Ulva lactuca

Lore & Background

Ulva lactuca is a thin flat green alga growing from a discoid holdfast, with a somewhat ruffled and often torn margin. The membrane is two cells thick, soft and translucent, and grows attached without a stipe to rocks or other algae by a small disc-shaped holdfast. Green to dark green in colour, it is formed of two layers of cells irregularly arranged in cross-section. The chloroplast is described as cup-shaped in some references but as a parietal plate in others, with one to three pyrenoids. Other species of Ulva are similar and not always easy to differentiate.

The life cycle involves sporangial and gametangial thalli that are morphologically alike. The diploid adult plant produces zoospores by meiosis, which settle and grow into haploid male and female plants. When these haploid plants release gametes, they unite to produce a zygote that germinates and grows into the diploid plant.

Ecologically, Ulva lactuca is very common on rocks and other algae in the littoral and sublittoral zones around the British Isles, the coast of France, the Low Countries, and up to Denmark. In the Pacific Northwest, it is distributed from Alaska to the Gulf of California. It is particularly prolific where nutrients are abundant, such as off the coast of Brittany, where high nitrate levels from intensive farming lead to large quantities washing up on beaches. Their decay produces methane, hydrogen sulfide, and other gases, and in August 2009 unprecedented levels caused a major public health scare in Brittany.

Reader's Guide

Ulva lactuca holds significance as the type species of the genus Ulva, providing a taxonomic reference point for related green algae. Its synonym U. fenestrata reflects its 'windowed' or 'holed' appearance, a characteristic noted in the literature. The species is notable for its worldwide distribution, spanning Europe, North America, Central America, the Caribbean, South America, Africa, Indian Ocean Islands, South-west Asia, China, Pacific Islands, Australia, and New Zealand.

Its ecological impact is substantial: in nutrient-rich waters, such as those off Brittany, it can proliferate massively, leading to public health incidents. In August 2009, rotting thalli on Brittany beaches produced hydrogen sulfide, a toxic gas that inhibits cytochrome c oxidase, causing cellular hypoxia. A horse rider lost consciousness and his horse died after breathing the fumes, and a truck driver hauling sea lettuce died from cardiac arrest resulting from pulmonary edema, indicative of possible hydrogen sulfide poisoning. Environmentalists linked these events to excessive fertilizer use and nitrate excretion from livestock.

Culturally, Ulva lactuca is used in Scotland in soups and salads, and in Hawaiʻi it is called limu pālahala and eaten mixed with other algae, salted with raw fish, boiled as soup, or served with chili pepper, onions, soy sauce, and sugar. It also serves as fertilizer and adornment for hula, where it is called limu pāpahapaha. As a biofuel source, it is suitable for transesterification in biodiesel production, grows in saline and waste water, and has a higher ability to sequester atmospheric carbon dioxide than terrestrial energy crops.

Did You Know?

• Ulva lactuca is the type species of the genus Ulva.
• Its synonym U. fenestrata refers to its 'windowed' or 'holed' appearance.
• In August 2009, unprecedented levels of U. lactuca on Brittany beaches caused a public health scare due to hydrogen sulfide gas.
• In Hawaiʻi, U. lactuca is used as adornment for hula, where it is called limu pāpahapaha.