Rhodophyta, Red Algae
Rhodophyta, Red Algae
The Rhodophyta - Red algae division
The red algae division includes about 4000-6000 species, mostly marine with only about 200 found in fresh water. Red algae occupy an important position in warm sea vegetation, although some types may be found throughout the world even in cold water. The red algae division is ancient, and fossilized examples have been found from the Silurian period. From a systematic point of view, and with regard to their reproductive system, red algae are considered to be among the more developed algae.
Red algae contain chlorophyll-a and often chlorophyll-d. They also contain carotenes, xanthophylls and phycobilins (see table 2). Their red colour is due to the phycoerythrin pigment, which envelopes the green of the chlorophyll-a. The phycoerythrin is capable of absorbing green light, providing red algae with an advantage in situations of low light characteristic of deep or turbid water.
Most types of red algae are multi-cellular and have a defined nucleus. The cell wall incorporates two layers: the inner layer is made of cellulose and the outer layer contains various ingredients, such as agar and carrageen. Some types, especially those belonging to the Corallinaceae family, have a calcified cell wall that contains high levels of calcium (1%
-30%). These types are capable of forming reefs, and the coarse sand (Zif-Zif) found along Israel’s and Lebanon’s northern shores contains fossilized red algae residue. About one tenth of the Rhodophyta macroalgae are of financial value and are used as food, spices and as a source for manufacturing agar and other stabilizing materials.
Formerly, the red algae division was divided into two classes: Bangiophycidae and Florideophycidae. At present, all red algae are incorporated into one class: the Rhodophyceae. Some divide this into two sub-classes: Bangiophycidae and Florideophycidae.
The Bangiophycidae sub-class includes between 120 to 200 species, mostly found on salty substrates in either fresh water or in the sea. The Bangiophycidae possess a comparatively simple structure, some being single-celled, some having either a flat, cylindrical or complex thallus. The thallus usually possesses a single or double line of cells. Sexual reproduction has not been observed yet in many species, but rather vegetative growth. Where sexual reproduction does occur, it is through direct division of the vegetative cell into several reproductive cells. Reproduction occurs through single spores (monosporangia) that contain a single reproductive cell (monospore).
The Florideophycidae sub-class is heterogeneous and includes most red species. The thallus is multi-celled and varied. Reproductive cells differ from regular cells, and often regenerate. A-sexual reproduction using single spores or clusters of four spores has been observed. In some species the gametophyte and the sporophyte resemble one another; in others they differ to the point that they have often been erroneously defined as separate species.
Lee, in his book, mentions twelve orders of red algae. Eight of these are found in the Eastern Mediterranean and are therefore mentioned in this book:
¿ Order Bangiales ¿ Order Nemaliales ¿ Order Gelidiales ¿ Order Gigartinales ¿ Order Bonnemaisoniales ¿ Order Coralliniales ¿ Order Ceramiales ¿ Order Rhodymeniales
This primitive order includes about seventy species belonging to fifteen genera. The Bangiales order includes algae of a simple structure. Some are single celled, communal, unicellular or posses a paranchimatic thallus. The thallus is comprised of a single or double layer of cells. In most species in this order there is no cytoplasmic connection between the cells (tip connection), as opposed to other red algae species. Sexual reproduction has not been observed in many species and, in those where it does exist, it occurs through single spores (an indication of primitive behaviour). Many species develop as epiphytes or parasites upon other algae. Two prominent genera are Porphyra and Bangia.
The order is characterized by species that contain a large number of growth cells (apical cells) in each branch. The central cells in the thallus are mostly colourless. The reproductive mechanism is developed, and in many species a three-stage life cycle may be observed. A complete cycle may extend over a year, and for some of the stages, conflicting reports exist from different locations around the world. This form of reproduction is also apparent in other red orders. The stages are:
a. The sexual generation (gametophyte) is unisexual, haploid and distinct for male and female plants (monosexual, monoecious). The female gametophyte develops female reproductive cells (egg), and the male gametophyte develops flagellum-less male reproductive cells (sperm), which are pushed towards the female gametophyte by the water current. b. An a-sexual, diploid generation (sporophyte) develops from the zygote that remains attached to the female gametophyte. This generation is a parasitic sporophyte on the gametophyte plant. The parasitic generation, referred to as a cystocarp, performs photosynthesis and produces a number of groups of a-sexual spores. c. The spores germinate a generation of sporophytes (asexual, diploid), independent and structurally resembling gametophytes. d. The sporophytic generation undergoes meiosis, generating male spores. These spores germinate haploid (male or female) gametophytes, and the cycle repeats itself.
Species from the Nemaliales order serve as important agar plants. Some deposit calcium through a mechanism that differs from that of the Coralliniales order. The calcium is secreted as an Argonite compound (not calcite). In this book, this order is represented by the Nemalion species.
Based on field observations, it seems that the members of this order effect a three-stage life cycle, although not all stages have been observed under laboratory conditions. Each of the thallus’ branches contains a growth cell - one apical cell. The order contains one family: Gelidiaceae. Several of the family’s species are important agar plants. Observed species include Pterocladiella, Gelidium and Gelidiella.
