What Are Zooxanthellae?

Ever wondered where corals get their vibrant colors and healthy growth from? This is all down to one of the most remarkable symbiotic pairings in the ocean. Behind your coral’s stunning polyps lies a microscopic being that makes coral reef ecosystems and our home aquariums what they are. 

This powerhouse is called zooxanthellae, and we are going to tell you everything you need to know about what they are and why zooxanthellae are important in reef aquariums. 

Zooxanthellae And Their Relationship With Corals

Zooxanthellae are single-celled dinoflagellate algae that live inside the tissues of corals, anemones, and other marine invertebrates. They act as nature’s solar panels, working tirelessly within coral tissues, converting light into energy via photosynthesis. 

Zooxanthellae is not a casual roommate situation - the coral provides the zooxanthellae with a safe home and with the nutrients needed to thrive, such as nitrogen and phosphorus to thrive. In return, the zooxanthellae provide oxygen and organic compounds, which provide the coral up to 90% of their energy needs. 

Understanding this relationship is key to providing the best care for your corals, from the coral lighting to coral feeding schedules and water chemistry management. 

Why Are Zooxanthellae Important In Reef Aquariums

Zooxanthellae Provide Energy To Corals

Most reef-building corals, like Acropora corals, primarily rely on zooxanthellae for nutrition. This beautiful symbiotic relationship is what allows corals to thrive in nutrient-poor tropical waters where other marine organisms may struggle. 

Healthy zooxanthellae populations provide oxygen during daylight hours (or when lighting is on in an aquarium's case), and help process dissolved nutrients that could cause unwanted algae growth in aquariums. 

Zooxanthellae Boost Coral Color

Zooxanthellae play a significant role in the stunning coral displays that we see in reef aquariums, like epic Zoa gardens. While corals have their own pigments, the golden-brown color pigments from zooxanthellae algae contribute to the overall coloration of corals, plus the zooxanthellae pigments can influence how coral pigments appear. 

When corals become stressed in aquariums, they expel the zooxanthellae during coral bleaching, which is why corals turn pale or start bleaching. To prevent coral bleaching, you should ensure the aquarium conditions meet your coral’s needs. 

Can Corals Live Without Zooxanthellae?

This is probably one of the most common questions asked in the hobby. While some corals can survive without zooxanthellae, most corals cannot thrive for a long time without their symbiotic partners.

Non-photosynthetic corals (NPS corals )like sun corals naturally lack zooxanthellae and rely on capturing water in the aquarium column. If you have NPS corals, you will need to frequently feed them and provide excellent water flow so they feel at home. 

Photosynthetic corals that have expelled their zooxanthellae due to coral bleaching can temporarily survive, but they are essentially starving in silence. So, being the good aquarium hobbyist you are, they will need immediate intervention to survive. This includes providing the coral with reduced lighting, stable water conditions, and supplemental coral feeding while the coral re-establishes a zooxanthellae population. 

The Importance Of Aquarium Lighting For Zooxanthellae

Zooxanthellae need specific lighting conditions to photosynthesize properly, and getting this wrong is one of the fastest ways to kill expensive corals. 

Light Intensity 

Zooxanthellae require adequate PAR to function, however, different coral species have adapted to different light intensities. For example

• High-light corals (most SPS corals): Most require PAR levels of 200-400. These corals naturally host more zooxanthellae than their coral relatives. 

• Medium-light corals (most LPS and some soft corals): These thrive with a PAR level of 100-200. 

• Low-light corals (soft corals and some LPS corals): These prefer PAR levels of 50-150. 

Light Spectrum 

Zooxanthellae are best supported with blue lighting with some red wavelengths. Your lighting should be around 440nm of blue light and 68nm of red light. Modern LED lighting systems, like lighting at Reefco Aquariums, provide a full spectrum of lighting. 

Photoperiod & Acclimation

Zooxanthellae need consistent light cycles to survive. At Reefco Aquariums, we recommend running the photoperiod for 8-10 hours. However, always check your coral’s specific lighting requirements. 

Also, for the newbie reefers out there, you must always gradually acclimate properly when introducing new corals or upgrading the aquarium lighting. 

Controlling Zooxanthellae Populations In Reef Tanks

If your corals are turning brown or looking dull, you may have too many zooxanthellae. To control zooxanthellae populations, you can:

• Lower nitrates and phosphates to gradually starve excess zooxanthellae without stressing corals

• Reducing the aquarium lighting intensity (PAR level) slightly to limit zooxanthellae reproduction - this should be done gradually over a few weeks to prevent coral stress

• Increase the water flow to prevent dead zones where nutrients often accumulate

The goal of controlling zooxanthellae is not to eliminate the zooxanthellae but to maintain healthy populations to support your corals. 

Zooxanthellae F&Qs

Can Zooxanthellae Be Bad For Corals?

Zooxanthellae are beneficial for corals; however, like any relationship, problems can happen if the balance is disrupted. Overpopulation of zooxanthellae can cause corals to turn brown when the zooxanthellae mask the coral’s natural fluorescent proteins. This is most common when the water temperature increases and the zooxanthellae become toxic, forcing the coral to expel the zooxanthellae in self-defense. This is why temperature stability is essential in reef tanks. 

Do Coral Fish Eat Zooxanthellae?

Yes, many reef fish do eat zooxanthellae, although it is not their ‘go-to’ meal. Fish that are known to graze on algae or nip on coral polyps, which contain zooxanthellae, include: 

• Tangs and surgeonfish

• Some angelfish

• Parrotfish

How To Add Zooxanthellae To Reef Tanks?

Most hobbyists don’t need to add zooxanthellae to their aquarium systems, as the algae are naturally present in healthy coral tissues. However, there may be situations where supplementation may be needed.

The best way to introduce zooxanthellae to reef tanks is through healthy corals that already have a thriving zooxanthellae population within their tissues. They will naturally release zooxanthellae into the water, where they can settle on surfaces or be absorbed by other corals. 

Conclusion

Zooxanthellae are the foundation of successful coral keeping. Every decision you make as a hobbyist, from selecting what aquarium lighting to how you feed your corals, will lead to your corals’ health success. 

If you would like to learn more about the role of zooxanthellae in reef aquariums or what lighting we recommend to support this delicate balance, reach out to the Reefco Aquariums team today.