Anabaena Cynobacteria


Anabaena

Classification

·         Domain – Bacteria
·         Phylum – Cyanobacteria
·         Class -  Cyanophyceae
·         Order – Nostocales
·         Family – Nostocaceae
·         Genus – Anabaena

Introduction

·         Anabaena is a genus of filamentous cyanobacteria, or blue-green algae.
·         It found as plankton. It is known for its nitrogen fixing abilities.
·         They form symbiotic relationship with certain plants, such as the mosquito ferns.
·         Some species of anabaena are endophytes. They live in the roots of Cycas and Azolla.

Ultra-structure

·         Anabaena has filamentous structure.
·         The filaments of anabaena consist of string of beaded cells.
·         Several intercalary heterocysts are present in the trichome.
·         The filaments are either straight or circulate or irregular.
·         Filaments occur within the sheath.
·         Sheath is always hyaline and watery.



Heterocyst

·         A heterocyst is a differentiated cyanobacterial cell that carries out nitrogen fixation.
·         The heterocyst functions as the site for nitrogen fixation under aerobic conditions.
·         They are formed in response to a lack of fixed nitrogen (NH4 or NO3).
·         They contain only photosystem 1, which enables them to carry out cyclic photophoshorylation and ATP regeneration
·         These changes provide the appropriate conditions for the functioning of oxygen-sensitive nitrogenase.
·         Heterocyst is of same shape as of vegetative cell and is mutually interdependent.
·         During times of low environmental nitrogen about one cell out of every ten will differentiate into a heterocyst. Heterocyst then supplies neighboring cells with fixed nitrogen in return for the products of photosynthesis that they can no longer perform.

Akinetes

·         An akinete is a thick walled dormant cell derived from enlargement of a vegetative cell.
·         It serves as a survival structure. It is a resting cell of cyanobacteria.
·         Akinetes appear thick walled with granular looking cytoplasm, under magnification.
·         The akinetes are filled with food reserves, and have a normal cell wall surrounded with 3 layer coat.
·         Development of akinetes from a vegetative cell involves: Increase in size, Gradual disappearance of gas vacuoles, Increase in cytoplasmic density, number of ribosomes and cyanophycin granules.

Reproduction

·         Anabaena is reproduced only by vegetative and asexual methods.
·         The sexual reproduction is completely absent.
·         Anabaena reproduce vegetatively by the following methods-
·         Fragmentation
·         Old trichome becomes very large and irregular due to which it gets to break up into short fragments.
·         These short fragments of trichome divide vegetative cells and develop into new trichome.
·         Hormogones are the short fragments of trichomes. Developed in the region of heterocyst.
·         Then they came out of the trichome due to some movement.
·         They divide vegetative cells and developed heterocyst and again surrounded by sheath. In this way new trichome is formed.

Anabaena reproduce asexually by following methods

Akinetes

·         The akinetes are produced in mature colonies. They are formed in unfavorable conditions.
·         They are also called arthrospore or resting spore. They are penetrating bodies.
·         In favorable conditions they directly or indirectly giving rise to new filaments.
·         The contents of akinete divide into bits prior to germination.

Heterocyst

·         Heterocyst cell divide transverly and form (2-4) celled hormogones.
·         These hormogones come out by bursting the thick wall of heterocyst and germination occurs to give rise to new trichome.

Significance

·         Anabaena and azolla have formed a symbiotic relationship in which the cyanobacterium receives carbon and nitrogen sources from the plant in exchange for fixed nitrogen. This relationship is useful to humans in production of food; especially in the fertilization of rice paddies.
·         Anabaena performs oxygenic photosynthesis very similar to that of eukaryotic plant and algae, by utilizing water as a reductant source, consequently producing molecular oxygen.
·         Anabaena provides a model for the study of gene differentiation in the formation of heterocyst. The recent sequencing of the genome of anabaena species strain PCC 7120 is yet another step towards understanding this process.

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