If an Earth-like exoplanet had a sufficiently extensive plant biosphere (i.e large forests, aquatic algal mats etc), is it at all conceivable that scientists could spectroscopically detect the presence of biochemicals, such as chlorophyll, anthocyanins etc?
If possible, how would the spectra of these biochemicals be distinguished from other chemicals?
Take a look at this article: http://www.nature.com/nature/journal/v513/n7519/full/nature13785.html
Every more or less stable compound has its identifying absorption lines, based, well, on what the compound is made of. The question is, what is the minimum amount for a compound in an given atmosphere at a given distance with a certain sun to be still detectable. In deed, this also depends on how "empty" the light's way to us is. But guessing, you want to know, if we propably could detect an exo-planet with vegetation on it - yes, this is possible under circumstances. Most of all, the area covered by the possible organisms plays an important role.
