Heterokontophyta V Phaeophyceae
Phaeophyceae - the brown algae
- Multicellular, from branched filaments to massive and complex kelp
- Because of their large size, the brown algae were long given their own
division, the Phaeophyta, but they are clearly members of the heterokonts,
and are clearly a comparable group to the other major heterokont groups.
- Major elements in the temperate marine coastal flora, but less conspicuous
in the tropics.
- Relative newcomers in the geologic record
- Unlike diatoms, brown algae do not have hard parts that fossilize
- Some ancient fossils have been interpreted as brown algae, but these
identifications are very tenuous
- About 265 genera, and 1500-2000 species.
- Amost exclusively marine
- Structure & metabolism
- Intercallary meristem located at the juncture between blade
- Meristoderm -- cortical tissue responsible for photosynthesis,
but also capable of meristematic activity
- Flagellate cells are either gametes or zoospores; the vegetative thallus
is never motile
- Flagellate stages are more or less typical heterokont cells, but with
the flagella inserted laterally
- A typical heterokont eyespot is present in most, but a few species
lack the eyespot
- Chloroplasts are generally discoid, and are secondary, with typical heterokont
- A CER is present
- Thylakoids are in groups of three, with girdle lamellae
- Pigmentation is Chlorphylls a and c, with fucoxanthin
giving them a golden-brown color
- No silaceous cell walls or scales are present
- Cell walls are cellulosic, with the cellulose stiffened by calcium alginate.
Mucilaginous substances, including fucoidan and other alginates form the
amorphous portion of the wall.
- During mitosis, the nuclear envelope does not break down until late anaphase,
and is thus considered to be semi-closed. The spindle is not persistant
in telophase, and the daughter nuclei lie relatively close to each other
- Distinctive sporangia are present, and are diagnostic for the Phaeophyceae
- Plurilocular sporangia are involved in gamete formation
- Unilocular sporangia release zoospores (also called zoids)
- Isogamous, anisogamous, or oogamous.
- Most species are diplohaplontic, with an alternation of free-living generations.
These may be isomorphic or heteromorphic.
- In the Laminariales -- the kelps -- the life cycle is a heteromorphic
alternation of generations
- The dioecious gametophytes are microscopic and filamentous, living
at the same depth as the sporophyte holdfast
- The sporophyte is the familar macroscopic kelp
- In the Fucales and Durvillaeales -- called rockweeds or "wracks"
-- the life cycle is diplontic and oogamous. Meiosis is gametic, and the
only haploid stages are the gametes.
- Organisms to know
- Pilayella littoralis (=Pylaiella) widely
used in laboratory studies of brown algae
- Laminariales -- Kelps
- Fucales Rockweeds
- Almost all are marine, although there are four or five small genera that
occur in fresh waters.
- Most abundant and diverse in temperate and polar waters, but a few do
occur in tropical waters.
- Most live attached to a solid substrate, either rock or other hard surfaces
such as barnacles and other molluscs, or sometimes as epiphytes on other
- The major exception are some species of Sargassum, which form a
huge floating mass in the mid-atlantic, the "Sargasso Sea". This
is a very unusual habitat, and there are many distinctive organisms that
depend upon the Sargassum for food and shelter. Nutrient cycling
in the Sargasso Sea needs further investigation; it is a region of unusually
high productivity in relatively nutrient-poor waters.
- Laminaria's distribution is limited in part by the opacity of the
water, the lower limit being the depth at which about 0.6% of the incident
sunlight reaches the sea floor.
- Somtimes live at great depth. According to Bold and Wynne (1985), living
Lobophora variegata was collected at 220 m near the Bahamas.
- Balance between kelp, sea urchins, sea otters
- Economic importance
- Iodine production
Required Reading: VdH Chapter 12, 13
Ricketts, E.F., J. Calvin, and J.W. Hedgpeth; revised by D.W. Phillips. Between
Pacific Tides, 5th edition.Stanford University Press, Stanford, CA. [First
published in 1939, the second edition of Between Pacific Tides had a
forward by John Steinbeck. The book emphasizes animals, but is fun to read and
has some nice information on intertidal zonation and kelps]