A coral reef has more in common with a forest than you might think. When sunlight strikes a group of trees, some parts tend to get more sun than others.

Leaf tissue somewhat compensates for this by scattering light outward, helping to illuminate other leaves. A similar thing happens with coral. When scientists shone a laser at a coral, the coral’s colorful tissue spread the light, generally redistributing it to other parts. Coral’s white calcium carbonate skeleton also gets in on the action. But it tends to spread light less, helping instead to focus it on specific areas that would otherwise be in the shade. 

Coral can also rearrange themselves depending on the circumstances, expanding to increase the spread of light or contracting to minimize it. Yet even with all of these tricks, light can only penetrate so far into coral tissue; it tends to drop off the deeper you go, as in forests, making getting by more difficult for cells at the bottom of tissue. 

So just as different cells in a leaf contain different amounts of chlorophyll, coral cells seem to house different amounts of the photosynthetic algae that makes their food, Symbiodinium. Cells specialized for low light to still make decent amounts of food in dim conditions, as measurements of photosynthesis, showed. Snorkelers visiting coral may not notice the canopy below, a much smaller and subtler affair than the lofty bowers of forests on land. But the researchers argue that understanding it is crucial for understanding how life gets by on the sea floor.
The Microbe That Invaded Caribbean Coral Reefs
Think of giant pythons from southeast Asia, ending up in the Florida everglades and suffocating any small mammal they could find. Think of cane toads from South America, relentlessly marching over ...
By Ed Yong

Think of giant pythons from southeast Asia, ending up in the Florida everglades and suffocating any small mammal they could find. Think of cane toads from South America, relentlessly marching over …

By Ed Yong

SKETCHY SUNDAY: Symbiodinium spp.

Corals have developed an important symbiotic relationship with photoautotrophic dinoflagellates, i.e. zooxanthellae, which live inside coral tissue for protection and inorganic nutrients. In return, the algae produce a photosynthate that provides the coral with energy to complete growth, reproduction, and calcification processes.

This is a drawing of an individual Symbiodinium, which populate a coral along the lines of hundreds of thousands per square centimeter.

Drawing by Stacy Peltier, 2013

Coral reefs: we know about the bleaching, but we’re learning about the healing.

I just read two article about coral bleaching. One tells the usual sad story, but the other is telling a “and they lived happily ever after…….surprise!” story.

The first one was published in Daily  Kos, which summarizes research into the process by which corals bleach in warm ocean water. The article includes a video. I found a related video which is a little longer and a little more gross…… looks like the coral is doing a major gag before it heaves. Really. Plus the coral is green, so my description is apt. If you want to read the article in Daily Kos, here’s the link.

Anyway, here’s what you see in the video:

“Our H. actiniformis used a pulsed inflation to expel Symbiodinium over time (seen as greenish plumes in the video) – inflating their bodies to as much as 340 per cent of their normal size before suddenly and violently contracting and ejecting Symbiodinium through their oral openings over the four to to eight day duration of the experiments,” Dr Nothdurft said.

Got that? No? Neither did I. But when I read that and then watched this video, I sort of got it.

But then I read an article in the New York Times entitled, “Giant Coral Reef in Protected Area Shows New Signs of Life.” Here’s the link. According to the article, scientists were pleasantly surprised to find a patch of coral reef in the Pacific Ocean that appears to be healing, even though the ocean temperatures are warmer than normal. The area is in a marine preserve, so the absence of a lot of human interference through commercial and recreational fishing may be helping. Excerpt:

In 2003, researchers declared Coral Castles dead.

On the floor of a remote island lagoon halfway between Hawaii and Fiji, the giant reef site had been devastated by unusually warm water. Its remains looked like a pile of drab dinner plates tossed into the sea. Research dives in 2009 and 2012 had shown little improvement in the coral colonies.

Then in 2015, a team of marine biologists was stunned and overjoyed to find Coral Castles, genus Acropora, once again teeming with life. But the rebound came with a big question: Could the enormous and presumably still fragile coral survive what would be the hottest year on record?

This month, the Massachusetts-based research team finished a new exploration of the reefs in the secluded Phoenix Islands, a tiny Pacific archipelago, and were thrilled by what they saw. When they splashed out of an inflatable dinghy to examine Coral Castles closely, they were greeted with a vista of bright greens and purples — unmistakable signs of life.

To understand the stresses facing corals — from pollution and climate change, for example — researchers would like to isolate each problem. Almost everywhere on Earth, corals must endure climate change and human activity. But not in the 157,626-square-mile Phoenix Islands Protected Area, created by the government in 2008. Shipping lanes skirt the preservation area. Commercial fishing there ceased last year.

Dr. Rotjan, who is also the chief scientist for the area’s conservation trust, said the recent protections might have fostered the coral rebound. The algae that live in corals may also be evolving to cope with warmer temperatures, or hardier coral species may be supplanting others, she said.

In a letter published in Nature earlier this year, another global team of researchers reported a similar coral recovery after they reduced the acidity in three lagoons in the southern Great Barrier Reef, off Queensland, Australia. Carbon emissions increase the acidity of seawater.