Chloropyhta and Dinoflagellata respectively.  Some even have a mix of both green and brown algae and some have no algae symbionts at all. 

Aggregating Anemone symbionts have been heavily researched in the past, but a new study has supported that there is a link between Aggregating Anemone’s symbionts and their microbiome. An anemone’s microbiome as it turns out is very important! In this new study by Morelan et al., 2019, they took Aggregating Anemones from beaches up the coast of California and Oregan to see if each anemone’s microbiome correlated to their symbionts inside or even the latitude it was sampled from.  Using RNA gene sequencing, they created a list of most abundant bacteria orders of each anemone’s symbiotic state: green, brown and aposymbiotic.  They found that green symbiont anemones had a higher number of different bacteria than brown anemones. They even found indicator bacteria species that are associated with each symbiotic state! This is an exciting development; maybe in the future we will be able to predict what kind of algae helpers are hiding with anemones based on what kind of bacteria they have. 

This new evidence also shines even more light on the anemone-algae-bacteria relationship, also called the holobiont.  Three different organisms working together to create a safe and sustainable life for all three involved. It’s beautiful isn’t it? 

Next time you’re down on the beach at low tide and you see an anemone sitting pretty, whisper to yourself: algae are the powerhouse of the anemone.

Holobiont: is the assemblage of the host organism, and the many other organisms that live on or inside the host.  All together they act as one single organism

Aposymbiotic: meaning no symbionts present.