Peter S Vroom & Celia M Smith. American Scientist. Volume 89, Issue 6. Nov/Dec 2001.
Imagine for a moment that an alien organism is after you. And suppose further that your otherworldly adversary defies being killed: You slash at it with a knife only to find that its severed limbs stop oozing blood and quickly regenerate. You chop it into pieces, but each tiny sliver regrows into a complete organism. This scene, which could have been taken from a 1950s science-fiction film, is actually taking place in the Mediterranean Sea, where the frightening alien is not a space monster but an exotic form of marine algae called Caulerpa taxifolia. Native species of marine plants are unable to compete with the ability of Caulerpa to proliferate. Indeed, this plant spreads so fast that its range in the Mediterranean expands on average by some 50 kilometers each year.
What makes this organism so unstoppable? The answer can be understood, in part, by first recognizing that Caulerpa belongs to a unique group of marine plants called siphonous green algae (order Bryopsidales). Athough they all possess the same qualities that make Caulerpa such an effective invader, most siphonous green algae do not overwhelm their aquatic habitats. Indeed, some species survive only under the most ideal conditions. We have been studying how siphonous green algae grow and reproduce in hopes of discerning better the evolutionary relationships between the different species, understanding how they affect the health of tropical ecosystems and figuring out what exactly causes the proliferation of “weedy” types in some environments.
Siphonous green algae are unlike most other plants, which typically use different compartments, or cells, for specific purposes. For instance, in an average house plant, millions of cells in the leaves carry out photosynthesis, the process that converts sunlight into food. Specialized, nonphotosynthetic cells transport water, move sugars around and provide structural support. To survive, such a plant requires millions of these cells functioning together. And as any gardener knows, the permanent loss of one part-say, the roots or the leaves-kills the plant.
Unlike roses or forsythia, marine plants have the luxury of living in an environment where such specialized structures are not required. Because water is so much denser than air, marine plants are buoyed upright and do not need the rigid internal supports typical of their terrestrial counterparts.
And being surrounded by the sea, they need no elaborate vascular system to transport water. Consequently, most of the algae one finds growing, say, on a tropical reef consist of little else than millions of photosynthetic “leaf” cells.
Siphonous green algae simplify their anatomy still further by forming themselves into one big compartment. With this configuration there is a lot less cell wall to make, potentially allowing growth to take place quite rapidly. Although the most efficient geometry in terms of minimizing cell wall would, in theory, be a large sphere, this shape is not common in reef settings, probably because it offers little opportunity for the plant to anchor itself to a surface and survive the turbulence that waves create. Rather, these organisms take the form of tubes (or “siphons”) arranged in elaborate networks that, in some genera, can affix to the sea floor at many points. Indeed, siphonous green algae use this general body plan to produce an amazing array of sophisticated structures-some resembling leaves and stems-all with a single cell.
Living Garden Hoses
An easy way to visualize these peculiar plants is to imagine a living garden hose. The outside of the hose is the cell wall. The interior provides room for all the other essential components needed in eukaryotic cells (complex cells that contain nuclei and other specialized organelles). Strangely, most of this space is taken up by nothing more than the central vacuole, essentially a bag filled with water and ions. The vacuole pushes against the inside of the cell wall, forming a characteristic shape for each species. The thin layer of cytoplasm sandwiched between the cell wall and the vacuole contains everything else: nuclei, chloroplasts, mitochondria and various other cellular constituents.
The hoses, or siphons, in 14 of the 32 genera of these algae are arranged rather simply, like delicate branches.
These genera are termed uniaxial by phycologists, the scientists who study marine algae. The least complicated geometry, seen for example in Derbesia, consists of siphons that divide in two again and again. In other uniaxial varieties, such as Bryopsis, the siphons look more like feathers than branches, whereas in Caulerpa, the siphons can assume a dizzying range of shapes: broad leaves, thin blades, cacti or even clusters of grapes. Internal extensions of the cell wall support these large, complex arrangements.
The 18 remaining genera are considered multiaxial, because their branching siphons weave around one another to form the fleshy bodies of these plants. The most common configuration (seen, for example, in the genus Avrainvillea) looks somewhat like a fan. But everything from umbrellas to stalked cups can also be found easily in a day’s exploration on many tropical reefs. Some multiaxial genera (notably, Halimeda) have developed bendable bodies in which broad, plate– shaped segments alternate with thin, flexible joints. Other genera (in particular Codium) appear as felty, green fingers. The variety is enormous, but even the most complex multiaxial species, if somehow unraveled, would reveal itself to be a single undivided siphon. So cytoplasm is free to move from one end of the plant to the other-which can be tens of meters away in some extreme cases.
