What is Sea Lettuce Seaweed? Ecology and Human History of Ulva lactuca

Description

Sea lettuce is classified as a green seaweed in the phylum Chlorophyta, which has about 4,300 known species.  Around 90% of all Chlorophytes grow in freshwater and many are unicellular.  Sea lettuce is the common name for a group of closely related multi-cellular marine species in the genus Ulva and class Ulvophyceae.

True to its name, sea lettuce resembles leaves of green lettuce.  The color is often bright green but can range from dark green to almost yellow.  Sea lettuce is not a particularly large seaweed, being no more than 40 cm (16 inches) in size, but its bright green color and abundance make it especially noticeable in the marine environment.  The blade is described as being a leaflike, flattened thallus, and at just two cell layers it’s quite thin, with a sheetlike appearance.  The leaves can be narrow or broad, and single or multi-lobed. They’re often rounded or oval with ruffled edges and riddled with holes or perforations. The blade lacks a stipe (stem) and instead emerges almost directly from the holdfast.

Sea lettuce usually grows attached by a small discoid holdfast to rocks and shells, but it can also grow in a free-floating, non-attached form, sometimes in prolific masses.  In the field Ulva may be mistaken for two other groups of green, sheetlike seaweeds; Monostroma and Ulvaria. The latter in particular can be confused as sea lettuce because it is common and often just as prolific.  However, both Monostroma and Ulvaria, at just one cell-layer thick, are thinner than Ulva.  If in doubt, one can apply the “fingerprint test”; if your fingerprints are visible through a translucent frond, then in all likelihood it’s either Monostroma or Ulvaria. If your fingerprints are not visible through the frond and the texture seems similar to wax paper, then it’s probably Ulva.

Scientific and Common Names

Sea lettuce is the most popular common name in the English language for Ulva. It’s also sometimes known as green laver.  In Japan, sea lettuce is known as aosa, most likely from the adjective aoi “blue-green”.  Edible sea lettuce was one of the first algae assigned a scientific name by Carl Linnaeus, the father of modern taxonomy, who named it Ulva lactuca. The word Ulva is thought to originate from the Latin word for “sedge” or “marsh plant” though an even older Proto-Indo-European word for “to grow” is also a possible origin. The word lactuca is Latin for “milk bearing” and it’s also the genus name for common salad lettuce (Lactuca sativa), which exudes a milk-like sap when cut.

Taxonomy, which is the scientific classification and naming of life forms, can be a tricky business. Sea lettuce contains no milky sap, and Linnaeus applied the name lactuca to sea lettuce because of its superficial resemblance to salad lettuce.  For most of the centuries since Carl Linnaeus first published Systema Naturae (1735), Philosophia Botanica (1751), and Species Plantarum (1753), scientists have relied on detailed descriptions and comparisons of physical traits to classify plants, animals, and other life forms.  In recent decades, however, genetic analysis has become a powerful and widely accepted tool for classifying life and delineating relationships.  Often, however, genetic analysis reveals that previously accepted relationships and classifications based on physical traits need revision.

Phycology, the science of seaweed, has not been immune to this issue and the naming of sea lettuce is a prime example.  When Linnaeus described and named Ulva lactuca, he archived a dried specimen, known as a type specimen or holotype, into his collection.  The type specimen was thought to have originated from European waters. However, in 2019 scientists genetically analyzed the holotype and found that it was nearly identical to a warm temperate to tropical species known as Ulva fasciata.  The northern sea lettuce plant that people had been calling Ulva lactuca since Linnaeus’s time turned out to actually be the species Ulva fenestrata, originally described from a location in Kamchatka, Russia.  It’s now suspected that Linnaeus’s type specimen was collected from the Indo-Pacific by a student or colleague during one of the many voyages of discovery of that era.

