Dear Ana,

from the modern methods may be DSC - Differential scanning calorimetry would be usefull for you.

Here the page

https://www.netzsch-thermal-analysis.com/ru/produkty-reshenija/differencialnaja-skanirujushchaja-kalorimetrija/

but in Russian. Try  in English proposals.

I know that Olga Shapchenkova from Belarus has several publications on the Question you are interested in. But she is not on RG and I have no her address. Try engines.

As well may be can try thermal gravimetry.

I faced once a problem of differential selection of different groups of periphyton (epiphyton). Giant job.

https://www.researchgate.net/publication/261364530_Involvement_of_Some_Components_of_Epiphyton_in_Organic_Matter_Decomposition

Andrey

Article Involvement of Some Components of Epiphyton in Organic Matte...

Perhaps the authors of this study could help?

https://www.researchgate.net/publication/226905036_The_potential_of_fish_production_based_on_periphyton

Article The potential of fish production based on periphyton

Would total DNA or total protein be a sensible proxy?

Those are easy...

thank you everybody!

http://www.tandfonline.com/doi/abs/10.1080/10454430802329762?src=recsys&journalCode=wjaa20

https://books.google.com.tr/books?id=RR_fCQAAQBAJ&pg=PR19&lpg=PR19&dq=the+analysis+of+periphyton%27s+energy+content&source=bl&ots=CvNaOHKd8g&sig=-HAk847RXSijYfFs6T6swLn7Z5U&hl=tr&sa=X&ved=0ahUKEwiAq9CovJPYAhXPh7QKHVTaBgsQ6AEIRDAE#v=onepage&q=the%20analysis%20of%20periphyton's%20energy%20content&f=false