Halimeda

Genus of algae

Halimeda is a genus of green macroalgae. The algal body (thallus) is composed of calcified green segments. Calcium carbonate is deposited in its tissues, making it inedible to most herbivores. However one species, Halimeda tuna, was described as pleasant to eat with oil, vinegar, and salt. As in other members of the order Bryopsidales, individual organisms are made up of single multi nucleate cells. Whole meadows may consist of a single individual alga connected by fine threads running through the substrate. Halimeda is responsible for distinctive circular deposits in various parts of the Great Barrier Reef on the north east coast of Queensland, Australia. Halimeda beds form in the western or lee side of outer shield reefs where flow of nutrient rich water from the open sea allows them to flourish, and are the most extensive, actively accumulating Halimeda beds in the world. The genus is one of the best studied examples of cryptic species pairs due to morphological convergence within the marine macroalgae. Some species grow so vigorously in tropical lagoons that the sediment is composed solely of the remains of their tissues, forming a calcareous "Halimeda sand". In fact some tropical reef systems, such as atolls, consist largely of Halimeda sand accumulated over the aeons. Overall, Halimeda represents the most common green algea large grains in the sediment of the lower latitudes.

The genus Halimeda J. V. Lamouroux belongs to the order Bryopsidales under the family Halimedaceae. It has five monophyletic sections Halimeda J. V. Lamouroux, Micronesicae Hillis Col, Opuntia J. Agardh ex De Toni, Pseudo opuntia J. Agardh ex De Toni, and Rhipsalis J. Agardh ex De Toni which were based on the differences in the fusions of medullary siphons. There are 71 species and 67 infraspecific names listed on Algaebase as of 2015. and are separated by nodes which are non calcified. and the Florida Keys. Some species (e. g. H. copiosa, discoidea, gracilis, opuntia, simulans, and tuna) have a global distribution. Halimeda tuna is found solely in the Mediterranean. Some species such as H. opuntia have been found to produce up to 54.37 g CaCO3 m−1 yr−1. The genus also contributes to reef building, as it is large producer of carbon sediments on reefs, generating a wide range of sediment sizes from coarse particles to silt and clay. Although it was largely assumed that its abundance on reefs is due to it being unpalatable to herbivores, more recent studies have found that Halimeda is in fact subject to grazing by some herbivores such as Scarus rivulatus, Hipposcarus longiceps, and Chlorurus microrhinos. Hard coral cover can actually play a key role in maintaining Halimeda biomass on reefs, as one study found that thalli growing under the canopy of Acropora colonies were larger than those in open areas exposed to herbivory.

Halimeda reproduces both sexually and asexually. Sexual reproduction is rarely observed because it is completed in 36 hours. Some species of Halimeda have been found to reproduce synchronously in mass spawning events similar to that of corals, albeit occurring over several months, with small portions of the population spawning each day. Therefore it is likely that the life span of the genus is limited to a few months to a year. Halimeda opuntia ethanol extract exhibited activity against hepatitis C virus (HCV) due to polymerase inhibitory (HCV 796) binding sites based on molecular docking simulation. Methanolic extracts of Halimeda macroloba have recently been found to exhibit cytotoxicity towards MCF 7 and HT 29 cells, which are derived from human breast cancer cell lines and colon cancer lines, respectively. These results therefore suggest the genus' potential for cultivation as a food source. Halimeda tuna appears to be used as fodder in the Philippines.

H. bikinensis
H. borneensis
H. cereidesmis
H. copiosa
H. cryptica
H. cuneata
H. cylindracea
H. discoidea
H. distorta
H. favulosa
H. fragilis
H. gigas
H. goreauii
H. gracilis
H. heteromorpha
H. howensis
H. hummii
H. incrassata
H. kanaloana
H. lacrimosa
H. lacunalis
H. macroloba
H. macrophysa
H. magnidisca
H. melanesica
H. micronesica
H. minima
H. monile
H. opuntia
H. pumila
H. pygmaea
H. renschii
H. scabra
H. simulans
H. stuposa
H. taenicola
H. tuna
H. velasquezii