Rhodophyta, Red Algae

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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.

Classification

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

Order Bangiales

 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.

Order Nemaliales

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:

 

 Introduction

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.

Order Gelidiales

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.

Order Gigartinales

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.

Order Bonnemaisoniales

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.

Order Coralliniales

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.

Order Ceramiales

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.

Order Rhodymeniales

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.

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