Ectocarpus


Ectocarpus is a genus of filamentous brown alga that includes a model organism for the genomics of multicellularity. Among possible model organisms in the brown algae, Ectocarpus was selected for the relatively small size of its mature thallus and the speed with which it completes its life cycle. Tools available for Ectocarpus as a model species include a high quality genome sequence and both forward and reverse genetic methodologies, the latter based on CRISPR-Cas9.

Taxonomy and Nomenclature

Brown algae are heterokonts, a group that also includes diatoms and oomycetes. Despite their simple filamentous thalli, the Ectocarpales are part of the crown group of brown algal orders and are a sister group to the order Laminariales. The type species for the genus is Ectocarpus siliculosus Lyngbye. In 1809, Dillwyn described Ectocarpus as Conferva siliculosa based on specimens collected by W.J. Hooker from Norfolk and East Sussex. In 1819, Lyngbye subsequently described Ectocarpus using a specimen from Denmark, citing C. siliculosa Dilwyn as its basionym.
Major taxonomic databases such as AlgaeBase, the World Register of Marine Species (WoRMS), and the Catalogue of Life (CoL) currently list more than 40 accepted species within Ectocarpus. Most of these names originate from nineteenth- and early-twentieth-century descriptions based solely on morphology, which is now known to be highly plastic and taxonomically unreliable in this group.
Historically, the commonly accepted European classification recognized only two widespread species: Ectocarpus siliculosus and Ectocarpus fasciculatus. Nevertheless, reproductive barriers have been reported between isolates from different geographic regions. Modern molecular studies have substantially revised the understanding of species diversity in the genus. In 2010 Peters et al. reinstated Ectocarpus crouaniorum Thuret as a third common species based on evidence of pre- and post-zygotic isolation and genetic divergence from E. siliculosus. E. subulatus Kützing was subsequently proposed for reinstatement in 2015, based on more than 6% 5′-COI sequence divergence from E. siliculosus and its distinctive high abiotic stress tolerance, including elevated temperature and low salinity.
Sequence-based analyses of 729 individuals in 2016 further revealed cryptic diversity within the siliculosi subgroup, and supported the recognition of at least 15 distinct species-level lineages. These included E. siliculosus and E. crouaniorum, and 13 unnamed lineages, which include the genomic model strain Ectocarpus sp.7 Ec32.
As a result, contemporary molecular research recognizes far fewer well-supported species than the numbers found in traditional nomenclatural databases. The taxonomic status of many historically described species remains unresolved, and ongoing revisions are expected as additional molecular data and type material assessments become available.

Morphology

Studies on morphology have been limited for Ectocarpus as only two species in the genera are well-described based on morphology and genetic sequence.
Ectocarpus is a filamentous alga that can grow up to 30 cm. Cultured specimens in the laboratory tend to be fertile when they are 1–3 cm in length. Ectocarpus'' has a normal, branched appearance in unialgal cultures, but in axenic cultures it has a ball-shaped appearance suggesting that bacterial symbionts are required for the alga to attain normal morphology.

Distribution

Ectocarpus can be found across the globe, in temperate shorelines growing as epiphytes on other flora or on rocky substrates. While commonly attached to a substrate, thalli of Ectocarpus may also survive while floating. Ectocarpus are more commonly found as epiphytes on marine macroflora rather than epilithic. E. fasciculatus is known as an endophyte of Laminaria digitata, but no study has documented how it bypasses the kelp's defense. E. crouaniorum are found in the intertidal zone while E. siliculosus and E. fasciculatus can be found in mid-intertidal and subtidal zones, respectively.

Ecology

Ectocarpus thalli tend to shelter several marine invertebrates and some protists. Temperature affects the life cycle of some strains. A study of the life cycles of natural populations in NW France and SW Italy found marked isomorphy between generations in some populations and evidence of populations with modified, asexual life cycles.

Life History

In the laboratory, the life history is an isomorphic to slightly heteromorphic alternation of generations, but asexual strains also exist. Ectocarpus has a haploid-diploid life cycle with both sporophyte and gametophyte generations. It can complete its whole life cycle within 3 months in the laboratory. Diploid sporophytes give rise to haploid meiospores which will then produce a haploid gametophyte generation. These gametophytes are dioecious, producing either male or female gametes, which fuse to produce diploid zygotes, restarting the sporophyte stage. Parthenogenesis may also occur when a gamete does not find a gamete of the opposite sex, producing a parthenosporophyte. Deployment of the sporophyte developmental program requires two TALE homeodomain transcription factors, OUROBOROS and SAMSARA. If either of the genes encoding these two proteins is dysfunctional, the alga develops as a gametophyte.

Cultivation and Exploitation

A protocol has been established to culture Ectocarpus in the laboratory. Ectocarpus is able to grow in artificial seawater although the standard medium is Provasoli-enriched seawater. Standard laboratory conditions are growth at 13 degrees Celsius under a 12h:12h light:dark cycle with irradiance at 20 μmol photons m−2 s−1.

Chemical Composition

Iodide originating from seawater can accumulate to high concentrations in several brown algae but high levels are not observed in Ectocarpus. Genes predicted to encode enzymes involved in iodine metabolism have been identified in Ectocarpus, including haloperoxidases, dehalogenases and haloalkane dehalogenases. These enzymes may be part of the defence mechanism of Ectocarpus against halogenated defenses of brown algal hosts when growing as an epiphyte.

Utilization and Management

Ectocarpus is vulnerable to an array of pathogens and parasites and is also sensitive to abiotic stresses such as shifts in temperature, light and salinity. Major modifications to the Ectocarpus subulatus transcriptome have been observed following stress treatments.

List of species

Some currently accepted species of Ectocarpus include: