Hypnea

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Hypnea
Hypnea musciformis
Scientific classification
Clade:	Archaeplastida
Division:	Rhodophyta
Class:	Florideophyceae
Order:	Gigartinales
Family:	Cystocloniaceae
Genus:	Hypnea J.V.Lamouroux, 1813
Species
See text

Hypnea is a genus of red algae, and a well known carrageenophyte (plant producing polysaccharide carrageenan).

Hypnea is a genus of red seaweed under class Florideophyceae, order Gigartinales, and family Cystocloniaceae. There are 99 species listed on Algaebase with 78 being accepted taxonomically.^[1] The genus was originally classified under family Hypneaceae,^[2] which was considered distinct from Cystoclonoiaceae on the basis of differences in their carposporophyte structure,^[3][4] and the shape of the thalli and cortical cells. Differences in carrageenan chemistry, with Hypneaceae species producing kappa-carrageenan and Cystocloniaceae producing lambda-carrageenan, was another reason for their distinction.^[5] However, these criteria were questioned and molecular analysis later showed that the two families had similar vegetative and reproductive characters.^[6][7] Furthermore, it was proven that lambda-carrageenan and kappa-carrageen coexist within the Gigartinales order.^[7] This led to the two families being merged into simply Cystocloniaceae.

Phylogeography of Hypnea valentiae in India indicates that nuclear ribosomal DNA Internal transcribed spacer-1 (ITS1) sequences had 4.35 x 10-1 Tamura 3-parameter (T3P) pairwise distance between them, which indicate significant evolutionary differences accumulated over time.^[8] In comparison, T3P distance between related genera Kappaphycus and Eucheuma was 1.85 x 10–1. In phylogeny reconstruction using Bayesian inference, both the isolates formed a well-supported clade along with the only available accession of this genus at ITS1 locus, indicating affiliation of both the isolates in this genus. The isolate from the west coast was more basal in the phylogram, which suggests phylogenetically primitive position of this population. Newly generated DNA barcodes of the geographic isolates of this native carrageenophyte in this study is expected to be a key in tracing its further dispersal routes, either natural or deliberate. This is the first report on the comparative morphological and molecular assessment of Hypnea from India.

Hypnea is characterized by membranous or cartilaginous thalli which can be either erect or prostrate. The thallus features a main axis with an apex of varying shapes (straight, curved, tendril or bifurcated). Various forms of branching occur, with the irregular type being most common, with dichotomous or lateral branching occurring in some species. Thallus color is highly variable, with yellow, green, pink, red, brown and black being observed. Habitat type can influence thallus length, which varies from 0.5 to 50 cm.^[9] The habit of the thallus is used to divide species into two major groups: those with >10 cm thallus height, and those that measure between 2–3 cm.^[9] Like many other algal species, Hypnea exhibits a high degree of phenotypic plasticity and cryptic diversity which makes species difficult to distinguish from each other morphologically since they share many characters.^[10][11]

The genus is found in the Atlantic, Pacific, and Indian Oceans, with the Atlantic having the most species. In terms of countries, China has the most number of species although none are endemic. Other countries with many endemic species of Hypnea include South Africa, Australia, Japan, Indonesia, Mauritius, the Philippines, India, and Brazil. Hypnea musciformis has the widest geographical distribution.^[9]

Hypnea is commonly present in various habitats, from intertidal and subtidal zones, as well as in sites with varying degrees of exposure. They can also be found growing as epiphytes on other organisms such as the brown alga Sargassum. The genus exhibits seasonal abundance which is highly influenced by similarly seasonal variations in light, temperature, and tidal levels.^[12]

Hypnea exhibits a dioecious thallus and a triphasic life cycle consisting of two diploid phases and a haploid phase. The diploid carposporophyte releases carpospores that develop into a diploid tetrasporophyte which undergoes meiosis to form four haploid tetraspores which will form 50/50 male and female gametophytes. After fertilization of gametes, the carposporophyte is formed and the cycle begins again.^[13][9]

Carrageenans are abundant in the cell walls of some red algae, where they provide the thallus with additional flexibility to protect it from the impacts of herbivory, wave action, and desiccation. Environmental variables such as temperature, amount of rainfall, and salinity likely influence the yield and viscosity of the carrageenan.

Hypnea is cultivated in many tropical countries such as the Philippines, Brazil, Bangladesh, India and Vietnam for food and for their kappa- carrageenan extracts.^[14] After Eucheuma/Kappaphycus, it is the second most important source of carrageenan in the tropics, however it is only cultivated in experimental settings and not commercially. The technology needed to do this is still being developed and requires further investigation.^[15] Most experimental cultivation activities have been done using Hypnea musciformis, and its highly variable morphology may be partly responsible for the inconsistency in the results of these experiments.^[16][17]

Carrageenan has a wide range of applications from food to pharmaceutical uses. The extracts of various Hypnea species have been investigated for their antiviral, antibacterial, antioxidant and antifungal properties.^[18][19][20]