Multiaxial species seem better able to endure turbulent settings than their uniaxial relatives, which favor sheltered lagoons. Why? Just as a single strand of rope or fishing line has limited strength in comparison with a braided line made of the same material, so too with these algae, which attach to the floor of the reef using a series of small colorless siphons, termed rhizoids (part of the overall unicellular structure). In some sandy locales, large bulbous aggregates anchor the algae to the seafloor. Where the substrate is harder, many individual rhizoids affix the plant to the bottom. Interestingly, experiments have shown that the rhizoids of at least some siphonous algae absorb nutrients, which can then move rapidly to other areas of the body, a feature paralleling the function of roots in terrestrial plants. But siphonous green algae, being composed of enormous cells, are unlike land plants in most other ways. Indeed, in their cellular construction, they differ markedly from most other eukaryotic organisms.
Most plant and animal cells, for example, are measured in micrometers, with one nucleus being normally sufficient to direct all the goings-on within. But the giant cells of siphonous green algae are millions of times larger. Caulerpa, for instance, readily grows 80 centimeters tall in the Mediterranean. And if all the siphons in a 20-gram Codium plant (one small enough to fit in a demitasse cup) were unwound and placed end to end, they would stretch almost 30 kilometers! Such an extended cell requires billions of nuclei. To attain the requisite number, these organisms have uncoupled the processes of nuclear and cell division. Nuclei divide rapidly, along with chloroplasts and mitochondria. Indeed, millions of nuclei flow constantly around these large single cells, which do not divide but rather grow by expanding their walls. How then do these peculiar organisms reproduce?
Giving Up on Sex?
Human beings, like most animals, procreate by producing gametes (eggs and sperm), which fuse to form an offspring who ends up looking like a small (often endearingly cute) version of its parents. Although this strategy works well for people, many siphonous green algae do not follow this time-tested formula. Rather, they produce gametes that fuse to form an organism quite unlike its “parents”-so different, in fact, that a casual observer would not guess they were related. The newly formed entity grows and produces asexual spores, which when released into the environment eventually become adult copies of the original organism. This type of life history has two independent, free-living forms, which can resemble each other so little that they challenge an investigator’s ability to figure out just what is related to what.
In some species of siphonous green algae, the large plants commonly found in reef settings represent a gamete-producing phase, whereas their asexual phase results in a plant that is microscopic. Other species exhibit exactly the opposite pattern: A dominant, asexual configuration alternates with a microscopic gamete producer. Yet other species exhibit life histories more like our own, with only a single (gamete-generating) form. In this regard, this one order of green algae displays more diversity than is found anywhere else in the entire plant kingdom.
Even more intriguing than the range of sexual life histories seen among these organisms are their clonal abilities, whereby a single individual can produce genetically identical offspring by breaking apart and regrowing. This means of procreation is quite surprising. After all, when a tree is chopped up into little bits, the pieces do not grow back. With these algae, they do. The ecological implications of this reproductive strategy are mind-boggling. Instead of being vulnerable to, say, storm damage, these plants benefit when they are smashed to pieces, each of which can then sprout and become an adult.
We began studying this curious trait in earnest in the early 1990s, when Takaaki Kobara of Senchu University in Japan demonstrated to us how to culture tiny fragments of Bryopsis, a simple, uniaxial alga that typically reproduces sexually. Because the life histories of Hawaiian representatives of Bryopsis had never been studied, we thought that culturing native species might reveal some new insights about their biology. We then observed that excised siphon tips as small as a millimeter across-chunks that weighed less than a milligram-grew into 60gram “pompons” within just weeks. (Indeed, we were so amazed by the clonal abilities of these species that we almost forgot to examine their sexual reproduction.)
About the same time that we were studying Bryopsis, a colleague in our lab, Linda Walters, was cutting up different kinds of reef algae to examine how readily invertebrates could settle on their surfaces. The aim of her project was to discern whether these plants had physical or chemical defenses that might be exploited to produce novel antifouling compounds for ships. Along the way, she discovered that tiny pieces of the siphonous green alga Halimeda remained alive after cutting and produced new attachment rhizoids. Walters then began laboratory and field experiments to determine the smallest possible fragment that could remain alive, and whether the type or orientation of initial injury played a part in determining survival. In the course of this study, she conducted some jaw-dropping experiments at the Waikiki Aquarium, where she documented that herbivorous fish will take bites of Halimeda but then spit them out, whereupon the tiny morsels can regrow into new plants. Who would have suspected that grazing fish might be helping to increase the population of a marine plant?
