Iron a trace metal? Not in many soils. Maybe its low solubility in soils and therefore limited uptake by organism is an issue.

From an evolutionary point of view it may be pure coincidence. Nature often sticks to a solution that works well, although it may not be the most effective, or energy-efficient. A good example is RuBisCO, it is the most abundant protein on Earth, but thermodynamically it is actually quite inefficient in fixing carbon. The two protein lineages may have acquired a certain cofactor and just stuck with it. Btw, crustaceans have hemocyanin as a cofactor to transport oxygen. It has two Cu atoms instead of Fe, and hence their blood looks blue.

Thanks, Emil, that's another hydrothermally induced metal (Cu) porfyrin, we can add. Here is what Konn et al. (2015) writes about the hadean ocean: "Hydrothermal activity is relevant to

Hadean and Archean Earth, as it began as soon as water

condensed to form oceans, and some kind of plate tectonics

(corresponding to crust formation) appeared 4.4 billion years

ago (Wilde et al., 2001). Also, hydrothermal systems as well

as ultramafic rocks were much more abundant on primitive

Earth than today (Russell et al., 1988)." "Different lines of evidence

indicate the presence of significant levels of CH4 (100–

1000 ppm) in the atmosphere in the Archean (Pavlov et al.,

2000; Kasting, 2005). Fiebig et al. (2007) proposed an

abiogenic origin of this CH4. Magnesium (Mg) as well as

transition metals such as iron (Fe) and nickel (Ni) must have

been abundant in the early ocean (Mloszewska et al., 2012).

Mg2 + together with Ca2 + would have been the prevalent

divalent cation, while the prevalent monovalent cation was

Na+ (Pontes-Buarque et al., 2000)." Konn et al., (2015). (my enhancement)

Furthermore, it is well known that these metal substances are often abundant in the vicinity of active hydrothermal systems even on today's ocean floor.  That's why I agree with Konn et al. (2015) in that life started in the vicinity of hydrothermal systems in the Archean ocean. 

Why have all plants porfyrins with magnesium, and all animals have porfyrins with iron or vanadium in their cells?

From the standpoint of evolutionary hierarchical thermodynamics and the principle of substance stability, we can give the following answer. In evolution, living beings are enriched with heavy elements. So the amount of nitrogen, phosphorus, sulfur increases during the transition from the plant world to the animal world. Similarly, thermodynamics explains the transition of plant porphyrins with magnesium to the porphrin of animals with iron, vanadium in their cells.

Have a look at please:

G. P. Gladyshev. Hierarchical thermodynamics and evolution of chemical and biological matter

https://www.researchgate.net/publication/309533110_Hierarchical_thermodynamics_and_evolution_of_chemical_and_biological_matter

and

1. On General Physical Principles of Biological Evolution https://www.researchgate.net/publication/314187646_On_General_Physical_Principles_of_Biological_Evolution

2. Hierarchical Thermodynamics: Foundation of Extended Darwinism 

Imperial Journal of Interdisciplinary Research (IJIR) • February 2017

https://www.researchgate.net/publication/314082150_Hierarchical_Thermodynamics_Foundation_of_Extended_Darwinism

3. Life - A Complex Spontaneous Process Takes Place against the Background of Non-Spontaneous Processes Initiated by the Environment

https://www.omicsonline.org/open-access/life--a-complex-spontaneous-process-takes-place-against-the-background-of-nonspontaneous-processes-initiated-by-the-environment-2157-7544-1000188.pdf

4. The principle of substance stability forms a structure and design of nucleic acids https://www.researchgate.net/publication/320016231_The_principle_of_substance_stability_forms_a_structure_and_design_of_nucleic_acids

5. Сайт RG - https://www.researchgate.net/profile/Georgi_Gladyshev/contributions

Research Hierarchical thermodynamics and evolution of chemical and bi...

Article On General Physical Principles of Biological Evolution

Article Hierarchical Thermodynamics: Foundation of Extended Darwinism

Working Paper The principle of substance stability forms a structure and d...

This looks like an interesting bunch of papers. Will check them out. Thanks Georgi.

Thanks, Georgi, looks very interesting.

To: Dr. Nina Nikolic

You're right. However, thermodynamics explains the fact that organisms are enriched with heavy elements in evolution. It, of course, does not prohibit the presence of relatively light elements in the simplest organisms.

On General Physical Principles of Biological Evolutionhttps:

//www.researchgate.net/publication/314187646_On_General_Physical_Principles_of_Biological_Evolution

Article On General Physical Principles of Biological Evolution

Thanks, Georgi, I will read that report.

Dear Dr. Nikolic,

Thank you. You have cited references to works that set forth various conflicting points of view with the use of the notions of entropy production and ambiguous applications of the second principle. These works do not allow us to make predictions from the position of thermodynamics, which uses concepts about the state functions of systems that (with good approximation) have real physical meaning.

In the late 70s of the last century, I substantiated the possibility of using the methods of JW Gibbs thermodynamics to describe evolution at all hierarchical structural levels of biological systems. I maintain that I. Prigogine's statements about living dissipative structures are erroneous. Descriptions of biological evolution from the point of view of increasing entropy are insufficient. I wrote about these considerations in a number of monographs and articles. In my works I explained many facts and made many predictions! Acquaintance with my work can start with an article:

On General Physical Principles of Biological Evolution https://www.researchgate.net/publication/314187646_On_General_Physical_Principles_of_Biological_Evolution

In this paper, I note some errors of a number of authors related to entropy and the second law. I do not know any facts that evolutionary hierarchical thermodynamics could not explain!

Thanks again.

Yours faithfully,

Georgi Gladyshev

Article On General Physical Principles of Biological Evolution

Magnesium is the key element in Chlorophyll, which is any of several closely related green pigments found in cyanobacteria and the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, chloros and φύλλον, phyllon.Wikipedia

Formula: C55H72O5N4Mg

Thanks Paul,

the question is why and where was magnesium picked up? I certainly believe it comes from periodotite, which is the rock type located in the upper mantle. You have to consider that the mantle was more-or-less exposed directly to the seawater about 2 billion years ago. Peridotite is a rock that contains abundant olivine, which is a complex Magnesium silicate. Since then, the exothermic driven reaction: serpentinization, whereby seawater reacts with periodotite to form the hydrous serpentinite, probably releases magnesium in some form together with hydrogen and CO2 which can have been picked up by the early plant cells performing early photosynthesis.

Hi all, here is an announcement for a session at next years EGU that may be of interest to some of you. The session topic is: Metal cofactors - From ancient to modern times:

http://meetingorganizer.copernicus.org/EGU2018/session/27183

Thanks, Emil, looks interesting.