Like many, we’ve long identified sea lettuce as Ulva lactuca, but in all likelihood sea lettuce collected from the northern Atlantic is actually Ulva fenestrata.  Other closely related Ulva species may also occasionally be gathered along with the more common form.  In acknowledgement of this uncertainty and the fact that most other companies and even many phycologists haven’t yet caught up with this change (or entirely confirmed it), we use only the genus name Ulva when discussing sea lettuce.  It should also be noted that scientific articles about Ulva lactuca published prior to 2020 may actually be about Ulva fenestrata!

Life History & Ecology

Sea lettuce is highly adaptable and can grow at all levels of the intertidal zone.  It’s thought to have originated as a species in the temperate North Pacific about 76 million years ago and spread from there.  In the modern era sea lettuce has been accidentally introduced by humans into regions where it wasn’t formerly present, most likely through discharged shipping ballast water.  It’s now found in almost every ocean of the world.

Sea lettuce can reproduce sexually, or vegetatively through fragmentation of the thallus.  Vegetative propagation helps Ulva rapidly proliferate under the right conditions.  This makes sea lettuce a good farming candidate for food, biofuels, or extraction of useful compounds, but it can also make sea lettuce a nuisance species.  This dual nature led to a 2019 review article about sea lettuce with the title “Ulva lactuca, A Source of Troubles and Potential Riches” (Herminia Dominguez and Erwann P. Loret, Marine Drugs, 17, 357; http://dx.doi.org/10.3390/md17060357)

The troubles ensue when blooms of sea lettuce, known as green tides, overwhelm bays, estuaries, and beaches. Other green algae species such as Enteromorpha contribute to these events. Green tides usually indicate nitrogen and phosphorous pollution, which sea lettuce uses as fertilizer to grow.  Nitrogen and phosphorous runoff into the marine environment usually come from agricultural activities or inadequate treatment of human waste.

As the plants proliferate, die and decay, they release the odiferous and potentially hazardous gas hydrogen sulfide (H₂S).   Hydrogen sulfide smells like rotten eggs and it can build up to such high levels that it’s been known to kill animals, including people.  In Brittany, France, green tides caused by the regions’ high concentration of dairy, swine, and poultry farms have killed wild boars, dogs, a horse, and at least two people in the past thirty years.  The earliest fatality may have occurred in 1989, when a dead jogger was found entangled in a mat of rotting sea lettuce on the beach at Saint-Michel-en-Greve.  The problem is not limited to Brittany.  The Emerald Isle of Ireland has been plagued with emerald seas, and the Salish Sea in the Pacific Northwest and the Three Bays Area on the south side of Cape Cod are other regions experiencing more frequent green tides in recent years.  In addition to being unsightly, smelly, and potentially lethal to beachgoers, green tides can suffocate other marine organisms and entangle fishing, aquaculture, and vessel equipment.

Normally, though, in an unpolluted marine environment such as the northern Gulf of Maine where we source it, sea lettuce is nutrient limited and it doesn’t proliferate to such extreme levels.  In this situation the life cycle is usually of the isomorphic, biphasic type.  This means sea lettuce goes through two morphologically similar but separate reproductive stages that differ in chromosome counts but are otherwise similar.  The stages alternate between haploid gametophyte plants and diploid sporophyte plants. This arrangement is one of the more common life cycles observed in marine algae.

Both sea lettuce life stages often occur simultaneously within any given population.  The reproductive season extends through much of spring, summer, and fall, with multiple variable factors triggering reproduction. These may include day length, temperature, lunar cycles, desiccation, and nutrient levels.  During reproduction haploid gametophytes release biflagellated gametes (zoospores) known as swarmers due to their copious numbers.  Swarmers swim until they fuse with another gamete to produce a diploid zygote.  The zygote settles onto rock or shell, germinates, and grows into a diploid sporophyte plant.  Mature diploid sporophytes then release haploid zoospores that settle directly onto suitable substrate, germinate, and grow into a haploid gametophyte plant to complete the cycle.  One of the interesting aspects of Ulva reproduction is that zoospores develop directly from normal vegetative cells rather than from specialized reproductive tissue, as is usually seen with other marine algae.

