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Algae: Phaeophyceae, Rhodophyceae, Practice Problems and FAQs

Algae are wonderful organisms with a variety of pigments. You must have seen ponds being covered with colourful layers of algae. Did you think algae exist as green coloured organisms only? Well, let me tell you that apart from the green coloured chlorophyll, algae also contain red and brown pigments which are not visible in green algae due to the abundance of chlorophyll.

But, what if I told you that there are algae which are red or brown in colour?? Do not believe me?? Keep reading if you want to know more about these red or brown algae.

Table of Contents

Phaeophyceae or Brown Algae

They are marine forms mostly. Some are present in freshwater and brackish waters also. They are of different types based on the complexity of the thallus:

Morphological form

Example

Image

Filamentous and branched

Ectocarpus

ectocarpus

Ribbon shaped

Sargassum

sargassum

Massive forms

 



Kelps




 

kelps

Common Characteristics of Brown Algae or Phaeophyceae

Plant body is differentiated into a holdfast, stipe and frond. 

  • Holdfast - It helps in attachment to the substratum.
  • Stipe - It acts as a stalk.
  • Frond - It is a leaf like photosynthetic organ. 
     

    ectocarpus

They possess a cellulosic cell wall which has a gelatinous coating of algin on the outer side. Photosynthetic pigments include chlorophyll a, chlorophyll c, carotenoids and xanthophylls (fucoxanthin). The concentration of fucoxanthin regulates the colour of these algae which ranges from olive green to various shades of brown. The reserve food material is laminarin or mannitol.

Reproduction in Brown Algae

Reproduction occurs by vegetaive, asexual and sexual methods.

  • Vegetative reproduction - It occurs by fragmentation.
  • Aexual reproduction - It occurs by pear shaped zoospores which possess two unequal lateral flagella. 
  • Sexual reproduction - It ocurs by isogamy, anisogamy or oogamy.

Life Cycle of Brown Algae

Life cycle is mostly haplontic, except in Fucus (diplontic), Ectocarpus (haplo-diplontic) and kelps (haplo-diplontic). Ectocarpus and Dictyota show isomorphic alternation in generation where gametophyte and sporophyte are morphologically similar and identical. 

Economic Importance of Brown Algae

1. They can be used as food. Examples include Laminaria, Sargassum etc. 
2. Phycocolloids such as algin are extracted from brown algae which are used as emulsifiers in ice creams, tooth pastes etc.
3. Some have medicinal properties. Examples include Laminaria which yields antibiotic-like substances, laminarin sulphate which is a blood coagulant.
4. Iodine is obtained from Fucus and Laminaria

Rhodophyceae or Red Algae

These algae mostly inhabit marine ecosystems but some forms are also found in freshwater and brackish water bodies. Red algae not only grows in the well-lit areas of the water bodies but also at depths which receive negligible amounts of light. They are found in deep waters as phycoerythrin can capture the blue-green part of the spectrum effectively. These are considered to be one of the most ancient algae as the oldest fossil found is around 1.6 billion years ago. 

These algae are mostly multicellular and are of different types based on the complexity of the thallus as follows: 

Common Characteristics of Rhodophyceae or Red Algae

Cell wall is made up of cellulose, pectin, poly sulphate esters etc. Photosynthetic pigments include chlorophyll a, chlorophyll d and phycoerythrin. The predominance of phycoerythrin makes them appear red. Nutrition in red algae is majorly photoautotrophic except for Harveyella which is parasitic in nature. Food is stored as floridean starch.

Reproduction in Red Algae

Reproduction in red algae involves vegetative, asexual and sexual methods. 

  • Vegetative reproduction - It occurs by fragmentation and regeneration of damaged holdfast. 
  • Aexual reproduction - It occurs by the production of non motile spores
  • Sexual reproduction - It ocurs by oogamy.

