Red Algae Introductory article Article Contents Carlos Frederico Deluqui Gurgel, Smithsonian Marine Station, Fort Pierce, . Florida, USA Introduction: Definition and Characterization . Sexual Reproduction University of Alabama, Tuscaloosa, Alabama, USA Juan Lopez-Bautista, . Vegetative Reproduction . Major Groups Red algae are ancient aquatic plants with simple organization, noteworthy colour . Ecological Importance variation, vast morphological plasticity, challenging taxonomy and most extant species . Economical Importance (about 6000 worldwide) are marine. They include species with complex life cycles, significant ecological importance and extensive economical applications. doi: 10.1002/9780470015902.a0000335 Introduction: Definition and carrageenan. Some taxa present calcium carbonate depos- Characterization its whose crystal state can be found in two forms, either calcite or aragonite. See also: Algal Calcification and Red algae (Rhodophyta) are a widespread group of uni- to Silification; Algal Cell Walls multicellular aquatic photoautotrophic plants. They ex- Red algae are one of the oldest eukaryotic groups in the hibit a broad range of morphologies, simple anatomy and world, with fossil evidence dating back from the late Pre- display a wide array of life cycles. About 98% of the species Cambrian, about 2 billion years ago (Tappan, 1976). The are marine, 2% freshwater and a few rare terrestrial/sub- oldest multicellular eukaryotic fossil record is of a red alga aerial representatives. Planktonic unicellular species have dated 1.8 billion years ago. We also know that red algae simple life cycles characterized by regular binary cell divi- share a single common ancestor with green algae (Chloro- sion. Advanced macroscopic species exhibit the character- phyta) and the land plants (Embryophyta), and these three istic trichogamy, triphasic, haplo-diplobiontic life cycle, groups, together with the Glaucophytes define the current with one haploid (gametophytic) and two diploid (carp- Plant Kingdom (Keeling, 2004). See also: Algal Taxon- osporophytic and tetrasporophytic) stages. Red algae are omy: Historical Overview; Eukaryotes and Multicells: true plants in the phylogenetic sense since they share, with Origin the green lineage (green algae and higher plants), a single common ancestor (Adl et al., 2005). However, the features that distinguish red algae from any other algal group refer Sexual Reproduction to the absolute lack of flagella and centrioles, presence of phycobilisomes and unstacked thylakoids in the chloro- The basic scheme of sexual reproduction includes the de- plast, absence of parenchyma and presence of pit-connec- velopment of a specialized female filament called the carp- tions between cells (i.e. incomplete cytokinesis). The char- ogonial branch. The female gamete (carpogonium) is easily acteristic red colour and its many variations are the result recognizable by the presence of the trichogyne, an elongated of a variety of photosynthetic pigments (chlorophylls and extension responsible for receiving the male gametes carotenoids) plus phycobilisomes, the light harvesting (spermatium). After fertilization, the zygotic nucleus de- complex composed of three main classes of water-soluble velops, directly or indirectly, into a diploid phase, the carp- protein-based pigments (phycobiliproteins): phycoery- osporophyte, which grows parasitically on the female thrin (red), phycocyanin (blue) and allophycocyanin gametophyte. During the direct development, the fertilized (blue–greenish) (Grossman et al., 1993). See also: Algal carpogonium matures into a carposporophyte usually com- Chloroplasts; Algal Metabolism; Algal Photosynthesis; posed of gonimoblastic filaments (vegetative diploid cells) Algal Pigments; Phycology and carposporangia (reproductive diploid cells). Mature The growth in length of macroscopic species is governed carposporangia release diploid carpospores into the water by a single apical cell or by a group of multiaxial apical column. However, in many groups with indirect develop- cells. Intercalary cell divisions are common and account for ment, the zygotic nucleus undergoes nuclear divisions and most of the growth in width and thickness of thalli in many these nuclei are transferred to other specialized cells, called red algal groups. No specialized growth tissues or meri- auxiliary cells, where they will, in turn, develop into carp- stems exist and, therefore, a true parenchyma is never de- osporophytes remote from the original fertilization site. veloped. In this view, all red algal thalli can be interpreted The auxiliary cell can be located or originated in close as a group of filaments held together by cell-wall interac- proximity to the carpogonial branch, in a short distance to tions, mucilage