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Class Phaeophyceae
Class Phaeophyceae
The Phaeophyta, or the brown algae, are a group of macroscopic, photosynthetic algae. They form some of the largest kelps in the ocean, serving as a crucial foundation for cold-water coastal ecosystems, usually on the west coast of continents. They have structures that are convergent with land plants, such as root-like and leaf-like structures, as well as phloem-like cells for moving sugars. The brown algae possess alginic acid which has many uses including as a thickening agent in edible and non-edible products.
Ecology and form
Ecology and form
Photosynthetic algae, which can be quite large
Brown algae use chlorophyll a & c pigments, as well as fucoxanthin, which is an accessory pigment in the chloroplasts
These pigments give these algae a brown color; they absorb light in the blue-green to yellow-green spectrum
Produce algin or alginic acid
Forms a gum-like substance
Capable of absorbing 200-300 times its own weight
Kelps
Kelps
Brown algae that can grow up to 200 ft long; many are highly differentiated and plant-like
Holdfast: root-like attachment to rocky strata
Stipe: short and stem-like with phloem-like cells for moving sugars
Blade: very long and conducts photosynthesis
Air bladders: maintain buoyancy for the kelp
Above: Macrocytis floating on top of the water
Above: the root-like holdfast and stipe of a kelp
Above: Fucus, rockweed, with air bladders
Above: Macrocytis under the water
Spore-forming phase (=sporophyte)
Spore-forming phase (=sporophyte)
Unilocular structure (1-chambered): produces either spores or gametes through meiosis.
Brown algae can be either haplo-diplontic or diplontic.
In haplo-diplontic algae, the unilocular sporangium will produce spores through meiosis that will germinate into the gametophyte phase.
In diplontic forms, there is no gametophyte. Instead, the unilocular structure produces gametes through meiosis.
Gamete-forming phase (=gametophyte)
Gamete-forming phase (=gametophyte)
Plurilocular gametangia (multi-chambered gametangia): produce gametes through mitosis.
Those gametes are released into the water, and will swim to find another to fuse with in fertilization.
These fused gametes will create a diploid zygote (sporophyte) which will anchor itself to the seafloor and grow into a macroscopic seaweed.
Above: unilocular sporangia of Ectocarpus
Above: plurilocular gametangia of Ectocarpus
Diversity
Diversity
1,500 - 2,000 species in 250 genera
e.g., Laminaria, Fucus, Macrocytis, Ectocarpus, Sargassum, etc.
Mostly marine (99.7%) but there are a few freshwater species such as Sphacelaria lacustris
Dominate rocky shores
e.g. seaweeds, rockweeds, kelps
Geologic Age
Geologic Age
Ordovician(?) - present
Kelp forests appear during Miocene (~15 million years ago)
Above: Bloom of Sargassum on the shores of south Florida
Above: Diversity of brown algae
Additional Resources
Additional Resources
Maine’s Once Abundant Kelp Forests Face an Array of Growing Threats (Inside Climate News 19Dec2025)
└Turf algae redefine the chemical landscape of temperate reefs, limiting kelp forest recovery (Farrell et al., 2025)
Sargasso Sea shift reveals dramatic decline in some historic seaweed populations (Phys.org 4Dec2025)
└Dramatic decline of Sargassum in the north Sargasso Sea since 2015 (Zhang et al., 2025)
Heat-resistant kelp cultivars for warmer seas: A novel triploid breeding method (Phys.org 19Nov2025)
└Breeding of triploid Undaria pinnatifida through crossing aposporous gametophytes derived from doubled haploid sporophytes with haploid gametophytes (Shan et al., 2025)
Main driver of Sargassum blooms in the Atlantic Ocean revealed (Phys.org 5Nov2025)
└Equatorial upwelling of phosphorus drives Atlantic N2 fixation and Sargassum blooms (Jung et al., 2025)
Seaweed-infused ceramic clay offers lighter, greener option for construction materials (Techxplore 28Aug2025)
└Lyra et al. (2025) Life Cycle Assessment of Lightweight Ceramic Clay Aggregates Sintered in a Microwave Oven with the Incorporation of Sargassum spp.
Telling the story of the Atlantic's Sargassum surge with 40 years of data (Phys.org 28Aug2025)
└Lapointe et al. (2025) Productivity, growth, and biogeochemistry of pelagic Sargassum in a changing world
Brown algae genomes reveal ancient origins and evolutionary fate of sex chromosomes (Phys.org 26Aug2025)
└Barrera-Redondo et al. (2025) Origin and evolutionary trajectories of brown algal sex chromosomes
Kelp could protect Scotland's coast from erosion (Phys.org 5Aug2025)
Pacific algae invade Algeria beaches, pushing humans and fish away (Phys.org 4Aug2025)
Students discover sunflower sea stars keep kelp-hungry urchins at bay (Phys.org 28Jul2025)
└Mancuso et al. (2025) Sunflower sea star chemical cues locally reduce kelp consumption by eliciting a flee response in red sea urchins
From kelp to whales: How marine heat waves are reshaping ocean life (Phys.org 18Jul2025)
└Starko et al. (2025) Ecological Responses to Extreme Climatic Events: A Systematic Review of the 2014–2016 Northeast Pacific Marine Heatwave
Restoring kelp forests by culling sea urchins makes financial and ecological sense (Phys.org 1Jul2025)
└Carnell et al. (2025) Prioritising investment in kelp forest restoration: A spatially explicit benefit-cost analysis in southern Australia
Killer whales make seaweed 'tools' to scratch each other's backs (Phys.org 23Jun2025)
└Weiss et al. (2025) Manufacture and use of allogrooming tools by wild killer whales
Kelp forest collapse alters food web and energy dynamics in the Gulf of Maine (Phys.org 6Jun2025)
└Yiu et al. (2025) Kelp forest loss and emergence of turf algae reshapes energy flow to predators in a rapidly warming ecosystem
Can kelp forests help tackle climate change? (Phys.org 3Jun2025)
How seaweed is a powerful, yet surprising, climate solution (Phys.org 2Jun2025)
How much stinky seaweed will South Florida see? (Phys.org 1May2025)
A 'surprising' cause of sargassum blooms in Caribbean: Study pinpoints changes in circulation and wind patterns (Phys.org 13Mar2025)
└Jouanno et al. (2025) An extreme North Atlantic Oscillation event drove the pelagic Sargassum tipping point
Hourglass model of complex multicellularity found in brown algae (Phys.org 26Oct2024)
└Tautz (2024) Brown-algae development joins the hourglass club
The Missing Mammal That May Have Shaped California’s Kelp Forests (NY Times 5Dec2022)
California's Crashing Kelp Forests (UC Davis, 2019)
Blooms of Sargassum along Mexico (Bloomberg 2019)
Blooms of Sargassum in the Caribbean (The Scientist July 2019)
Riding the Kelp Highway (Ocean Wise 10Nov2017)
World's Kelp Forests are Disappearing (YaleEnvironment360, 2017)
Beer made from Sugar Kelp (NPR 16Jul2014)
Brown alga and bacteria produce biofuel (Nature 19Jan2012)
Questions for Thought
Questions for Thought
How are the brown algae similar in structure to land plants?
What do the terms "unilocular" and "plurilocular" refer to?
How is the life cycle similar to land plants?
Why do kelp forests exist on the West coast of North America, but not the eastern coast?
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