Sea lettuce is considered a perennial plant because it dies back to a small bud in winter months that regrows the following season.  However, it’s unknown how long an individual plant lives, and non-attached plants do not overwinter.  Although it sometimes becomes a nuisance when it over-proliferates, sea lettuce provides valuable ecosystem services such as sheltering invertebrates and recycling nutrients.

History of Use

Ulva’s attractive, bright green color, thin tender fronds, and ease of harvest make it an attractive snack for adventurous beachcombers, fishermen, and coastal residents, and it’s probably been eaten since long before recorded history.  Historical references to sea lettuce are sparse, but in a 1709 Scottish text, Provision for the Poor in Time of Dearth and Scarcity, Sir Robert Sibbald writes "In the North, and in many other places of the coast of this Countrey, People feed upon Slake, that is, the sea lettuce; They make Broath with it and sometimes serve it up with Butter: it groweth upon the Rocks washen with the Sea.”

In Japan, which has a culinary seaweed tradition stretching back thousands of years, sea lettuce and closely related seaweeds such as Enteromorpha were used to make “poor man’s nori”, a less costly alternative to the classic nori sheet made from red seaweeds. 

In recent years Ulva has become a popular feedstock for farmed abalone and sea urchins, as well as being incorporated into diets for farmed fish such as barramundi.  Sea lettuce is also being investigated as a source of useful substances such as ulvan, protein, and biofuels.

Culinary Attributes

As implied by its name, sea lettuce can be eaten in its fresh, raw form in a salad; either alone, mixed with other fresh seaweeds, or tossed with land greens.  Fresh sea lettuce has a soft texture but strong flavor variously described as vibrant, ocean, slightly salty, and similar to sorrel.

Drying concentrates these flavors but also brings out a rather pungent bitterness, especially when dried sea lettuce is cooked.  For this reason, it’s best used as a seasoning in soups or salads rather than as a main ingredient.  One chef we know of said the flavor reminded them of truffles. Others like to pair sea lettuce with sharp cheeses, or mixed in with spinach. In Japan, Ulva and other green seaweeds are often used as an edible and nutritious garnish in miso soup. 

Dried sea lettuce leaf can be reconstituted in cold fresh water and used in salads. Our sea lettuce flakes and powder make it convenient to use sea lettuce as a seasoning to add mineral nutrition as well as flavor to any recipe, for example a Dijon vinaigrette.  We also offer a delightful tea made of sea lettuce paired with African Honeybush. 

Nutritional & Medicinal Attributes

All seaweeds are mineral rich but Ulva is especially abundant in calcium, iron, and magnesium.  Calcium is essential for bone health and magnesium helps us absorb and retain calcium.  Sea lettuce can contribute to healthy bones because it has high levels of both; up to 12% of the RDI for calcium and 38% of the RDI for magnesium in a 7g serving. The same serving may contain as much as 44% of the RDI for iron.  Dried sea lettuce has a respectable protein content of about 15% that’s well balanced with every essential amino acid.  The protein content can actually be quite high at times; up to 47% has been reported. The iodine content of sea lettuce, at about 45ppm, is the lowest of all our sea vegetables.  While this is still high enough for sea lettuce to meaningfully contribute to dietary iodine, sea lettuce can be consumed in relatively large portions, such as in a salad, without exceeding the tolerable upper intake level for iodine.

Sea lettuce contains the sulfated polysaccharide ulvan and other sulfur compounds.  Sulfur compounds impart a distinctive aroma to sea lettuce and a somewhat bitter flavor, but they also offer potential health benefits.  Ulvan is currently receiving much scientific attention owing to its physiochemical and biological properties for agricultural and pharmaceutical applications.  Ulvan shows antioxidant properties in lab studies.  It’s also been shown to reduce total serum cholesterol, triglycerides, and low-density lipoprotein (LDL) cholesterol (all bad for coronary health), while at the same time elevating high-density lipoprotein (HDL) cholesterol (considered heart-healthy).  Finally, ulvan has been shown to demonstrate antiviral properties, notably against influenza A and herpes simplex viruses.