and pit-connections (pseudoparenchyma). receive the zygotic nucleus (procarpic condition), or away Red algal cell walls are composed of cellulose fibrils (rarely from it, in another vegetative independent filament (non- xylan fibrils) and a matrix of hydrocolloids. Cell wall procarpic condition). In the latter case, a complex network hydrocolloid matrixes in red algae are formed by sulfated of connecting filaments (ooblast filaments) can develop polysaccharides classified in two main groups: agar and to deliver the diploid nuclei to several auxiliary cells. ENCYCLOPEDIA OF LIFE SCIENCES & 2007, John Wiley & Sons, Ltd. www.els.net 1 Red Algae The arrangement, morphology and number of cells that algae at higher levels of biological organization were pro- make up the carpogonial branch, and the shape, origin and posed by Saunders and Hommersand (2004) and Yoon location of auxiliary cells are prime characters for the tax- et al. (2006). Both works recognized the red algae as the first onomy of red algae. In many red algae the fully mature first divergence in the Plant Kingdom, with at least five major diploid stage is called carposporophyte. The cystocarp is red algal internal evolutionary lineages, and the mon- composed of the carposporophyte plus all protective sterile ophyly between the Florideophyceae and a redefined haploid tissue of the female gametophyte encircling and Bangiophyceae sensu stricto (Bangiales only). However, interacting with it (pericarp). Carpospores develop into a the phylogenetic relationships among the unicellular red second free-living phase called tetrasporophyte, which can algal groups (e.g. Porphyridiales lineages) remain partially be morphologically similar (isomorphic alternation of gen- unresolved, despite a plethora of physiological, morpho- erations) or different (heteromorphic alternation of gener- logical and molecular data available. Currently, there are ations) from the gametophytes. Tetrasporophytic plants approximately 32 orders of red algae but more are expected produce tetrasporangia by meiosis, which release tetra- to be proposed. The evolutionary relationships and taxo- spores. This pattern of meiotic cell division in the tetras- nomic classification systems in red algae, across all levels of porangium is stable in red algae and can be one of three biological organization, have been extensively advanced in types: cruciate (including decussate), tetrahedral and zon- the last 15 years with the aid of molecular-based phyloge- ate. When released, each tetraspore will give rise to either a netic analyses. Among the most common phylogenetic male or a female haploid gametophyte. markers used with this purpose are the chloroplast- encoded rbcL (1467 base pair single copy gene that codes for the large subunit of the ribulose 1,5-bisphosphate car- Vegetative Reproduction boxylase/oxygenase enzyme), and the nuclear-encoded ribosomal cistron encoding 18S, ITS1, 5.8S, ITS2 and the Vegetative reproduction is quite common in red algae. 28S rDNA (e.g. Freshwater et al., 1994; Harper and Thallus fragmentation is considered by many as the most Saunders, 2001). significant kind of vegetative reproduction in red algae due to the huge drifting biomass mats observed, many times accumulating and piling high at beaches. In many places, such as the intracoastal waterways along the Eastern coast Ecological Importance of the USA, bottom deposits of drifting populations of seaweeds are constant components of the benthic commu- General importance nity. Key species in this case include Hypnea cervicornis, In many tropical and subtropical intertidal communities Spyridia hypneoides and Acanthophora spicifera. Some spe- red algae are the primary component of the flora in terms of cies produce propagules such as spores from bisporangia biomass and species diversity. As primary producers and (e.g. Caloglossa apomeiotica) monospores from mono- conspicuous members of the benthic marine community, sporangia (e.g. Monosporus sp., Ceramiaceae), and frail red algae serve as a source of oxygen for the environment, branchlets designed to break apart, disperse and develop food for heterotrophic species, substrate for many epiphy- into new plants (e.g. star-shaped branchlets of Hypnea tic species of animals and plants and as a refuge, nesting cornuta). Vegetative growth and fragmentation is the pri- and egg depository site for many other organisms. In mary mode of reproduction in many invasive species of red the marine environment red algae can be found inhabiting seaweeds such as Gracilaria salicornia and Hypnea mu- the upper littoral zone to the deepest benthic region of the sciformis in Hawaii (Davidson et al., 2003). See also: Algal continental shelves. As littoral organisms, many species Spores can endure severe abiotic