The brown algae are multicellular, large and complex seaweeds, which dominate rocky shores in temperate and polar regions. Apart from one or two freshwater types, they are all marine. The presence of fucoxanthin masks the presence of chlorophylls a and c. (In this context it must be stated here that not all ‘brown’ seaweeds look brown, norindeed do all the ‘red’ ones look red). Unlike the higher plants and green algae, which use starch as a food reserve, the phaeophytes use an unusual polysaccharide calledlaminarin (β-1,3-glucan).
The level of tissue organisation in the brown algae is greatly in advance of any of the types we’ve discussed so far. The simplest thallus of a brown alga resembles the most complex found in the greens.
The phaeophytes also represent an advance in terms of sexual reproduction; here oogamy is the usual state of affairs and alternation of generations has developed to such an extent that diploid and haploid stages frequentlyassume separate morphological forms. Again, we shall use two examples to illustrate life cycle diversity in the brown algae.
Laminaria is one of the kelps, the largest group of brown algae. It grows attachedto underwater rocks or other objects by means of holdfasts, root-like structures which anchor the plant. The thallus is further subdivided into a stalk-like stipe and a broader, blade-like lamina. Reproduction in Laminaria involves sporophyte and gametophyte plants that are morphologically quite distinct; (heteromorphic alternation of genera-tions). Reproductive areas called sori develop on the blade of the diploid sporophyte at certain times of year (Figure 9.9). These consist of many sporangia, interspersed with thick protective hairs called paraphyses. As the sori develop, meiosis occurs, leading to the production of haploid zoospores. These in turn develop into haploid filamentous gametophyte plants, much smaller and quite different in morphology from the more highly organised sporophyte. Indeed, in contrast to the large sporophyte the gameto-phyte is a microscopic structure. The gametophytes are dioecious, that is the male and female reproductive structures are borne on separate individuals. The female plant bears a number of oogonia, each of which produces a single egg, which escapes through a pore at the apex of the oogonium, but remains attached in a sort of cup, formed by the surrounds of the pore. In similar fashion the male plant bears several antheridia, each liberating a single antherozoid; this however is motile by means of flagella and fertilises the egg. The diploid zygote so produced grows immediately into a new sporophyte plant.
In our second example of a phaeophyte life cycle, there is no alternation of generations at all, the gametophyte generation having been completely lost. The wracks are familiar seaweeds found in the intertidal zone, and Fucus vesiculosus, known commonly as the bladder wrack, is one of the best known (Figure 9.10). It gets its name from the air bladders distributed on its surface, which assist buoyancy.
The adult has reproductive structures called receptacles, slight swellings situated at the tip of the thallus; within these are flask-like invaginations called conceptacles which contain the male or female gametangia, again interspersed with sterile paraphyses. F.vesiculosus is monoecious but some other Fucus species are dioecious. Each antheridium
undergoes meiosis, followed by mitosis to produce 64 antherozoids or sperm, while by the same means eight eggs are produced in the oogonium. At high tide these gametes are released into the open water. Fertilisation results in a diploid zygote, which continues to drift quite free, while secreting a mucilaginous covering. It eventually settles, becoming anchored by the mucilage, and germinates into an adult individual. Here then, we have a life cycle in which there is no gametophyte generation, and no specialised asexual reproduction (although in certain conditions fragments may regenerate to form adults).