Brown algae are a photosynthetic lineage of heterokonts. They derived their golden brown chloroplasts from secondary endosymbiosis. In this event, an ancestral oomycete engulfed a red alga. As in primary endosymbiosis, instead of being digested, overtime the red alga degenerated into a chloroplast, this time with 4 membranes -- the engulfing membrane from the oomycete, the red alga’s plasma membrane, and the two membranes of the original chloroplast within the red alga. In many groups derived from secondary endosymbiosis, the chloroplast has lost one of these membranes.
Figure \(\PageIndex{1}\): In the diagram above, we see a unicellular photosynthetic eukaryote with a 2-membrane chloroplast. In step one, this organism is engulfed by a heterotrophic eukaryote. In step two, we see the photosynthetic organism inside the heterotrophic organism. In step three, the original photosynthetic organism has been reduced to a chloroplast with 4 membranes. Artwork by Nikki Harris CC-BY-NC with added labels by Maria Morrow.
Brown Algae, Phylum Phaeophyta
Brown algae are brown due to the large amounts of carotenoids they produce, primarily one called fucoxanthin. These organisms are exclusively multicellular and can get so large that they require special conductive cells to transport photosynthates from their blades down to the rest of their tissues. These conductive cells are called trumpet hyphae and have sieve plates and resemble sieve tubes found in flowering plants.
Much like Saprolegnia, the body of an alga is termed a thallus because it is not differentiated into specialized tissues. The general morphology of a brown alga includes a holdfast, stipe, gas bladder(s), and blade(s).
Kelp
Figure \(\PageIndex{2}\): In the diagram above, there are two kelp thalli. The one on the left side is labeled. At the bottom of the thallus is a network of root-like projections that make up the holdfast. The stem-like structure that travels up from the holdfast is the stipe, which terminates in an inflated gas bladder. There are several leaf-like structures attached to the gas bladder. These are blades. The thallus on the right has all of these components, but in a slightly different arrangement. Can you find them? Artwork by Nikki Harris CC-BY-NC with added labels by Maria Morrow.Figure \(\PageIndex{3}\): A bull kelp thallus that was washed up on the beach and arranged in a spiral so all parts would be visible in one image. At the center of the spiral is the holdfast. This would be attached to the sea floor. A long stipe connects the holdfast to a gas bladder that is obscured by many thin blades, which all attach to the top of the gas bladder. Photo by Maria Morrow, CC-BY-NC.
Figure \(\PageIndex{4}\): A piece of feather boa kelp with several small gas bladders. The four gas bladders shown all attach to the edge of the stipe. If the full thallus were visible, gas bladders would be attached down the entire length of both sides of the flattened stipe. Do you see any blades present? Photo by Maria Morrow, CC-BY-NC.
Figure \(\PageIndex{5}\): This kelp thallus was found in a tide pool. It has a prominent blade and stipe, but no gas bladder is visible. Where would the holdfast be? Photo by Maria Morrow, CC-BY-NC.
Fucus
A model organism for the Phaeophyta life cycle is Fucus (rockweed), which, like its relative Saprolegnia, has a diplontic life cycle.
The Fucus thallus has dichotomous branching (forking into two equal branches) and swollen, heart-shaped reproductive tips of the branches. These swollen branch tips are called receptacles.
Figure \(\PageIndex{6}\): The image on the left shows a Fucus thallus (though a bit of the holdfast remained attached to the rock). The thallus branches dichotomously, making Y shapes each time it branches and forks into two equal pieces. The image on the right is a closer view of the ends of the branches. The swollen, heart-shaped ends are called receptacles. The bumps on these receptacles are the tops of the conceptacles--vase-like chambers embedded within the receptacle. Photos by Maria Morrow, CC-BY-NC.
The receptacles are covered in small bumps, each with a pore at the center of the bump called an ostiole. The bumps are conceptacles, chambers that house the male and female gametangia.
Figure \(\PageIndex{7}\): A microscope slide of a cross section through a Fucus receptacle. The cross section reveals the conceptacles, which appear as spherical chambers within the receptacle. In each conceptacle, there are globose structures (oogonia) and branching, tree-like structures (antheridia). Photo by Melissa Ha, CC-BY-NC.Figure \(\PageIndex{8}\): A cross section through a Fucus receptacle shows inside a conceptacle. The male and female gametangia are housed within the conceptacle chamber. The antheridia are branched structures that look like small trees. These produce sperm with heterokont flagella (not visible in this image). The oogonia are globose structures divided into sections as eggs are produced. The eggs will be fertilized by sperm that swim in through the ostiole, forming a diploid zygote that will be released in the marine water. This zygote will grow by mitosis into a multicellular, diploid thallus. Photo by Maria Morrow, CC-BY-NC.Figure \(\PageIndex{9}\): A closer view of the antherida and oogonium inside a Fucus conceptacle. There are no eggs distinguishable within the oogonium. However, individual sperm cells can be seen within the antheridia. These sperm cells would each have the typical heterokont flagella. Photo by Melissa Ha, CC-BY-NC.
Full Life Cycle Diagram
Figure \(\PageIndex{10}\): The diplontic life cycle of Fucus. If you start at the bottom of the diagram, there is a diploid thallus. The ends of the thallus branches are receptacles. A cross section through a receptacle shows us the inside of a conceptacle: the opening of the conceptacle is an ostiole. Within the conceptacle, there is a globose oogonium that has been divided into many compartments. These are egg cells and are the result of meiosis within the oogonium. The branched antheridia produce sperm by meiosis, all of which have heterokont flagella. The haploid sperm are released and swim to fertilize an egg, forming a diploid zygote. The zygote grows by mitosis into a diploid thallus, which brings us back to the beginning. Diagram by Nikki Harris, CC-BY-NC with labels added by Maria Morrow.
