The Incredible Ulva
Possible ‘slender’ mutation of Ulva lactuca grown in a lab Photo: Seas of Norway
Seaweed are primitive plants, but their leaves are actually called thalli or blades. Ulva thalli are only two cells thick (about ¼ thickness of paper) but with the help of their bacteria buddies, thalli can grow as much as 40 % in one day! This kind of prolific growth has earned the species, Ulva prolifera its name and reputation as the major former of “green-tides”. If you haven’t heard of green-tides before, they’re algal blooms that are sparked by an overload of nutrients in the water. These days, such large amounts of nitrogen and phosphorous often have anthropomorphic origins like agricultural runoff or wastewater. It’s called eutrophication and it allows fast-growing algae such as Ulva to explode in numbers and take over entire ecosystems. This super-growth can have devastating impacts on biodiversity and lead to anoxic environments that are completely depleted of oxygen, but it also has many people wondering, could Ulva’s explosive growth rate be harnessed?
Green-tide smothers Qingdao coastline in China before the 2008 Olympics. Photo: SCMP
So, for good or for evil, Ulva is a super-alga, but it also has a kind of kryptonite. For Ulva to unlock it’s full potential, the key is a compound called Thallusin. Unfortunately for Ulva, they cannot create it themselves, so they need to get it from an external source. And it’s really quite essential, a lot more serious than your average need for coffee in the morning. Without it, they literally struggle on a cellular level. Thallusin is necessary for proper gene expression in Ulva, it stimulates cell division, cell growth and rhizoid and cell wall formation. In its absence, such as in sterile laboratory environments, several kinds of mutations have been observed. Some species become hollowed out slender tubes, others form a callus-like morphology or a bubble and some even grow to resemble miniature pincushions. There are many possible abnormalities that can occur, all a long-way from the mighty lettuce leaves we see in green-tides.
So, you find yourself reading about seaweed. Brilliant! There’s a lot more to it than meets the eye. Yes, seaweed is algae, like the slimy stuff that makes rocks slippery but seaweed refers to the larger types such as kelp and sea lettuce. Surely you’ve heard of sea lettuce? If it was a superhero, it would be The Incredible Hulk. It grows super fast, it’s been known to mutate, and of course, its bright green. Ulva is it’s scientific name but it gets its common name “Sea Lettuce”, because, you guessed it, it resembles a head of lettuce.
Not as popular in salad as land lettuces (though it contains a lot more protein than most terrestrial plants), Ulva is a common along coastlines around the globe. It provides important habitat for small creatures and is a food source for those of all shapes and sizes, including people. Our interest in sea lettuce goes beyond the sushi bowl, it has been used to make biofuels and any number of products after a bit of processing. Ulva has also gained the attention of evolution scientists when research revealed an intimate relationship between the algae and certain marine bacteria, without which, The Incredible Ulva mutates into something… unexpected.
Growth represented by percentage coverage of lab grown Ulva. Photo: Seas of Norway
For its source of Thallusin, Ulva has developed a cross-kingdom relationship, and like all successful relationships it begins with communication. When zoids are released from the sporophyte Ulva, they swim around searching for chemical signals that will lead them to specific marine bacteria. When they sense the bacteria is nearby, they stop swimming and start tumbling. This usually leads to a bunch of zoids tumbling on the same patch of bacteria, usually on a hard surface like a rock. If the chemistry is right then this is where the romance begins. As the zoids germinate they form a mucilage layer which the bacteria use as a medium to enter the Ulva cells. Remarkably, the Ulva cells can regulate and control growth of inappropriate bacteria and other epiphytes by producing a kind of antibiotic.
Symbiotic relationships are not uncommon in nature (you can read about them here). However, it’s not every day you see two species from different kingdoms communicating on a cellular level and assisting with something as crucial as gene expression. This alliance is no doubt very old. It therefore offers intriguing insights into the origins of multicellularity and cellular differentiation. Certainly, there is a lot more to the common sea lettuce than meets the eye. As our understanding of it improves, so do our chances of harnessing its potential as a sustainable resource, and avoiding destructive and smelly green-tides.