What is the Difference Between Algae and Seaweed

The main difference between algae and seaweed is that algae are a large, diverse group of photosynthetic eukaryotic plants, ranging from unicellular microalgae, such as Chlorella and diatoms, to macroalgae, such as the giant kelp and large brown algae. But, seaweed refers to the macroalgae that are macroscopic, multicellular, and marine. Furthermore, algae are prominent in water bodies, while seaweed mostly grows in shallow marine waters, under 100 m. Moreover, algae serve as primary producers in aquatic food chains, while seaweed provides essential nursery habitat for fisheries and other marine species.  

Algae and seaweed are two groups of photosynthetic organisms that grow in aquatic habitats such as marine and freshwater habitats. Generally, they play a key role in ecosystems as primary producers. 

Key Areas Covered 

1. What are Algae
     – Definition, Characteristics, Importance
2. What is Seaweed
     – Definition, Characteristics, Importance
3. What are the Similarities Between Algae and Seaweed
     – Outline of Common Features
4. What is the Difference Between Algae and Seaweed
     – Comparison of Key Differences

Key Terms 

Algae, Brown Algae, Green Algae, Kelp, Photosynthetic, Red Algae, Seaweed

What are Algae 

Algae are plant-like, unicellular or multicellular organisms with a thallic plant body. They belong to the kingdom Protista. Also, they only live in aquatic habitats; in both freshwater and marine water. Although most algae are autotrophs, some of them can be heterotrophs. Of these, autotrophic algae contain chlorophyll and undergo photosynthesis. Hence, photosynthetic algae serve as primary producers of most of the aquatic food chains. Besides, they also produce 70% of the atmospheric oxygen. 

Figure 1: Algae on the Seabed

Furthermore, the three divisions of algae are Chlorophyta (green algae), Rhodophyta (red algae), and Phaeophyta (brown algae). Basically, they differ by the combinations of photosynthetic pigments present in the plant body. That is, green algae are a diverse group of algae that contain chlorophyll, beta-carotene, and xanthophyll. Meanwhile, phycoerythrin is the main type of photosynthetic pigment in red algae. On the other hand, chlorophyll c and fucoxanthin are the two main photosynthetic pigments in brown algae.   

What is Seaweed 

Seaweed is essentially macroalgae that exclusively grows in marine habitats. However, it has members in all three groups of algae; Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae. Of these, brown algae, also known as dusky plants, are the most common type of seaweeds. And, these have a brown or yellow-green in color. However, the appearance of the seaweed resembles non-arboreal, terrestrial plants.

Also, holdfasts are the root-like structures, which anchor the brown algae to the surface. Besides, the algal body is a thallus. Its flattened, leaf-like structures are called blades. And, stipe refers to the stem-like structure. Furthermore, the large brown algae are also known as kelp. There are around 30 kelp varieties.

Figure 2: Giant Kelp

Moreover, kelp is the most commonly consuming seaweeds, containing many important vitamins and minerals such as vitamin K, vitamin A, vitamin C, folate, vitamin E, vitamin B12, vitamin B6, niacin, thiamin, riboflavin, pantothenic acid, magnesium, iron, iodine, calcium, sodium, phosphorus, zinc, etc.  

Similarities Between Algae and Seaweed  

• Algae and seaweed are diverse groups of aquatic organisms that are photosynthetic.  
• Also, both are eukaryotes classified under the kingdom Protista.  
• Their cell walls contain cellulose.  
• Furthermore, their plant body does not differentiate into the stem, root, or leaves, and it does not contain specialized tissues for conducting water or nutrients through the plant body.  
• Both extract nutrients from water by diffusion.  
• The three main types of them are  Chlorophyta, Rhodophyta, and Phaeophyta.  
• Although they contain different combinations of photosynthetic pigments, all of them commonly contain chlorophyll and beta-carotene.  
• Both mainly serve as primary producers of moat aquatic as well as some terrestrial food chains. 

Difference Between Algae and Seaweed 

Definition 

Algae refer to a simple, non-flowering, and typically aquatic plant of a large group, which includes seaweed and many single-celled forms. Here, the seaweed refers to the large algae, growing in the sea or on rocks below the high watermark. 

Number of Species 

Algae include a large number of species, while seaweed includes around 10000 species. 

Examples 

Algae range from unicellular microalgae, such as Chlorella and diatoms, to macroalgae, such as the giant kelp and large brown algae. But, seaweed refers to the macroalgae. 

Habitat 

Algae grow in both freshwater and marine habitats, while seaweed exclusively grows in marine habitats. 

Mode of Living 

Algae can either benthic or free-floating on the water, while seaweed is benthic. 

Depth of Water 

Algae are prominent in water bodies, while seaweed mostly grows in shallow marine waters, under 100 m. 

Unicellular or Multicellular 

Algae include both unicellular and multicellular organisms while seaweed is exclusively multicellular. 

Size 

Algae are either microscopic or macroscopic, but seaweed is exclusively macroscopic. 

Mode of Nutrition 

Algae can be either autotrophic or heterotrophic, but seaweed is autotrophic. 

Importance 

Algae serve as primary producers in aquatic food chains, but seaweed provides essential nursery habitat for fisheries and other marine species.  

Conclusion 

Algae are a diverse group of aquatic organisms that are photosynthetic. They can be either unicellular microalgae or multicellular macroalgae. Among them, unicellular algae grow in both marine and freshwater habitats, and they are either benthic or free-floating. On the other hand, seaweed refers to the macroalgae, which are both multicellular and marine. Also, they are benthic and grow in shallow water. Additionally, while algae commonly serve as primary producers in aquatic food chains, seaweed especially provides essential nursery habitat for fisheries and other marine organisms. Therefore, the main difference between algae and seaweed is their size, habitat, and importance. 

References:

1. Vidyasagar, Aparna. “What Are Algae?” LiveScience, Purch, Available Here.
2. “Seaweeds: Plants or Algae?” Seaweeds: Plants or Algae? | Point Reyes National Seashore Association, Available Here.

Image Courtesy:

1. “NSW seabed 1” By Toby Hudson – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “Giantkelp2 300” (Public Domain) via Commons Wikimedia