An Old Grass and the Sea
The oldest-known organism in the world has been constantly cloning itself on the floor of the Mediterranean Sea for some 200,000 years. Neptune grass was colonizing coastal and offshore waters when Woolly mammoths reigned, during Alexander the Great’s wars, and while da Vinci dabbed at The Mona Lisa. Rootstalks of Posidonia oceanica creep horizontally by, at most, two inches per year. Yet today, plants that began with a single ribbon-like shoot weigh more than 13 million pounds and span 10 square miles. But even their long and steady growth can’t make up for a disturbing modern trend: for the past two decades, coastal seagrass beds worldwide have been shrinking. P. oceanica has been dying off at a rate of 5 percent per year.
In an effort to understand why Neptune grass is disappearing so quickly, marine biologist Sophie Arnaud-Haond and colleagues at the French Research Institute for the Exploration of the Sea began studying its DNA. They discovered that the oldest clones of P. oceanica predate -- by as much as 160,000 years -- a species of Tasmanian seagrass previously believed to be the world’s longest-living organism. That means that Neptune grass probably emerged in the late Pleistocene and survived the last ice age, when sea temperatures were 10 to 15 degrees Celsius cooler than they are today and ocean levels were hundreds of feet lower. Cloned shoots of the plant carry exact replicas of the original DNA, so while individual grass blades die out, the plant itself keeps growing, multiplying its bright-green tendrils into vast meadows. Those meadows provide a crucial habitat for a host of creatures such as mussels, crabs, fish, sea anemones, and sponges. Seagrass also acts as a major carbon sink, slowing global warming by storing CO2 that oceans absorb from the air.
P. oceanica plants are capable of reproducing sexually -- by flowering, fertilizing one another, and releasing seeds. So the fact that the original plants haven’t coupled with others to recombine their DNA is noteworthy. It suggests that Neptune grass is remarkably adaptable to a wide range of environmental conditions. It also means that, evolutionarily speaking, cloning may be a better reproductive strategy than scientists thought. In theory, asexual reproduction -- practiced by single-celled organisms like bacteria, as well as a number of plants and fungi -- should be "an evolutionary dead end," Arnaud-Haond says. Any genetic mutations that arise over time will multiply unchecked, even if they make a species less fit, and that can lead the entire population straight to extinction. Sex, on the other hand, presumably evolved to give complex organisms a chance to create healthier offspring: natural selection constantly determines which new gene combinations are best-equipped to survive in a specific habitat. But P. oceanica seems to have found a way to produce thousands of generations of clones that never accumulate flawed DNA. At least for this seagrass, sex may do more harm than good: why add and subtract ingredients from what appears, based on the species’ persistence, to be a perfect genetic recipe?
Unfortunately, it looks as though humans may be changing the environment perilously fast even for plants that, as Arnaud-Haond puts it, have been "perfecting their strategy for millennia." She says that the decline in Neptune grass "was likely triggered by increased pollution in the environment," including industrial, agricultural, and human-waste runoff from shore. But rising temperatures are making the problem worse. Recent climate change projections suggest the Mediterranean Sea will be at least one degree hotter by the end of the century. Ocean acidification, which is also caused by increased CO2 in the atmosphere, is on track to surpass levels seen in the past 300 million years. Indeed, we may kill off P. oceanica before we learn its prehistoric, DNA-preserving secrets.