Kelp: Laminaria, a brown algae

Like Fucus (rockweed) Laminaria is representative of an interesting group of organisms that are commonly seen in the intertidal zone and washed up on beaches. Most of the organisms called ‘seaweeds’ are brown algae, although some are red algae and a few are green algae. Like most (but not all) brown algae, Laminaria is a large, multicellular organism that well-adapted to life in intertidal and shallow coastal waters, most commonly in relatively cool waters.

Taxonomy and Phylogeny

The brown algae are a small group of eukaryotes who traditionally were placed in their own phylum, the Phaeophyta, sometimes along with two groups of primarily unicellular algae, the golden algae and the yellow-green algae, based on similarities in pigmentation and other factors. Most modern treatments do not elevate this group to the phylum level but combine it with other ‘heterokonts’, a group defined by having two characteristic flagella, one longer than the other. The heterokonts (sometimes called the Stramenopiles) also includes diatoms and the heterotrophic water molds (Oomycota). The pigmentation of photosynthetic stramenopiles is similar to that of the haptophytes like coccolithophores  and the cryptophytes. This may not represent a common phylogeny but instead that all three groups separately became photosynthetic by acquiring the same photosynthetic endosymbiont. The photosynthetic members of these groups are thought to have been  produced by secondary endosymbiosis and their chloroplasts have four membranes (see discussion in the article on diatoms).

Structure

Typical of most brown algae Laminaria is truly multicellular, and has three distinct  multicellular organs: a holdfast, that attaches the organism to a substrate, a broad flat blade that carries out the bulk of photosynthesis, and a stipe (stalk) that connects the blade to the holdfast and is long enough to allow the blade to obtain light. Many brown algae also have ‘floats’, air filled bladders that cause the blade to be elevated. Brown algae have transport systems that allow photosynthate from the blade to be distributed throughout the plant.

Reproduction

Most brown algae are sexual and exhibit alternation of generations. The ‘dominant’ (i.e. larger and more visible) stage is usually the sporophyte (diploid) stage but there are some brown algae that show isomorphic alternation of generations (the sporophyte and gametophyte look identical) and a few where the gametophyte stage is dominant. Another brown algae, rockweed (Fucus), shows a life cycle like humans, with no alternation of generations and where the only haploid cells are gametes. Gametes are sometimes distinct from one another (egg and sperm) and sometimes all look the same (isogametes). Flagellated cells are common and include sperm, isogametes and zoospores, mobile cells that can attach to a substrate and grow into a new organism.

Matter and energy

Laminaria is a typical photoautotroph, using the energy of sunlight to synthesize carbohydrates from carbon dioxide and then using the carbohydrates as an energy source in cellular respiration and as building materials to synthesize a variety of biomolecules. The brown algae produce a distinct form of a carbohydrate storage polysaccharide called laminaran, made up of glucose units connected by a beta 1-3 linkage, rather than the alpha  1-4 linkage found in starch. They also have high concentrations of mannitol that serves as a transport carbohydrate, a role occupied by sucrose in most plants.

Interactions

Laminaria is an important member of cool, shallow coastal waters. Along with other members of the brown algae they form ‘kelp forests’, providing food for a number of organisms and habitats for others. Rockweed (Fucus) is especially important in the intertidal zone for similar reasons.