Life Cycle of Red Algae 

Life cycle is mostly haplontic, except for Polysiphonia which has a haplo-diplontic life cycle.

Economic Importance of Red Algae

  • They can be used as food. Examples include Porphyra, Chondrus etc. 
  • Agar is obtained from Gracilaria and Gelidium. It is used as a thickener in preparations. It is used as a solidifying agent in culture media.
  • Carrageenan obtained from Chondrus is used as a clearing agent and emulsifier.
  • Some red algae like Corallina have medicinal value. 
  • Bromine can be taken from Rhodomela

Practice Problems of  Algae

Question 1. Identify the group in which chl a, chl d, and phycoerythrin pigments are found.

(A). Cyanophyceae
(B). Rhodophyceae
(C). Chlorophyceae
(D). Phaeophyceae

Solution : Rhodophyceae include red algae like Gelidium, Gracilaria etc. They possess pigments like chlorophyll a, chlorophyll d and phycoerythrin.

Question 2. Match the following

Column I

Column II

A. Ribbon shaped

I. Gelidium

B. Parenchymatous sheets

II. Sargassum

C. Lace like

III. Porphyra

(A). A - II, B - I, C - III 

(B).A - III, B - II, C - I

(C) A - II, B - III, C - I

(D) A - I, B - III, C - II

Solution : Gelidium is a red algae in which the thallus is lace-like.

Sargassum is a ribbon-shaped brown algae.

Porphyra is a red algae in which the thallus is arranged as parenchymatous sheets.

Thus, the correct option is c.

Question 3. An algal species stores food in the form of Floridean starch. Which of the following classes does it belong to?

(A). Rhodophyceae

(B). Phaeophyceae

(C) Chlorophyceae

(D) Both B and C are correct

Solution : Algae belonging to the class Rhodophyceae store food in the form of Floridean starch.

Algae belonging to class Phaeophyceae store food in the form of mannitol and laminarin. Chlorophyceae store food in the form of starch.

Thus, the correct option is A.

Question 4. The pigments found in brown algae are

(A) Fucoxanthin and chlorophyll c

(B) Phycobilins and chlorophyll d

(C) Chlorophyll a and chlorophyll b

(D) Phycoerythrin and chlorophyll d

Solution : Photosynthetic pigments present in brown algae include chlorophyll a, chlorophyll c, carotenoids and xanthophylls (fucoxanthin). 

Thus, the correct option is A.

FAQs of  Algae

Question 1. Write down the differences between red algae, brown algae and green algae?

Solution: 

Characteristics

Red algae

Brown algae

Green algae

Habitat

Mostly marine 

Mostly marine

Freshwater, brackish water and marine

Photosynthetic pigments

Chlorophyll a, chlorophyll d and phycoerythrin

Chlorophyll a, chlorophyll c and fucoxanthin

Chlorophyll a and chlorophyll b

Cell wall

Cellulose, pectin and polyester sulphates

Cellulose and algin

Cellulose

Stored food

Floridean starch

Mannitol and laminarin

Starch

Motile stages

Absent

Present

Present

Flagellar number, position of insertion

Absent 

2 unequal flagella arise laterally

2-8 equal flagella arise apically

Examples

Porphyra

Fucus

Chlorella

Question 2. What is an algal bloom? 

Solution: An algal bloom is an excessive growth of algae over a water body. It results in discolouration of the water body. It can lead to the death of fishes and other aquatic creatures. 

Question 3. Define agar. Where is it used?

Solution: Agar is a polysaccharide obtained from red algae like Gracilaria and Gelidium. It is used as a thickener in preparations. It is also used as a solidifying agent in culture media.

Question 4. What are phycocolloids? 

Solution: Phycocolloids are a type of polysaccharides derived from seaweeds. Once dissolved in water, they become viscous and sticky. Alginates, agars, and carrageenans are the common phycocolloids. Alginates are obtained from brown algae. Agar and carrageenan are obtained from red algae.

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