In the midst of the dazzling outcrops of Australia’s Great Barrier Reef lies a vast but obscure structure. Unlike its more famous neighbor, this edifice is not made by coral, the little tentacled animals that build the giant, colorful, calcium carbonate structures we know as reefs. Instead, this neighboring feature is a thick pile of the vacated calcium carbonate skeletons of, of all things, a plant.

The drifts of empty skeletons can reach 65 feet thick, sit about 100 feet beneath the surface, and, based on new lidar and sonar data, assume strange patterns: complex nets, rings, and ripples.

Perhaps you’d like to see this plant responsible for all this for yourself. Halimeda is its name and it does a wonderful prickly pear cactus impression. It's also quite popular with home aquarium owners, who find it adapts well to their tanks.

Halimeda is a green alga, an early-evolved plant that makes the same set of chlorophyll pigments that we seen in land-based plants, giving it its comforting hue. So common is this plant, apparently, that there are even pipefish that specialize in imitating it.

Even for an alga, Halimeda’s biology is bizarre. Unlike most terrestrial plants, it builds itself a hard mineral skeleton and suit of armor, making it a sort of vertebrate of the plant world. But instead of calcium phosphate, the mineral that hardens our bones, the skeleton of Halimeda is fortified by calcium carbonate in a sharp crystalline form called aragonite. Calcium carbonate, in other forms, is famous for its presence in cave- and concrete-producing limestone rock, eggshells, seashells and the skeleton of coral.

If a plant with a mineral skeleton is not weird enough, it gets weirder. Inside this skeleton you will not find tightly packed tiny cells, as you might see in a land plant. Instead, the plant grows long, highly branched tubes called siphons (a bit like fungal hyphae), but without any sort of cell walls or divisions.

The plant still divides its DNA and the storage compartment that contains it (the nucleus), but the organism never walls off the daughter nuclei into separate cells as happens in pretty much the rest of life on Earth. That is, the plant is coenocytic – effectively one huge cell -- and all cellular organelles are free to mill about the property.

And they do. One of the more astonishing abilities of Halimeda is its habit of draining its light-harvesting, food-producing chloroplasts into internal storehouses overnight, quite literally turning a bright green plant ghostly white within the space of an hour. The reverse occurs just after dawn.

Credit: Drew and Abel 1992

The plant goes to all this trouble in order to thwart grazing animals like chloroplast-stealing sea slugs that might fancy a midnight snack (such slugs have the amazing ability to hijack the chloroplasts and use them to feed and oxygenate themselves.) Halimeda underlines its disapproval of this behavior by producing foul-tasting toxic chemicals, as well as studding its tissue with the aforementioned nasty sharp pointy aragonite crystals.

There are only a few other organisms on the planet that have the same sort of freewheeling acellular lifestyle as Halimeda, and they are all quite weird. Plasmodial slime molds are most famous, but giant seafloor protists called xenophyophores, glass sponges, and some fungi also possess open offices. I think someone could make a fascinating Ph.D. dissertation (perhaps they already have!) by studying the disparate and distantly related coencytic life forms and attempting to find what commonalities favor this biology.

Another amazing quirk of Halimeda biology is their incredible growth —they double their biomass every 15 days. One individual produced 359 new segments in just 68 days. This is all the more remarkable when you consider these plants often live 100 feet or more below the surface of the water, where light is filtered and waning.

Thus, it is easy to see how giant Halimeda drifts accumulate. As a result of this frantic growth (and also the fact that 90% of the plant is aragonite), great piles of cast off skeletons of Halimeda form on the seafloor in places like the Great Barrier Reef, reaching dozens of feet deep and forming “bioherms”.

This fascinating and vaguely Greco-Roman term refers to any reef-like biological structure made from piles of cast-off, crumbling, and broken shells, spines, skeletons, or other hard bits of marine invertebrates or algae. Sitting atop an active Halimeda bioherm is a green encrustation of living plants which continue building the pile. As the bits accumulate over thousands of years they may eventually cement into rock, becoming a place where the biological rubber meets the geological road.

The incredible volume of abandoned plant parts also means that when they disintegrate, they contribute mightily – sometimes more so than the nearby coral reef -- to the beautiful white carbonate sand beaches that outline the tropics of our planet.

Halimeda bioherms aren’t found exclusively near the Great Barrier Reef. They’ve also been sighted in the Eastern Java Sea near Indonesia, the south-west Caribbean Sea, and in the Timor Sea north of Australia. They seem to form in places where upwelling of cool, nutrient-rich water occurs next to a continental shelf. In the Great Barrier Reef, the necessary injections of fertile colder water occur through narrow breaks in the outer-shelf reefs.

Though the bioherms in the Great Barrier Reef have been known of for decades, no modern survey of their extent had been done until that detailed by a new report in the journal Coral Reefs. The new data indicate they encompass some 6,000 square kilometers -- about the same size as the state of Delaware. The bioherms exceed the area and volume of the adjacent coral reefs, though not of the entire Great Barrier Reef, which measures a more titanic 345,000 square kilometers.

Credit: McNeil et al. 2016

Still, 6000 square kilometers is an impressive size, and three times as large as previous estimates last made 30 years ago. The underestimate was partly due to inaccurate eyeballing of the old maps, which actually enclosed 3474 square kilometers, a fact only uncovered when the old maps were digitized. But the new surveys also revealed a previously unknown large area of bioherms some 1740 square miles in extent on the north end of previously mapped areas.

The Great Barrier Reef bioherms are divided by a conspicuous gap at Princess Charlotte Bay, where the Australian coast takes a sharp turn from north-south to east-west. The scientists hypothesize that this sudden turn disrupts the northerly long-shore current, interfering with the cold water nutrient injections that appear crucial to bioherm formation.

The new maps and scans also revealed startling new structures. Previously the piles of dead Halimeda skeletons were thought to form wavelike parallel ridges and troughs some five to 20 meters thick, with an occasional mound thrown in. New data from airborne lidar -- aircraft laser surveys of the seafloor -- and multibeam echosounders show that the terrain is far more complex.

a) old interpretation of Great Barrier Reef bioherm structure using parallel echo-sounder profiles b) new topgrophy based on 3D lidar bathymetry of the same area. Credit: McNeil et al. 2016

The underwater landscape appears to be a fabric of reticulations, rings, and finally undulations moving from west to east.

A honeycomb-like network with sharply higher, sinuous crests prevailed near the outer barrier reef, and represented about 16% of bioherm surface area. Another 16% constituted ring-shaped mounds around 200-250 meters across. Toward the west as the rings grew sparser they also grew larger. Some reached 500 meters wide – the length of more than five football fields. On the western side of the bioherms lay an undulating landscape of low wave-like ridges. This terrain type made up 68% of the bioherms.

Credit: McNeil et al. 2016

There are many possible explanations for these peculiar shapes, though scientists don’t yet really have any idea which one is correct because they don’t yet have enough data to test them. The contours of the seafloor before bioherm formation may have helped create these shapes. It is also possible the bioherms are prone to collapse in a way that produces networks and rings.

These shapes both puzzle scientists and hint that something very interesting is going on in the lee of the Great Reef. That such an enormous structure could be both virtually unknown by the outside world and constructed of what is essentially a reef-building plant is, in my opinion, simply wonderful.

Reference

McNeil, Mardi A., Jody M. Webster, Robin J. Beaman, and Trevor L. Graham. "New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef, Australia." Coral Reefs: 1-13.