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Let’s talk about the biofilm – WS Mexico

- Mexico 2015
As a coral larva you have a once-in-a-lifetime chance to find the perfect spot to settle and start growing into a thriving big colony. A bad choice cannot be undone, because as soon as you have glued yourself to the ground, you’ll stick to it, no matter what. So the question for SECORE scientists is: What makes a good spot to settle?

What are coral larvae looking for, when they choose their future home? And can we provide a perfect substrate to promote their settlement and growth?

“Coral larvae definitely own sensors for light and hydrostatic pressure, which help them to find a reef in their preferred water depth” says coral expert Prof. Daniel F. Gleason from Georgia Southern University. But just like humans do, the larvae do not pick any area in reef town. The naked surface of a rock for instance would not make it onto their top 3 list. The animals look for a prepared space instead – most likely for a rocky spot with a pink crust of coating.

“This pinky-reddish crust is made by crustose coralline algae, which play an essential role in a coral reef. They grow as a crust over and between the fragments and gaps in the reef and cement the coral bricks together,“ says Dr. Eugenio Carpizo Ituarte. “They are also key inducers of coral settlement and metamorphosis, sending out chemical signals to floating coral larvae, stating: ‘Settle here – this is the perfect spot’.”

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However, crustose coralline algae are not the only organisms, growing on rocky surfaces. Together with bacteria, green algae, and many other microorganisms, they form a complex biofilm that is one of the main requirements for a successful larvae settlement for most of the coral species.

So when SECORE scientists were planning this year’s Puerto Morelos restoration project, they counted in at least two months to get a sufficient amount of biofilm on their substrates. “In May, our partner at UNAM field station put 2000 new cement substrates into the sea. When we checked them today, we saw that their former grey surface was dotted with some green and many pinky-reddish blots – just the way we want them to be,” says SECORE coral restoration specialist Dr. Dirk Petersen.

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The green dots made by green algae had to be removed: “Green algae have long filaments, in which a coral larvae might get tangled up. Furthermore they compete with coral larvae for space and light, so we have to get rid of them”, says Dirk Petersen.

One substrate was saved for further inspection under a microscope. All of the students looked at its impressive biofilm and learned about its function. The other 1999 substrates were moved closer to the reef. “We are going to keep them there until next week. This way we’ll get a biofilm cover that is similar to the one in the reef and will hopefully help us to maintain a high settlement success”, says Dirk Petersen.

While talking to the students about the difference between a biofilm made of green algae and a biofilm mostly made of crustose coralline algae, Eugenio Carpizo Ituarte used a great metaphor we want to share with you. He said: “Think of a coral larva as a human, who is going for a parachute jump at night. When you look out of the plane, you see a flat, soft golf course beneath your feet, which is surrounded by a lot of forest. For coral larvae the patches of green algae feel like a forest. You definitely do not want to get tangled up there. You would rather land on the flat golf course with enough space to be safe and furthermore catch as much sunlight as possible during daytime – and that is what crustose coralline algae are offering: a perfect spot to settle, turn into a polyp and start growing into a strong new colony.” That is easy to remember, isn’t it? 

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Columbus Zoo and Aquarium
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California Academy of Science
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