Macroalgae (or seaweeds) contain high amounts of carbohydrates (up to 60%), medium/high amounts of proteins (10–47%) and low amounts of lipids (1–3%) with a variable content of mineral ash (7–38%) [
5]. With decreasing available land and fresh-water resources, the oceans have become attractive alternatives for the production of valuable biomass, comparable to terrestrial crops. Macroalgae cultivated under controlled and sustainable cultivation systems are probably the future method for supplying biomass to meet market development needs [
6].
The high carbohydrate fraction includes a large variety of easily-soluble polysaccharides, such as laminarin, alginate, mannitol or fucoidan in brown types; starch, mannans and sulphated galactans in red types and Ulvan in green types [
7]. Alginate, one of the main structural polymers of brown seaweeds, provides both stability and flexibility for the specimens exposed to flowing water, and is one of the industrially-relevant carbohydrate compounds found in seaweed biomass, as are other hydrocolloids, such as agar and carrageenans, which are commonly used as thickeners, gelling agents or emulsifiers. Various other non-carbohydrate products obtained from seaweeds include protein, lipids, phenols and terpenoids, and minerals such as iodine, potash and phosphorus—useful for human and animal nutrition [
8]. The harvesting of macroalgae—a valuable raw material for food—before they beach could well be developed into an effective solution [
9]. The interest of macroalgae in human nutrition is due to their high mineral concentrations (such as calcium, magnesium and potassium) and glutamic acid, which makes them also useful as taste enhancers. Algae could help to address one of the biggest challenges currently faced by the food industry, which is the ever growing human population. Algae are also a source of active principles largely unexplored for pharmaceutical products [
7]. The gelling properties of polysaccharides are well known, and the therapeutic applications are in development [
10,
11]. Algal polysaccharides, pigments, proteins, amino acids and phenolic compounds are potential functional food ingredients for health maintenance and the prevention of chronic diseases, with increasing potential uses in pharmaceutical industries [
12].
Ulva lactuca polymorphism dependent upon the environment led to the proposal that different species may exist, such as
Ulva armoricana, rigida, prolifera, pertusa, fasciata or
rotundata [
1,
2,
3,
4]. However, genetic analysis revealed that the different phenotypes observed were not based on genetic variations that would justify the existence of different species other than
Ulva lactuca [
13]. Taxonomy is challenging for the genus
Ulva, which belongs to the phylum of
Chlorophyta, constituting four traditional classes (
Ulvophyceae,
Trebouxiophyceae,
Chlorophyceae and
Chlorodendrophyceae) that evolved from unicellular marine planktonic prasinophyte algae in the Neoproterozoic [
1].
Trebouxiophyceae and
Chlorophyceae are found mainly in fresh water while
Ulvophyceaecolonize mainly shallow marine environments, similar to the Australian
Collerpa that has colonized the Mediterranean Sea since 20 years ago. Linnaeus was the first to observe that
Ulva lactuca could have different phenotypes with a tubular or a sheet-like tail, but taxonomists in the nineteenth century proposed that the tubular green algae were a distinct genus called
Enteromorpha. However, molecular approaches demonstrated that Linnaeus was correct—
Ulva and
Enteromorpha are not distinct genera [
13]. Different
Ulva species are still described in the literature [
14], but it turns out that reproduction is possible between these different “species”, which therefore should be described as
Ulva lactuca variants or clades.
Ulva blooms damage marine ecosystems and impair local tourism [
9].
Ulva principally invades beaches, and its biodegradation can produce acidic vapors that have induced the death of animals and possibly humans since a horse was reported dead in 2009 on the Brittany coasts (located at the west of France) due to
Ulva biodegradation [
2]. The first
Ulva bloom to be described was in Belfast (in the north of Ireland) at the end of the nineteenth century [
15].
Ulva blooms were well-studied in the Laguna of Venice from 1930, with an unexplained decrease observed after 1990 [
16,
17]. Since 1980,
Ulva blooms have been observed worldwide, from Galicia [
18] to Tokyo Bay [
19], including the American [
20] and Australian continents [
14]. However the largest events in the world to date have been the green tides observed in the Yellow Sea for ten consecutive years from 2007 and covering 10% of the Yellow Sea [
21]. In Europe, Brittany’s north coasts have the biggest
Ulva blooms [
3]. While there is no doubt that
Ulva blooms are due to human activities, it is generally farmers who are accused of being fully responsible for
Ulvablooms because of their use of fertilizers [
2,
3]. However, for the green tides of Belfast and Venice, a correlation was established with human waste, due to an increase of workers [
15] or tourists [
17]. Furthermore, abundant sources of nitrogen and phosphorus are important for
Ulva blooms, but phosphorus does not come from agricultural activity [
2] and it is difficult to determine what part of nitrogen is due to fertilizers or human waste.
Preliminary experiments suggest that in the Mediterranean seas there is a virus that might provide a natural and ecological way to control
Ulva blooms.
Ulva blooms occur in the Mediterranean seas but disappear rapidly, and an enzyme activity related to giant viruses was observed in the bay of Marseille on denatured
Ulva tissues (manuscript in preparation). Viruses are well known to participate in the control of microalgae blooms, but this is not demonstrated for macroalgae. Virus controls of microalgae blooms were recently observed in USA, with the two macroalgae
Aureococcus anophagefferens inducing harmful bloom algae on the east coast [
22] or
Tetraselmis in Hawaii [
23]. In these two cases, it was because of viruses that have been recently discovered and have been called giant viruses. Giant viruses were first discovered in amoebae [
24]. While most viruses known over the past century have a size < 200 nm, such as 160 nm for HIV or 20 nm for the smallest virus (
Parvoviridae infecting pigs), giant viruses have a size of up to 1 µm. Since then, giant viruses have been discovered all across the world, infecting many species, particularly marine species [
25].
Ulva contains commercially valuable components susceptible of being exploited for cosmetic, pharmaceutical, chemical, food and energy applications [
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26] and this review provides a background on products that might be obtained from
Ulva, as well as the processing technologies used to date.