diatoms(algae)

Diatoms:algae

division:chrysophyta/bacillariophytaclass:baccillariophyceaegenus:diatoms

• pigment diatomin present(golden brown)algae
• diatoms are classified into 2 orders on the basis of their symmetry.

order 1:pannales

it shows bilateral symmetry

order 2:centrales

it shows radial symmetry

occurence:

• found in aqutic environment both fresh water and marine water.
• some occur as planktons.
• some species are found in terrestial habitat.
• some species are epiphytic on other algae.

thallus structure

• diatoms are unicellular algae,the cells may sometimes get ambeded in a common gelations matrix to fodrm simple colonies.
• the cell wall shows the deposition of silica.
• cell wall is made up of 2 overlapping theca,the smaller fitting into the larger.
• it is called as frustuies.
• the outer/upper half is the epitheca and inner/lower half is hypotheca.
• each theca has 2 main parts.
• 1)valve:a more or less flattened plate like structure the surface of the theca.
• 2)the connecting band(cingulum):the incurved margins of the valves.it form girdle.
• when the frustules lies in such a way that upperside/lowerside/valveside is seen it is called as valve view or top view and when connecting bands are uppermost it is called girdle view/side view.
• frustules is made up of pectin.in valve region,frustules is silicified.
• this siliceous material is laid down in certain pattern that makes the wall ornamented.
• in pinnate diatoms,ornamentition is symmetrically that arranged on either side of the central line.in the middle of theca there is a longitudial slit called 'raphe'.
• raphe extends form one end to the other end.
• each pole the raphe terminated into a  swelling called as 'polar nodule'.
• raphe is interrupted at the mid point by a swelling called central point by a swelling called central nodule.
• protoplast is surrounded by plasma membrane.inside is a thick layer of cytoplasm surrounding a large central vacuole.
• the cells are uninucleate,oval or spherical nucleas embedded in the pariential cytoplasm.
• chloroplast are large pariental plate like arranged parallel to each other.
• movement occur through gliding.

vegetative reproduction 

• occurs by cell division.
• before the cell division,the protoplast of the cell increases in size pushing away epitheca from hypotheca.
• the nucleus divides mitotically followed by division of protoplast into two daughter cell produces a new hypotheca.
• the daughter cell which has the epitheca nearly of same size as that of the parent cell is nearly of same size as that of the parent cell.

asexual reproduction

• it takes place by means of auxospore.
• by continuous multipliction by cell division there is reduction in cell size in the successive generation.
• the reduction can continue only upto certain limit and beyond this limit either the cell die or convert into an auxospore.
• in auxospore formation,the original size of cell is restored.
• the cell secretes a large amount of mucilage which pushes the 2 valve apart.thus the protoplast is set free.
• it grows in size and attains the normal cell size.
• then it secrets around itself a silicified membrane'perizonium' new valves are secreted inside and auxospore becomes an independent cell.

    sexual reproduction 

• mostly homothalic,isogamous in pennate diatoms.
• this process occurs when the cell is reduced to a certain critical point after the cell division.
• two such cells come close to each other and get enveloped with in a common muucilaginous mass.
• the diploid nuclei of thes cells undergo meiosis and produces 4n nuclei.
• out of 4 nuclei in a cell,2 degenerates and thr another two remain function.
• the protoplast of the cell divides in 2 segments, each carrying a single haploid nucleus.
• two gametes from different cell fuse to form a zygote.
• the two zygote formed will grow in size to from auxospore.
• the auxospores secretes a perizonium layer and siliceous valve are formed.

  life cycle

•  life cycle is diploid predominant.
• the vegetative cell is diploid.
• the haploid phase is represented only by gametes formed by meiotic division of diploid cells.
• these gametes fuse to form zygotes which are diploid.
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