The Chlorophyta - Green algae division

The green algae division is the largest, insofar as number of species is concerned. It includes 500 genera and 8,000 species: single cell, multi-cell and sometimes multi-nuclear. Most of the species develop in fresh water and only one tenth in the sea. Most continental species have a cosmopolitan distribution, and few are endemic. Most sea species are also widely distributed. The division is characterized by a high level of inter-species divergence, due to both their cosmopolitan nature and evolutionary duration.

Most freshwater species are haploid usually reproducing asexually. Only towards the end of the season can sexual reproduction be observed. The zygote generated is wrapped in a cover that enables it to withstand a dry season or even frost outside the water. Most marine species are diploid, and only before sexual reproduction does meiosis occur. In some species sexual reproduction was not observed, but only asexual reproduction. Zoospores (asexual reproduction cells) are common in many species. The green division is the most primitive taxon in which the Heteromorphic generations life cycle has been observed.

The thallus’ green colour indicates a high level of chlorophyll-

a. Plants contain both chlorophyll-b and carotenoides. The most common storage substances among green algae are starch and sometimes fat. The composition of photosynthetic pigments and storage substances resemble those of higher plants. And, indeed, according to many researchers, green algae are the progenitors of continental vegetation. The cell wall contains cellulose, sometimes accompanied by calcium, but some species have no cell wall. In spite of their promulgation, and in contrast to red algae, green algae have relatively low economic value. Some species are used to recycle water or as fodder for fish and invertebrates. As mentioned, the taxonomy of the green division is complex and controversial. The classifications within the taxon were determined on the basis of the presence of certain enzymes (de Hydrogenesse), based on the location of flagellum and according to certain stages in the mitotic division. In the past the green division was classified into two taxons (or evolutionary genealogies): the Bryophytan line and the Chlorophycea line. Based on more recent research using electronic microscopy and comparative cytological research, the division was divided into four classes (Mattox and Stewart, 1984): The Ulvophyceae class - reproductive cells in members of this class incorporate several zoospores, often two or four, which are interconnected by unique microtubular roots. Nearly all the species in this class are marine. The Micromonadophyceae class, which embodies green algae, is primitive on various levels, and has rudimentary zoospores. It would seem that the other species evolved from this class. The Chlorophyceae class is divided into

nine orders and mostly includes freshwater algae. The Charophyceae class includes freshwater species, mostly microscopic. Some researchers believe the green Charophyceae division should encompass an independent division. The class divides into four orders, none of them related to a saline environment.

Class Ulvophyceae

Various publications contain different classifications into families and classes. The green division - more than any other - demonstrates the taxonomy of those groups. I will describe five of the orders mentioned in this book:

Order Ulvales

Ulvales are green algae, very common in the upper and intertidal zone. The leaf is comprised of only two cellular layers; in Ulva, they are attached to one another, and in the Enteromorpha the thallus is cylindrical, the two cellular layers separated. Group members are favoured by fish. Although eaten by people, as well, their economic value is low. Certain species can enrich wetlands used for recycling and treatment of sewage. Due to their elemental structure, their cosmopolitan distribution and their resemblance to continental plants, some Ulvales are used for research. Their life cycle is often isomorphic (male and female look the same). Reproductive cells are often released in accordance with the lunar cycle. The gametes converge in water, generating four-zoospore zygotes that germinate on a suitable substrate. The young sporophytes are filamentous, and resemble regular plants upon maturing. Along Eastern Mediterranean shores, the species of this order belong to two genera: Ulva and Enteromorpha. Recently, it was proposed that both be combined into a single species -Ulva. (Hayden et al., 2003). In this edition, we are abiding by the traditional system, both Enteromorphas and Ulvas appearing as distinct species. Nevertheless, in the list of species the new name appears as well.

Order Cladophorales

This order has one family, the Cladophoraceae – filamentous, branched and straight. The meaning of the name ‘carrying branches’ refers to the branching out of the thallus. Individuals connect to a substrate, often forming bushes or sponge-like clumps. Upon maturing, fresh water species sometimes detach from the substrate and float freely. The cells have multi-cellular nuclei (coenocyte), and in each cell a large protoplast is encircled by a chromatophore and often also by a large vacuole. Regeneration is isomorphic: the zoospores and the gametes generate at the end of the filament. Reproductive cells are mobile and bi-zoosporic. The Cladophoraceae order comprises many species, most very similar and difficult to discern. 300 species are estimated

 

 Introduction

to live in the Mediterranean. The Cladophoraceae family includes the Cladophora and the Chaetomorpha, both of which can be found along the coast.

