Chapter 6 Multicellular Primary Producers: Seaweeds and Plants
Seaweeds transform solar energy into chemical energy in the form of organic matter and make it available a
long list of consumers.
Seaweed also produces oxygen for organisms both on land and in the ocean.
Multicellular Algae: The Seaweeds
 Macrophytes or macroalgae
 Multicellular, eukaryotic organisms
 Large variety of adaptations to their environment
A. General Structure
 Lack true leaves, stems, or roots of plants
 Complete body of seaweed is known as thallus
 The leaf-like portions of the thallus are known as the blades, which are the main regions of
photosynthesis
 Gas-filled bladders are known as pneumatocysts. These help keep the blades close to the surface
 The stem-like structure that provides support is called the stipe, from which the blade originates.
 The holdfast, looks like roots, attaches the thallus to the bottom. They are not involved in significant
absorption of nutrients or water.
B. Types of Seaweeds
 There are three types of seaweeds; the green, brown, and red algae.
 Green Algae
o Division Cholophyta, live in freshwater and terrestrial environments
o Multicellular green algae are common in some marine environments.
o Bright green due to the chlorophyll in their cells
o Ulva, or sea lettuce
 Brown Algae
o Division Phaeophyta
o Varies from olive green to dark brown
o Yellow-brown pigment, fucoxanthin
o Dominant primary producers in temperate or polar rocky coasts, also the most largest and
structurally complex seaweeds
o Fucus, rockweed
o Kelps are the most complex and largest of all brown algae
o Macrocystis, largest of the kelp, estimated to grow 20in a day
o Kelp beds are the richest, most productive environments in the marine realm
 Red Algae
o Division Rhodophyta
o More species than the green or brown algae
o Red pigments called phycobilins
o Parasites of other seaweeds
o Porphyra common in rocky shores
C. Life History
 Reproduction in seaweeds is by asexual and sexual means.
 Sexual reproduction may involve an alternation of haploid and diploid generation.
D. Economic Importance
 Mariculture
 Phycocolloids, gelatinous chemicals that can form viscous suspensions or gels at low concentrations
(algin, carrageenan, and agar)

Algin can be found in dairy products such as ice cream and cheese. Also used in frosting, Rx, shampoo,
shaving cream, rubber products, paints, and cosmetics. Major producers of algin is the giant kelp
(Macrocystis)
 Carrageenan is taken from red algae and is used as an emulsifier.
 Agar is used to protect ham, fish, and meats during canning. It’s also used as laxatives, Rx, and in
cosmetics. It’s also used as a nutrient medium for bacteria, and the analysis of DNA.
 Seaweed can also be used as a fertilizer, food additive, and dressing wounds.
Flowering Plants
 Flowering plants, dominant on land, have few marine representatives.
 Seagrasses, which are true marine species, and salt-tolerant plants like salt-marsh plants and mangroves,
are exceptions.
 These plants have successfully adapted to soft-bottom coastal regions, developing as highly productive
meadow and, in the case of mangroves, forest along the shore.