In the past, the Gigartinales order was divided into two orders - Cryptonemiales and Gigartinales. Since the distinctions were insignificant, the two were combined (Kraft and Robins, 1985). Gigartinales is the largest order, incorporating 40 families. These are homogenous families, but quite distinct from one another. It should come as no surprise if the group’s taxonomy undergoes further changes in the future. Gigartinales includes species of great ecological and economical significance, and species in this family produce most of the world’s agar.
The Gigartinaceae family includes four to six species of multi-layered algae, each containing six to eight layers. Many species feature dichotomous development, incorporating a three-stage life cycle and tetra sporangia, single nuclear elliptical cells. Many species are a source for hydrocolloid, agar and carrageenan production. Among these are Gracilaria, Gigartina and Solieria.
The Gracilariaceae family is represented in the Eastern Mediterranean basin by several species of Gracilaria, all of them with potential economic value. Some are used in research at the IOLR Institute in Israel and some are already being used in aquatic agriculture.
The Solieriaceae family includes one local species, the Solieria filiformis. The inner layer of internal cells (the medulla) contains elongated cells. The Solieria filiformis is used in marine agriculture, is very tasty and used for hydrocolloid production.
The Phyllophoraceae family includes two Eastern Mediterranean species, the Schottera nicae.nsis, which grows alongside the upper, shady parts of potholes, and the rarer Gymnogongrus griffithsiae. The family features (usually dichotomously) split thallus that have several layers, the innermost of which are characterized by larger cells, them to the outermost cells.
Illustration 18: Three-staged regeneration, characterizes some red species. A diploid sporophyte develops as a parasite upon a female gametophyte.
Seaweeds of the Eastern Mediterranean Coast
At least two species of the Hypnea genus represent the Hypneaceae family along the Eastern Mediterranean coast. They are more abundant at the upper part of the intertidal zone.
The Halymeniaceae family includes extremely attractive red algae, fleshy and brightly coloured. Some species of the Halymenia genus have been described in Israeli literature, but it is doubtful whether any have survived along these coasts or anywhere in the Eastern Mediterranean over the past few years.
The Peyssonneliaceae family is taxonomically problematic: it is located between the Coralliniales order and the Gigartinales. Both orders probably stem from a single parent. Some researchers propose combining the two orders. Conflict also exists regarding the classification of Peyssonnelia, which can be seen as a transitory link, since not the entire thallus but rather parts of the alga - those clinging to an outside substrate - are calcified. Some claim that Peyssonnelia is a distinct family within the Gigartinales order, and some prefer to combine it with Coralliniales.
The order contains branched plants, sometimes with crawling holdfast. The thallus’ branches are thin or thick, soft and pleasant to touch. Side branches are arranged around a central axis. Until now, only one representative has been observed in the region -Asparagopsis taxiformis.
The order contains a single family - Corallinaceae, whose species all are able to deposit calcium. The calcium is absorbed from the marine environment and contained as a calcite within the cell wall. The rate of secretion is related to the rate of photosynthesis, and the two processes may be inter-linked. Changes in acidity (pH) in the algal vicinity, a result of the photosynthesis, affect the calcium’s solubility and sedimentary rate. Theories on the matter exist, but the calcium secreting mechanism’s ecological benefit is unknown, as is the reason why only some species deposit calcium.
What is pertinent is that along the Eastern Mediterranean coast, at depths of less than one or two metres, calcified forms abound. Apparently, under specific conditions, this strengthens the alga against waves and currents and, of course, affects the preferences of herbivores (fish and others) as well as the ability to absorb carbon dioxide from the water. Regarding this, one should mention the Corallina Amphiroa and Jania species, which populate the subtidal and intertidal zones, as well as the attractive Lithophyllum frondosum that forms colonies in alcoves.
The Ceramiales order is the most highly developed among the red algae division from an evolutionary point of view. Most species are delicate and structured in a complex manner. The order is divided into four families, distinguished from one another primarily by the shape of their thallus. The Ceramiaceae family is the most primitive in the order, and the others seem to have evolved from it. Most plants are tiny, and one requires a magnifying glass or microscope to observe their structure. Along the pipe-shaped thallus, condensed rings resemble bracelets or bangles. It is represented by the Ceramium, Centroceras and Spyridia species.
The Polysiphonia, the principal among the Rhodomelaceae family, is a delicate, filamentous alga that is an important food element for sea fish. Many species in this order follow a three-stage life cycle, as described for the Nemaliales order. The Rhodomelaceae family is represented by many common species along the Eastern Mediterranean coast, including Acanthophora, Laurencia, Chondria, Halopithys, Pterosiphonia and Rytiphlaea.
The Dasyaceae family is represented by members of the Dasya genus. The plants are branched, delicate and attractive, with hairy branches. Like many other species, they await more in-depth research.
The order includes twenty five genera and one hundred thirty species divided into two families. The reproductive mechanism for many species in this order has not yet been examined. In certain regions along the North American coast Rhodymenia possessing only male antheridia have been found, but their life cycles in the Eastern Mediterranean remain unknown. It is worth mentioning that the individuals in this region are smaller than in other places, as are many marine species of plants and animals (the nanism syndrome). In northern countries, members of this order serve as animal fodder. Among the species belonging to this order are the Rhodymenia and the Botryocladia, both members of the Rhodymeniaceae family.