Order Dasycladales

All the species in this order live in tropical and sub-tropical seas. The order is very ancient, and fossilized examples have been found from the Silurian period. The order is simple to identify thanks to the characteristic structure of its species. The thallus is radial, and the laterals are arranged in whorls along the main axis.

The species we are concerned with belong to two families - Dasycladaceae and Polyphysaceae. Members of the Dasycladus and Acetabularia genera can be found along the coast, the latter, though, quite rare but often used for scientific research.

Order Caulerpales

Algae belonging to this order lack cellulose in their cell walls. Their thallus has no partition (coenocytes) between its cells. It does, however, contain two pigments (siphonoxanthin and siphonein) that are not usually found in green algae, but which do characterize this specific order. Species live in the sea, are comparatively large, and individuals are found in warm water. The order is referred to in some literature as Siphonales or Codiales, and includes a number of families, of which I will mention the more important ones:

Bryopsidaceae: This family is sometimes referred to as Derbesiaceae, and includes three genera. The coenocytic thallus does not accumulate calcium, is branched and usually soft. It contains a large contractile vacuole (an expanding bubble) surrounded by cytoplasm and many small nuclei. The cell wall is only apparent in reproductive cells, and cells often divide using a fragmentation process. The female reproductive cells are darker and contain more chloroplasts compared to the male reproductive cells, which are smaller and brighter. The release of gametes is cyclical, precipitated by environmental factors, such as light and turgor pressure. The zygote settles upon a substrate and germinates soon afterwards. The most prominent member of the family found in the intertidal zone of the Eastern Mediterranean and around the rest of world is the Bryopsis, a delicate, dark green alga that resembles a feather.

Codiaceae: species belonging to this family constitute a large percentage of the tropical sea population. These species have a complex and interesting structure, and they have acquired many popular names: ‘Sea fan’ (Flabellia petiolata), ‘Dead men fingers’ (Codium) and ‘Neptune’s shaving brush’ (Penicillus). The life cycle is universal (see introduction) and mostly comprised of the diploid stage. The gametes have flagella, and the female ones are larger

and usually darker than the male ones. The Codiaceae family is one of the most important among the ‘reef-builders’. Its calcium secreting process is unique and characterized by the external proteins that deposit calcium. This was found also on fossils of prehistoric species of the Codiaceae family. Common species along the Eastern Mediterranean coast belong to the genera Codium, Halimeda, Flabellia (=Udotea) and Boodleopsis.

Caulerpaceae: The Caulerpaceae family has one single genus -Caulerpa. They are multi-nuclear plants (coenocyte) with two types of plastides: chloroplast and amyloplast. This genus is most common in tropic and sub-tropic regions at the intertidal zone and deeper. The algae in this genus are green, attractive and simple to define. Caulerpa means ‘crawling stalk’, and was chosen thanks to the unique structure of the thallus, which resembles a stalk extending horizontally or the stalk of a root.

Along this ‘pseudo-root’, groups of colourless rhizoids are located that enable the alga to cling to substrate. This structure allows the Caulerpa to develop on sandy substrates as well, a habitat where few alga species can subsist, leaving the Caulerpa to grow unhindered. From the central stalk, erect leaf-like units rise that are different from one species to the next. The algal body is large and relatively hard, and forms a continuous, partitionless, multinuclear unit. Its stability is maintained by hard fibres located inside the alga, forming a sort of inner skeleton. Some species are consumed by the inhabitants of the Philippine Islands. One species, the Caulerpa taxifolia, which resembles our Caulerpa mexicana, recently developed along the northwest Mediterranean coast at an alarming rate (see internet reportage). All the species in this group are easily dried for research, and so have been subject to many experiments.

Order Siphonocladales

Marine algae common primarily in tropical seas. The thallus is multinuclear (coenocyte). Apparently, sexual reproduction is mostly isogamous. Members of the Valoniaceae genus have been found to contain zygotes, but the entire life cycle has not yet been observed. Young individuals contain an apical cell that resembles a vesicle. The vesicles condense, press against one another, then develop lengthwise, developing into secondary branches. Each cyst, including the holdfast, develops from a single apical cell. Species mentioned in this book belong to the Valoniaceae family: Anadyomene, Valonia and Cladophoropsis.