While these are all laboratory-based studies using extracted ulvans and the findings haven’t yet been replicated in human subjects, there’s no doubt that sea lettuce can promote health when included in a nutritionally balanced diet.

Wild Harvest & Processing

The Ulva harvest typically begins in Maine and Maritime Canada in April or May and extends through the end of October. The plant is cut above the holdfast to regenerate, and because it is so prolific up to 75% of a bed can be safely harvested.  Hand harvesting is usually done at low tide when the plant is exposed and lying flat on the substrate.  Some harvesters have experimented, with varying degrees of success, with using homemade roller cutters towed behind them through shallow water so that the plants are harvested while still upright in the water column.  This helps reduce grit and the need to rinse the leaves with salt water before drying.  Sea lettuce is usually sun dried in open air.

Heavy harvesting is sometimes desired in areas experiencing green algae blooms. Sea lettuce can harm shellfish production when it forms extensive mats over shellfish beds or aquaculture facilities in estuaries or shallow bays.  In these cases, harvesting is encouraged at any time and in any quantities.

Large amounts of sea lettuce harvested from blooms are often processed into animal feedstocks, but new uses have recently been proposed as well. These include using sea lettuce as a biofuel stock or processing it to extract protein and/or ulvans.  The protein content of sea lettuce can exceed 40% when it grows in nitrogen enriched waters.  Green algae blooms offer an opportunistic supply of large quantities of biomass for these types of industrial applications, but since the blooms can’t be easily predicted it becomes challenging to develop a sustainable business model based on this practice. 

All of the sea lettuce sold by MCSV is traditionally harvested from a few remote rocky coastlines in the northern Gulf of Maine. These locations are certified for organic production and are far from any major urban centers or agricultural activity. We do not sell sea lettuce sourced from green tides.

Aquaculture

Ulva is considered a prime candidate for tank-based aquaculture because it grows prolifically as an unattached plant.  This means it can be grown suspended with aeration in the water column and doesn’t have to be seeded onto nets or lines. Vegetative propagation is another desirable aquaculture trait of sea lettuce. However, attempts to commercialize Ulva tank farming are still recent and relatively small-scale endeavors. One company that has found success is Blue Evolution, which grows Ulva in tanks in Mexico in conjunction with the University of Baja California.

A promising approach to Ulva aquaculture is in land-based integrated multi-trophic aquaculture (IMTA) systems.  In IMTA, two or more commercially important species from different trophic levels are integrated into a common culture system. A trophic level is the position an organism occupies in the food web. In IMTA, lower trophic level species can utilize the waste stream of higher trophic level species for growth, and in so doing remove waste products, improve water quality, and provide another crop.

Ulva, as a photosynthetic alga, is a primary producer that occupies the first trophic level.  In addition to sunlight, Ulva requires nutrients such as nitrogen and phosphorous.  These also happen to be two of the primary wastes produced from fish aquaculture, and Ulva is very proficient at mopping them up for its own purposes.  This same proficiency leads to the nuisance green algae blooms discussed earlier, but in IMTA it’s put to good use.  Various IMTA systems have been tested at research scale growing Ulva in combination with species such as gilthead seabream, sole, shrimp, and sea urchins.  After the culture water passes through the Ulva beds it can be discharged into the environment with fewer adverse impacts, or be recycled back into the culture system.  Ulva grown in IMTA typically has a higher protein content than wild Ulva and it can be harvested and processed into various useful products, including as a feed ingredient for fish or other higher trophic level organisms grown in the IMTA system.  This YouTube video shows an experimental IMTA system containing Ulva and lumpsuckers.