If i understood your question, i guess the speciation affects the "methabolism" because metals must be bio-available. For instance and generally speaking, Cr could be needed as Cr(III) or Cr(II) but as Cr(VI) it might be toxic. Iron might be needed as Fe(II) but not as Fe(III) or Fe(0). Therefore conditions in terms of Eh and pH might modify the speciation of metals and make them bio-available or not. Under anaerobic conditions i guess the problem about speciation might be a low potential due to the absence of oxygen and then some metallic nutrient might be present under some reduced form such as the metal one.

The bio-availability depends to a certain extent on the type of metals. According to my observation, with some exceptions such as zinc, anaerobic digestion is seen to decrease the organically bound fraction and increase the solid residue and reducible fractions.The conversion of organic matter in the anaerobic process seems to result in mineralization of the metals. The reducible component increase is due probably to the oxidation process (without oxygen)  involved in the anaerobic process. However, other authors suggested almost the contrary and they may be right. For example, see the link:

http://link.springer.com/article/10.2478%2Fs11696-011-0128-9 )

I have carried out speciation analysis of metals on anaerobically digested sludge and on sludge that did not undergo anaerobic digestion. For metals such as lead, chromium, titanium and nickel ,  the organically bound fraction decreased while the solids residue fraction increased. The reducible portion also increased to some extent. Fore zinc on the contrary anaerobic digestion increased the exchangeable component while it reduced the oxidisable component as well as the solids residue component. This means relatively speaking zinc is more bio available after anaerobic digestion. For iron the organic fraction decreased somewhat while the solids residue increased. However, the change in the exchangeable and reducible fractions was not significant for iron and manganese.

Thank you very much for your comments

@ Ababu Teklemariam Tiruneh

I took a look at your paper, is fractiontion equivalent to speciation?

Dear Eric

Thank you for bringing to my attention the semantic differences between the two. I used the terms assuming  they are equivalent and many papers do use the term interchangeably. However, i looked at the literature and there is a difference. Speciation is used to describe individual chemical species of an element (isotope, oxidation state, complex, etc) and may theoretically be applicable in the case of meals speciation analysis as the species analyzed more or less represent independent chemical species. But the analytical procedure looks more like a fractionation step as the procedure being a sequential (serial) process  may not be sufficiently accurate to identify the species.

Besides pH and EH being the master variables that control speciation, the ability to precipitate will also be important.  Sulfide is a common component in anaerobic digesters and sulfide salts are very insoluble. Thus, sulfide precipitate should be considered, as well as other highly insoluble salts for cations of interest.  Recent research is demonstrating the silver, from nanosilver products, primarily forms silver sulfide in wastewater systems and therefore is released as a soluble cation.