Multicellular Primary Producers: Seaweeds and Algae Chapter 6

Many fascinating photosynthetic organisms populate the oceans, and for the most part, they are very different from the land plants that surround us. Most, in fact, are not considered plants at all and are therefore NOT members of the kingdom Plantae. Despite this, some biologists consider some or all seaweeds to be plants.

Multicellular Algae: The Seaweeds The most familiar types of marine algae are seaweeds. Seaweeds share a general structure, although they show a wide range of growth forms and complexity of structures: Thallus ~ complete body Blades ~ leaf-like, flattened portions of the thallus Pneumatocysts ~ gas-filled bladders that keep blades close to the surface Stipe ~ stem-like structure that provides support Holdfast ~ structure that resembles roots, attaching the thallus to the bottom

Types of Seaweeds There are three types of seaweeds: Green Algae Brown Algae Red Algae It is not always easy to identify the groups by their colors because nature intervenes, through proportions of chlorophyll and other pigments

Green Algae Belongs to Phylum Chlorophyta Common Characteristics: Most live in freshwater Estimated 7,000 species, only about 10% are marine Most are unicellular Dominate in environments with wide variation of salinity – like bays, estuaries, and isolated tide pools on rocky shorelines

Structure of Green Algae Most have a simple thallus in comparison to the other two groups of seaweeds. Pigments and food reserve are same as plants, it is believed that land plants evolved from green algae Chlorophyll is not masked by other pigments, so the thallus is bright green typically

Examples of Green Algae Codium – “Dead Man’s Fingers” Caulerpa Halimeda – Calcareous Green Alga, which means it consists of numerous segments with deposits of calcium carbonate used in formation of coral reefs

Brown Algae Belongs to Phylum Heterokontophyta Common Characteristics: Color varies from olive green to dark brown due to yellow-brown pigments over chlorophyll Approximately 1500 species – almost all marine Dominant primary producer on temperate and polar rocky coasts Brown algae include the most complex seaweeds Largest of all brown algae are the kelps

Structure of Brown Algae The most simple brown algae have a finely filamentous thallus that is flat and branched out Most brown algae are not considered “simple” and have a thick, leathery thallus that can stand exposure to air Many species have gas-filled bladders to keep them afloat near the surface

Examples of Brown Algae Ectocarpus – simplest form of brown algae Fucus – commonly named Rockweed Sargassum – dominant on the California coast and the Gulf of Mexico, has spherical air bladders that keep the small blades afloat at the sea surface

Red Algae Belongs to Phylum Rhodophyta Common Characteristics: More species than the marine green and brown algae combined (approximately 4,000) Most are marine Inhabit most of the shallow-water marine environments Red pigments called phycobilins, which mask chlorophyll Most species are actually red; however, some may have a different color depending on light exposure Some harvested for food

Structure of Red Algae Structure of thallus doesn’t have wide variations in complexity and size as seen in the brown algae Greatly simplified by becoming parasites of other seaweeds Few have traces of chlorophyll and have become heterotrophs, depending solely on their host for nutrients Most are filamentous, but thickness, width, and arrangement of filaments varies

Examples of Red Algae Gigartina– most massive red algae Chondrus – “Irish Moss” Corallina – red algae that deposit calcium carbonate within their cell walls. Important in formation and development of coral reefs

Economic Importance Around the world, workers harvest seaweeds to be used in many ways. Mariculture (the farming of seaweed) is big business in China, Japan, Korea, and other nations throughout the world. The most obvious use is a food source as people from different cultures have discovered that many seaweeds are edible, especially some of the red and brown algae.

Other Economic Uses Food processing Stabilization of dairy products – ice cream, cheese, smooth toppings (caramel, frosting), instant puddings Shampoo, shaving cream Pesticides Production of paper, paints, and cosmetics Print processing for sharper images Canning of ham, fish, and meats Laxatives Fertilizer, animal feeds, reduction of soil acidity