Seaweeds: Plants or Algae?

Did you know seaweeds are technically not plants, but a type of algae? You'll often find them washed up on the beach, or waving in tidal pools near the shore. Wade into the ocean and you might feel some slippery against your feet and legs. But what are they seaweeds or algae? 

What is Seaweed?
Seaweeds inhabit seawaters, and are primitive plants belonging to algae family. However, there is no particular definition for the term seaweed, as there is no one common ancestor to seaweeds, but is a more common term to describe a certain group of plants with distinctive properties. Marine macroalgae, or seaweeds, are plant-like organisms that generally live attached to rock or other hard substrata in coastal areas. They belong to three different groups with over 10,000 species: brown algae (phylum Ochrophyta, class Phaeophyceae), red algae (phylum Rhodophyta), and green algae (phylum Chlorophyta, classes Bryopsidophyceae, Chlorophyceae, Dasycladophyceae, Prasinophyceae, and Ulvophyceae). Red and brown algae are almost exclusively marine, whilst green algae are also common in freshwater (rivers and lakes), and even in terrestrial (rocks, walls, houses, and tree bark in damp places) situations. Many of these algae are very ancient organisms, and although lumped together as "algae", are not really very closely related, having representatives in 4 of the 5 or 6 kingdoms of organisms. Seaweeds have been useful for humans in many ways viz. food, medicine, fertilizer, and industrial products, as those are rich in vitamins and other nutrients. Carrageenan, agar, and many other gelatinous products come from seaweeds.

What is Algae?
Algae include one of the most primitive organisms on the Earth, with fossils evidences dating back to more than three billion years. Earlier, algae included both prokaryotic and eukaryotic organisms, but now only the eukaryotes are included in the classification. They also do not have a common ancestor. They could be either unicellular or multi-cellular in their structure, and accordingly microscopic as well as macroscopic. They inhabit any aquatic ecosystem including freshwater, saltwater, and brackish water. Almost all the algal species are photosynthetic and show autotrophy. Algae collectively produce the largest amount of energy through photosynthesis. However, they are very simple in cellular structures including the gigantic kelps, without many complex organs (leaves, roots… etc.) as in terrestrial plants. This is an extremely diverse group with an unthinkable number of species. According to the US National Herbarium, there are 320,500 collected specimens, but there is no correct estimation on the number of algal species in the world.

Seaweeds differ from plants in important ways:

  • Seaweeds absorb water and nutrients in all their tissues, directly from the surrounding water. They don't have the complex system of roots, specialized tissues and leaves that help plants move water and nutrients in their body.
  • Seaweeds can do photosynthesis in all their tissues; most plants photosynthesize only in their leaves.

What is the difference between seaweed and algae?

  • Seaweeds are a group of algae, and have some special characteristics viz. macroscopic, multi-cellular, benthic, and marine.
  • Diversity of algae is extremely high and incomparable with that of seaweeds.
  • Algae could be both unicellular and multi-cellular, whereas seaweeds are necessarily multi-cellular.
  • All the seaweed species are autotrophic, whereas some algal species rely on other external food materials.
  • Algae inhabit both freshwater and marine waters, while seaweeds inhabit only seawaters.
  • Marine algae can distribute over shallow as well as deep waters, while seaweeds mostly inhabit shallow waters.

Below are some resources to help you learn more about this topic and handy reference tools for your next tidepool adventure. 

Algae and Marine Plants of Point Reyes National Seashore Species List

Common Seaweeds of Point Reyes (Photos) 

San Francisco Bay Area National Parks Science and Learning Website: Marine Plants and Algae

 The Seaweed Site