The cup-shaped chloroplasts, on at least some of them, would suggest Chlamydomonas. The size is consistent with that as well.  I kind of get a hint of flagellae on some of the cells. Are you able to see any evidence of flagella - even if you can't see them are they motile?  Do you have access to phase-contrast or Nomarski DIC illumination - flagella should be readily visible when viewing with these.

There are countless spherical, unicellular algae species of this size and appearance. To obtain any morphological details you would need a much higher resolution (electron microscopy). Getting physiological or ecological data would be helpful but it is a laborious thing. Nowadays the best way to unambiguously identify a microalgae species is DNA barcoding (using for example ITS-2 regions).

thanks for all the suggestions and that will help me to go ahead. 

Hi!

Yes this algae could be Chlamydomonas but I have serious doubt abouit It!

In your micrograf can't be seen the two characteristic flagella charactheristic for this tax.group .

Could be this cells of  some speciimen of Chlorella also if you haven't seen any flagella

Could be also some specimen of Scenedesmus or Desmodesmus  despite of ovoid shape of the cell , because many algal spcies can change their external morfological aspect dramaticaly if their environmental conditions .eg. water chemistryor  other abiotic factors has benn changed considerably -see the case of different algal cell cutlures-   external  or even internal  morfological shape changes of cultured cells durring different culturing condition-culturing medium composition ( chemistry) or as the effect of  shifting other environmental culturing condition-eg illumination related changes.

So I suggest carefull examination of those algae taking in the account some of the above mentioned suggestion regarding other LM teqniques and DNA analysis or send this algae to a taxonomist specialised   in sphaerical green algae-Chlorococcales and Volvocales-taxonomy!.

Hope my anwer was helpfull 4you

Best wishes Zsolt Nagy-László

Dear Sandeep, I  agree with Nagy that the specimen  could be Chlorella vulgaris  Beyerinck [Beijerinck] 1890, belonging to the class Trebouxiophyceae. I got  almost the  similar photomicrograph from my collections. I also recommend molecular identification using ITS–2 as a marker if the sample is pure.

May I suggest a paper for your reference:

Taxonomic reassessment of the genus Chlorella (Trebouxiophyceae) using

molecular signatures (barcodes), including description of seven new species

Good luck for greater works ahead!

thank you Zsolt Nagy-László and Jackson for your suggestions ...

Good luck and all the best wishes for you and  for your's work

with kind regard Zsolt

This alga has spherical unicellular cell with a cup-shaped chloroplasts. Dimension and presence of flagella (a few cell on graph have) are required, would suggest Chlamydomonas. Morever, ecological preference of the species would be helpful but it is a laborious thing. Chlamydomonas species prefer high nitrate and phosphate at warm water bodies. Best regards

This alga has no flagella and its cell wall is rather hard according to the photo. So it might not be Chlamydomonas. I agree with Nady-Laszlo and Jakson.

thank you all for the valuable suggestions..

I not agree, mostly this is not Chlamydomonas. Is this culture have any mucilage?

With these magnification it is difficult to identify. Perhaps Oocystis

cf. Schizochlamydella delicatula