[Cantacoxinha]

I'm looking for an organic compound or a class of compounds naturally present in bodies of water (lakes, seas) which might be able to bind transition metals. So far I was able to find only humic acids as an example. Any suggestion on some other compound? Or a good book or a review on types of organic compounds of natural origin in aquatic ecosystems?

[wildfyr]

The number of compounds is pretty much too numerous to count... but an easy one would be all the various porphyrins

[Babcock_Hall]

Citrate is a possibility, although it would not surprise me to learn that it is labile to microbial uptake.  Siderophores are likely to be present, as well.

[phth]

Researchers spent many moons figuring out ligand-metal equilibrium constants in aqueous media through electrochemistry. Very useful database with tens of thousands of ligand-metal combinations to my recollection. Check out this database: https://www.nist.gov/srd/nist46
Many publications not listed in SciFinder, for example. There were 5 thousand ish ligands for the metal I was interested in. as a function of pH which was very useful for the aqueous system IMHO. Imagine being able to screen 5,000 ligands and choose the best one...