The Phaeophyta - Brown algae division

 

The brown algae division is a distinct, homogenous and definable group. Within the division, variety manifests in many aspects, including - among others, the shape of the thallus: some are filamental, some are flattened, and some are highly differentiated within the thallus, resembling higher plants. Some species are large and can extend to 1 metre in length. The brown division includes many species of 5 metres and more. Some reach 30 metres, with an individual weighing as much as 300 kg.

Brown algae contain chlorophyll-a , chlorophyll-c, auxiliary pigments .-carotene, pocoxantin and other xanthophylls. The cell wall is two-layered. The inner layer contains cellulose, and the exterior layer contains gelatinous pectin, used in industry. The most common storage substances are Laminarin (a soluble carbohydrate), mennitol and sometimes fat. Brown algae are highly valuable to the economy. Alginin and other stabilizing and emulsifying materials used in many industries are produced from the larger species, especially Laminariales which does not exist in the Eastern Mediterranean. Thanks to the size of these algae, extremely stable and strong storage substances have evolved within the brown algae division, such as various Alginin derivatives. Their strong spatial structure enables them to sustain the heavy algal load amidst strong currents. Some species are used as food and fodder, as well as manure and fertilizers.

Many brown species can be found in the intertidal zone and the upper layers of the water. Most are immobile, attached to a substrate, and some species may be found at depth of 200 m. Most, however, are to be found in the upper 50 m region. Species worthy of mention that form wide stands in the ocean include: Sargassum, which floats upon wide areas in the Saragossa sea (for which it is named) in the Atlantic Ocean; also noteworthy is the Kelp forest that accompanies the Gulf Stream along the American coast and includes brown algae species whose impressive size sometimes reaches dozens of metres.

The brown division includes about 1500 species. In the past, some people divided the group into three classes, and there are those who feel that the division should be according to regeneration: Isogeneratae: The class includes those algae that regenerate isomorphicly, where the sporophytes and the gametophytes are similar in size and shape. Heterogeneratae: Algae with hetromorphic regeneration belong to this class; the sporophytes and gametophytes are distinct from one another. In Laminaria, for example, the gametophyte is reduced to a series of microscopic strands. There are also differences between the male and female gametophytes. Some of these orders have representatives along the Eastern Mediterranean coasts. Cyclosporae: This class includes algae that do not undergo regeneration, and in which the gametophytic stage has been repressed. The species incorporate a single diploid generation, during which reproduction cells are generated following meiosis. Many species (Sargassum and Cystoseira) have a developed thallus that resembles higher plants. Many species of this class are prevalent along the Eastern Mediterranean shores.

Order Ectocarpales

The Ectocarpales order is considered the most primitive amongst brown algae. The thallus is made of filaments, or strands, or a cluster of strands. Ectocarpus species belonging to the Ectocarpaceae family are common along the coast. Reproduction is quite complex, and haploid, diploid, triploid and tetrahaploid plants have been discovered, each with its own reproductive process.

Order Scytosiphonales

Members of this order belonging to the Scytosiphonaceae family are un-branched, flat or hollow and sometimes full of gases. Each cell contains a single large chloroplast. The genera mentioned in this book belong to the Scytosiphonaceae family, and are the Scytosiphon, Colpomenia, Hydroclathrus and Petalonia.

Order Sphacelariales

This order contains the extremely widespread Sphacelaria, algae attached to a rocky substrate or as epiphytes. During its reproductive process, a large number of spores are released simultaneously through the sporangial perforation. The life cycle stages depend upon environmental conditions: light, temperature and day-length. The order’s distinctive feature is the presence of especially large apical cells. Order members bear some resemblance to species of the Ectocarpaceae family. The species mentioned in the book include Sphacelaria, from the Sphacelariaceae family and Stypocaulon (=Halopteris) from the Stypocaulaceae family.

Another important family that was in the past considered a separate (single-family) order is the Dictyotaceae family, which includes numerous species found in the region. The development of the thallus occurs through a single, central apical cell or through a row of apical cells. In many species, dichotomous growth can be discerned. The male reproductive cells have a single, flagellum with no supplements. On the other hand, the female gamete is relatively large and has no zoospores. The zygote coalesces in water and develops into a diploid plant. In the diploid and haploid generations, male and female plants look similar. Several representatives of the Dictyotaceae family display cyclical generation of gametes, and the period of their release is related to the lunar cycle. Species along our coast include: Dictyopteris, Dictyota, Dilophus -seen in the past and now affiliated with the Dictya genus, Lobophora, Padina, Spatoglossum and Taonia.

Order Fucales

This order is one of the brown division’s most developed. The haploid stage is limited to reproductive cells only, and the rest of the life cycle is diploid (a universal reproductive cycle - see Introduction). Its members are distributed throughout the globe, but species prevalent in the northern seas differ from those of the tropic and sub-tropic.

Fucales members, compared to other algae, have a complex morphological and anatomical structure. The thallus

contains embryonic tissue and tissue that resemble vascular cells in higher plants. Externally, the translocation ‘system’ is surrounded by simple epidermis cells. The outer layer in some species resembles the form of a higher plant, having a ‘stem’ and ‘leaves’. Many species have an air vesicle containing a mixture of gasses that resemble the composition of atmospheric air. At their base, the plants develop a rootlike holdfast. Many species survive for lengthy periods, some individuals for as long as a year or two. At the sea’s depths, 5 to 15-year old individuals have been found. Often, only the upper section of the plant commutes during the year.

Members of the Fucales in the Eastern Mediterranean belong to two families: Cystoseiraceae, which includes several species of Cystoseira, and Sargassaceae, that includes several species of Sargassum.