As our laboratory work continued, Mother Nature stepped in and provided a natural experiment: Hurricane Iniki passed south of Oahu on September 11, 1992, roiling the local waters. Within hours, members of our group were out in the field looking for hurricane–generated fragments of Halimeda. To our delight, many such pieces drifted to shore. When collected and cultured, they remained healthy and quickly developed attachment rhizoids. So we knew that, for Halimeda at least, clonal reproduction through fragmentation must take place naturally. We then wanted to find out how often such episodes happen and how much proliferation ensues.
Getting to the Bottom
The opportunity to probe those questions in a meaningful way came in 1994, when Walters and one of us (Smith) began a research project that would galvanize our lab for the next seven years-studying Halimeda at the National Undersea Research Center in Key Largo, Florida. This center, a joint venture of the National Oceanic and Atmospheric Administration and the University of North Carolina at Wilmington, operates Aquarius, the world’s only underwater laboratory. It is located on Conch Reef, which lies several miles offshore. For our project there, we tapped scientists from nine other institutions, including the University of Groningen, the University of Tampa, the University of Central Florida, the University of California, Santa Cruz, the University of Alaska, Fairbanks and Moss Landing Marine Laboratory.
Four members of the team lived and worked in and around the Aquarius habitat at a depth of about 40 feet for 10-day intervals. The great advantage of working from a submerged habitat is that a diver’s “bottom time” is not limited by the risk of suffering from the bends, the malady that ensues when nitrogen slowly dissolves into the blood at depth and then bubbles out dangerously after he or she surfaces. Normally, a scuba diver can remain at 40 feet for only 130 minutes. But living in Aquarius gave members of our group access to deeper-water populations of Halimeda for up to nine hours a day. At the same time, a team of surface divers studied Halimeda growing at a depth of about 20 feet. The technical support provided through this federally sponsored research facility, combined with the chance to work on the relatively undisturbed reefs of the Florida Keys National Marine Sanctuary, offered us an unparalleled opportunity to examine the ecology of this plant.
Halimeda is special because it is one of a few genera of siphonous green algae that produce calcium carbonate, the same material that clams use to construct their shells and corals use to build their skeletons. The observation that Halimeda contains calcium carbonate caused much confusion during the early 1800s, leading scientists of the time mistakenly to place this alga along with coral in the animal kingdom. The calcium carbonate, which fills gaps between the siphons of the plant, gives Halimeda a stony texture, allowing it and its evolutionary ancestors to survive in the fossil record. Indeed, fragments of dead Halimeda are quite robust: The tropical white-sand beaches one finds in Florida and elsewhere are normally thought to be the remnants of pulverized corals, but this is not always the case. While diving we see many patches of sand made up primarily of whitened fragments of Halimeda. The tiny pieces often remain relatively intact, allowing us sometimes to identify the particular species from the distincive shape of the sand grain.
Our research in the Florida Keys, which is far from complete, aims to answer several questions about this algal species. How, for example does the abundance of Halimeda affect the many other organisms living on the reef? How does this alga respond to changing environmental conditions? How does depth affect growth rate? And does clonal propagation increase the density of Halimeda after large storms, as our Hawaiian experience would suggest?
To address such unknowns, we began a suite of observations and experimental manipulations. Perhaps the most straightforward component of our work has been in monitoring the abundance of this alga. Some scientists concerned with the degradation of coral reefs around the world have suggested that increases of Halimeda and other algae indicate deteriorating ecosystems. But in monitoring the reef floor at least once and often twice a year, we found that although coral cover has declined since 1994, the abundance of Halimeda has remained relatively stable. Thus it does not appear to be displacing other organisms. But the observation that the plant is maintaining an equilibrium says nothing about whether its means of reproduction is sexual, clonal or a combination of both.
To test whether clonal reproduction could be significant, Walters gauged the intensity of fish-grazing and counted the number of Halimeda fragments in prescribed study areas. Naturally produced fragments could be found at low levels under most conditions, and we observed an elevated number immediately following the passing of Hurricane Irene in October 1999. Walters found that some fragments could survive being buried shallowly in sand for months before conditions were right for their growth. Our work on Conch Reef indicates that at least part of the population is clonal and leads us to wonder about how much genetic diversity is, in fact, present on a single reef.
