sir i m not sure but i think these are Aspergillus flavus. u can pure these colonies on SDA media and than identified by microscopy.

First pick material from a colony and check it under the microscope. You might easily see if it is a yeast (as you suggested) or a fungus or a bacterium. If you are interested in the pigment you might perform an acetone/methanol extraction and run a absorption spectrum to get a clue on the type.

A list of pigment-forming microorganisms can be found in:

Malik et al. (2012) Microbial pigments: a review. International Journal of Microbial Resource Technology 1(4):361-365

Yes, sezer is right. they seems to be psuedomonas or even flavobacterium... but spp aeroginosa is also pathogen you cans test simply pathogenic tests

It could be the pseudomonas species probably P. aeruginosa.

They produces green pigment as well as grows in LB agar and at 4 degrees Celcius too.But production of only green pigment is not the confirmatory test, further analysis through at least biochemical test should be carried out . Molecular methods like 16S rRNA sequencing is consider as gold standard for the bacterial identification.

For this biochemical tests will be sufficient.

Don't you think they might be algae? From my experience I would say that green coccoid algae look very similar, especially from the colour. Green algae are eukaryotes, they have bigger cells than those of bacteria; you can also check if they have red autofluorescence of chlorophyll.

I have not worked with algae, so can't say anything. But guess what, our lab is just 40 meters away from an estuary and there is every possibility of an algal contamination.

I agree with Daria, that the green organism/colonies can be algae - green algae or blue-green algae (cyanobacteria). I have seen the growth of the blue-green algae on marine-water agar plates when I studied the distribution of marine bacteria. Now we performe toxicity tests with the algae Pseudokirchneriella subcapitata, which grow very well on a mineral agar medium (form green colonies). I don´t believe that it can be heterotrophic bacterium. I have sampled different soils and waters and never seen such green colony of a bacterium. The organism must be photosyntetic. There exist photosynthetic sulfurbacteria in the environment, bus they are strong anaerobes.

@ Naveen: Your LB plate was kept at 4 deg. C. that means it must have been kept in a refrigerator without any source of light. If it is so then you can count out any algae contamination as they would require light to grow. Also there is small possibility of fungi contamination on LB because the medium is used for bacteria but in any case you put the thing on a slide and observe under a microscope. That would help in zeroing the group of the contaminant. And I dont think it would be right for anybody to identify a microorganism just by looking at the photograph. There are procedures laid done for identification and if you are interested in identifying the organism because you think it is producing a green pigment then you should carefully follow the protocols for bacteria or fungi which ever organism that may be.