Seaweeds The Multicellular Marine Algae

General Structure

This is a very diverse group, and although they lack true leaves, stems, and roots, they have a few common structures.

The complete body is called the thallus no matter the growth form. Usually, all regions of the thallus can photosynthesize.

The leaf-like flattened portions are called blades. The blades are usually the main photosynthetic region.These are not true leaves because they lack veins.

Most seaweeds have gas-filled bladders or floats that will help them maximize sunlight exposure. Sometime these floats contain carbon monoxide.

Some seaweeds have a stem-like structure called the stipe Some seaweeds have a stem-like structure called the stipe. This structure is not found on all seaweeds.

The stipe also allows a place for the attachment of the blades. The stipe provides support and can be long and tough, as in the Giant Kelp. The stipe also allows a place for the attachment of the blades.

A holdfast is a root-like structure that holds the seaweed to the bottom. This structure does not aid in gathering nutrients.

The holdfast does not penetrate through sand or mud, so like this Sea Palm, most macroalgae are only found on hard sediments.

Types of Macroalgae

The are three types of macroalgae Green and Red Brown

Green Algae About 700 of the 7,000 species of green algae are marine.

Green Algae Green algae can be very common where the salinities vary a lot. ( Bays, Estuaries, Tide Pools)

Green Algae They are usually bright green because the chlorophyll is not masked by other pigments.

Green Algae Green algae have a simple thallus when compared to red and brown algae.

Many forms are filamentous or form paper-thin sheets. Green Algae Many forms are filamentous or form paper-thin sheets.

Others form spongy fingers as in this example of Dead Man’s Fingers. Green Algae Others form spongy fingers as in this example of Dead Man’s Fingers.

Brown Algae Almost all species of brown algae are marine. The best know and the most complex are the kelps.

Brown Algae Color varies from olive green to dark brown because there are many yellow-brown pigments that mask the color of chlorophyll.

Brown Algae Besides the well known kelps, Sargasso weed is also a well known brown algae.

Brown Algae Sargasso weed often forms into large mats in the mid Atlantic where unique animals and communities can be found.

Can you see the Sargassum Angler Fish? Brown Algae Notice anything here? Can you see the Sargassum Angler Fish?

There are more species of marine red algae than green or brown. Most are actually red!

Red Algae Some red algae have calcium carbonate within their cell walls. These, help form coral reefs.