We also conducted studies to determine whether sexual reproduction is taking place. Halimeda, like many other siphonous green algae, exhibits a curious reproductive strategy. Gamete-producing structures called gametangia develop along the upper margins of each segment over a one-day period, and the cellular constituents of the entire plant are diverted into them. The gametangia then turn a deep green while the rest of the plant fades to white. We found that about 5 percent of the population develop these reproductive structures over a period of several hours. Then, just before dawn, all reproductive individuals release their gametes within minutes of one another, leaving behind nothing but blanched husks. The whole process lasts about 36 hours, and once the plants have reproduced, they die. Sexual reproduction thus removes adults from the population. The whole process takes place so quickly that few investigators have witnessed it in the field: If you quit early one day from field work, you miss the entire cycle and find only your tags lying in the sand where adults once stood.
In other experiments, we colored the calcium carbonate of plants pink by enclosing certain specimens overnight in bags of stain. New, green segments that developed after staining could easily be distinguished from the older pink segments, giving us a way to assess their growth. Using this technique, we discovered that plants from different parts of the reef grow at different rates, which can be attributed perhaps to variations in dissolved nutrients or in their ability to harvest the available light.
Some of the plants we studied grew at fantastic speeds (adding as many as 25 segments a week in some cases), which goes to show that not having to produce internal cell walls allows these algae to increase in size extraordinarily rapidly. The growth takes place in a manner akin to a long balloon inflating: Just as air forces the balloon to expand, pressure from the central vacuole forces a siphon to increase in size. This internal pressurization is most evident when a siphon is cut. Green “blood” shoots forcefully out of the wound, just as soda shoots out of a shaken bottle. But miraculously within 2 to 3 seconds of such an injury, the cell plugs the rupture, and regrowth slowly ensues. Siphonous green algae are indeed a hardy lot.
Ecological Success Story
So what does all this tell us about the ecological importance of these organisms? Clearly, these algae have advantages not held by other marine plants. For example, at night, when photosynthesis does not take place, Halimeda pulls all of its chloroplasts from the siphon tips, which protrude out of its central core of calcium carbonate. The one-compartment structure of the plant allows it to shift cellular constituents for this strategic withdrawal, which is thought to minimize what the many marine animals grazing at night are able to consume. How many organisms can so elegantly and effectively preserve their most important assets?
Such tactics, along with their ability to fragment and regrow, give siphonous green algae a competitive edge over many multicellular marine creatures. Nowhere is their success more apparent than in the explosive spread of Caulerpa taxifolia in the Mediterranean and of Codium fragile in the nearshore waters of the American Northeast and off the coasts of New Zealand. Both these algae are exotic to those locales, where they can aggressively outcompete a diverse array of native species.
Such invasions are worrisome because they upset ecological balances in natural communities. They can also have some serious economic consequences. For example, the Codium introduced into New England waters often settle on the oyster and scallop shells found in commercial fishing beds. Codium attain heights of 20 centimeters and weigh tens of grams. Their shape and size cause them to be swept away during storms, taking the underlying scallops and oysters with them. They are thus responsible for episodically decimating the local shellfish industries.
Caulerpa taxifolia now dominates much of the subtidal terrain in the Mediterranean, where it was accidentally released from a Monaco aquarium in 1984. Although the same species is common in Hawaiian waters, it remains sparsely distributed; what keeps it in check is a mystery. Many regulators and environmentalists are concerned about the discovery of this species at two sites in California in 1999, because it is impossible to guess whether a massive proliferation could ensue there too. Both Caulerpa and Codium can reproduce clonally, making eradication efforts difficult: Unless every single microscopic fragment is removed from an area, the plants quickly reappear.
These two species are indeed weedy pests in some regions, yet other species of siphonous green algae are so specialized that they are truly rare. Boodleopsis hawaiiensis, an endemic Hawaiian alga, lives only in splash-zones underneath overhangs of coral islands and requires pristine conditions. In the places where it resides, it forms dense mats, but because suitable habitats are rare, this species is now quite limited-possibly to one site in the world. B. hawaiiensis could well become the first alga to be listed as an endangered species.
Beginning students of marine biology may think that all the easy questions have already been answered. For these important reef algae at least, nothing could be further from the truth: Descriptions of their life histories, reproductive structures and ecological distributions are still completely lacking for about a third of the known genera. Environmental managers need such fundamental information to assess the health of tropical ecosystems. Without such knowledge, how will scientists ever understand what is natural for a reef? And without a good handle on the ecology of undisturbed areas, how can people ever hope to control the weedy varieties? Society could indeed benefit from more scientists doing research in this field. The pay is not great, but the fringe benefits are excellent, especially for people who like working in idyllic tropical locations. The biggest challenge, we have discovered, is diving every day in cobalt-blue waters without becoming too distracted by all the dazzling fish.