A Seaweed Divided Against Itself Upsets Oceanic Order

Baltic Sea Offshoot Encroaches on a Vital Counterpart

By Kari Lydersen
Washington Post Staff Writer
Monday, August 3, 2009

STOCKHOLM -- Something is happening in the Baltic Sea's underwater forests.

A new kind of seaweed is spreading over hundreds of miles in the north. Scientists call it a "super female clone" because most of the new plants are genetically identical females. Called Fucus radicans, or narrow wrack, it has apparently branched off in the past 400 to 1,000 years from Fucus vesiculosus, or bladder wrack, the tree-like vegetation common in the Baltic and other oceans worldwide.

Researchers are concerned about the ecological implications, since the new species -- or subspecies; some scientists dispute the classification -- is smaller and much less ideal for fish, crustaceans, barnacles and other organisms that populate the sea.

The new seaweed has developed at an "unparalleled speed," according to Lena Kautsky, a marine ecologist based in Stockholm. Kautsky worries that the loss of genetic diversity inherent in the spread of the new species could mean that disease or changing conditions will more easily wipe out large swaths of the older seaweed.

In ecological importance and function, bladder wrack is akin to the sea grass beds in the Chesapeake Bay, whose disappearance has withered the shellfish industry. In addition, Bladder wrack is used widely as fertilizer and in nutritional supplements, since its high iodine content is thought to combat hypothyroidism, and another compound helps reduce heartburn.

Bladder wrack is an indicator of the Baltic's overall health, since it grows well when the water is clear and light gets through. But it is stunted or dies when nutrient-fueled algal blooms block out the light. Kautsky and her brother Hans, also a marine ecologist, have spent years studying the depth at which bladder wrack occurs. (The deeper it grows, the healthier the sea.)

Bladder wrack growth came close to the surface in the 1970s. Since then, the Baltic's Fucus forests have regained some ground in the deep, but with nutrient pollution leading to low-oxygen zones, competition and the unknown ramifications of the narrow wrack clone, their future is still tenuous.

"It's a slowly dying forest," Kautsky said.

Kautsky describes the love lives of bladder wrack in almost poetic terms -- a moonlit ritual not shared by the newcomer. "It's quite romantic," she says.

Male and female bladder wrack reproduce primarily by releasing sperm and eggs into the water twice a month, in the evening, two days before the new and full moons. This precise timing allows sperm to fertilize eggs in the water, then the new embryo sinks, anchors itself to substrate on the sea floor and forms a new plant.

There is virtually no tide in the Baltic, so it is a mystery how the bladder wrack know exactly when to release their gametes. Moonlight and gravity likely play a role.

But narrow wrack shuns this fertilization method. Though it exists in male and female genders and is capable of sexual reproduction, it normally prefers asexual reproduction, spinning off small fragments of seaweed that form new plants.

The northern reaches of the Baltic, far away from the inland sea's narrow connection to the Atlantic Ocean, have a very low salinity that impedes an egg's ability to make sure only one sperm enters it; in such environments, multiple sperm are likely to fertilize an egg, killing it.

Salinity in the Gulf of Bothnia in the northern Baltic is only one-tenth of the level in average ocean water, so it favors asexual reproduction. Scientists think climate change may decrease salinity significantly across the entire Baltic, thanks to much heavier rains diluting the brackish sea. That means the narrow wrack could potentially spread throughout the sea and compete with bladder wrack for space and light.

In a paper published in the peer-reviewed journal BMC Evolutionary Biology in March, Kautsky and her colleagues argued that the rapid development of F. radicans defies traditional evolutionary knowledge. New species generally take thousands of years to differentiate from their forebears, and do so in geographically isolated places such as islands.

"Although the Baltic is a very species-poor sea, it is actually very interesting from the evolutionary perspective, as the odd environment, low salinity, has changed many species rather dramatically over the short history of the sea," said Kerstin Johannesson, a marine ecologist at Sweden's University of Gothenburg. "Sometimes I actually make a parallel to the Galapagos Islands -- an extreme habitat which has forced species to evolve differently in a relatively short period of time."

However other researchers think Kautsky and Johannesson are jumping the gun in declaring F. radicans a new species rather than a subspecies. They say more research is needed on just how different it is from F. vesiculosus. And they point out that there are various other types of Fucus around the world; if F. radicans deserves to be called a species, maybe those do as well.

"This is certainly a distinctive population," said Susan Brawley, a marine biologist at the University of Maine who studied Fucus in the Baltic in the 1990s. "Is it a species? That's where there's far more room for speculation and study."

She thinks F. radicans is reproducing sexually more often than Kautsky and her colleagues realize. She said fertilized embryos are actually attaching to the seaweed itself and then forming new plants when pieces break off, fooling scientists into thinking it is reproducing asexually. Researchers have missed this in part, Brawley said, because narrow wrack does most of its sexual reproducing in July, when Swedes are typically on holiday.

"I wouldn't have done it," she said of designating F. radicans a new species. "But I'm more of a lumper" in terms of classifying species. "Being a splitter is also a valid position," she said.

Ester Serrão, a marine biologist at the University of Algarve in Portugal who studied under Brawley and has collaborated with Kautsky, thinks the species classification is somewhat beside the point.

"The boundary that defines when a differentiated entity can be considered a new species is often fuzzy," she said. "Genetic data show that there is some other type of barrier preventing gene flow between them and causing them to have different genetic characteristics. This is very interesting -- in my view much more interesting than discussing the species name."

Meanwhile, bladder wrack also has another nemesis, the blue mussel, which happens to be the research focus of Kautsky's husband Nils for the past 35 years. Blue mussels and bladder wrack have something of a love-hate relationship, as the mussels filter phytoplankton out of the water, which allows more life-giving light to reach bladder wrack.

But blue mussels compete with bladder wrack for space to anchor on the rocky sea bottom, and in recent decades the mussels have gained the upper hand thanks to an excess of nutrients from agricultural and sewage flows into the Baltic that feed the algal blooms that blue mussels love to eat. These algal blooms are also responsible for eutrophication, the existence of large low-oxygen "dead zones" in the Baltic.

http://www.washingtonpost.com/wp-dyn/content/article/2009/08/02/AR2009080202128.html