The nitrogen/carbon content of DNA bases are inverse to each other. Thus while the pyrimidine base cytosine contains one more N than thymine, the latter contains one more C than cytosine. Similarly, the purine base guanine has the extra N , while adenine has the extra C.
G/C are therefore nitrogen rich, while A/T are carbon rich.
Evolution of life in oceans reflects the relative availability of N & C. Thus, microbes living in shallower ocean waters have access to abundant carbon due to fixation of the latter by photosynthetic plants dwelling at the surface of the ocean. However, these microbes are relatively nitrogen poor. The reverse situation applies to bacteria and archaea living at the depths, where plants are much less productive (meaning less photosynthesis), and are thus carbon poor. However, heterotrophic bacteria living in ocean depths have access to greater amounts of nitrogen from decaying plant and animal matter at the bottom.
This differential availability of nitrogen and carbon is reflected in the size and content of microbial genomes in oceans. Thus, microbes living in surface waters have smaller genomes due to the relative scarcity of nitrogen. Furthermore, surface microbes have a low GC content, compared with depth dwelling microbes, but are relatively enriched in AT, the carbon rich-nitrogen poor bases.
This of course has an effect on the absolute amount of proteins synthesised by microbes. Exons are over-represented in GC rich areas of the genome. More exons means translation of more proteins and therefore more nitrogen usage.
Nutritional constraints caused by availability of nitrogen and carbon is reflected even more vividly in the proteins produced by ocean dwelling microbes, and indeed all life forms in general. Redundancy of the genetic code means that there are multiple codons for most amino acids. The choice of a favoured codon also reflects nutritional pressures. Thus, a point mutation in the favoured codon will almost always give rise to an amino acid with a similar nitrogen or carbon content to the original amino acid rather than one with a higher nitrogen or carbon content. The non-favoured codons are not "used" by the mRNA (dictated by the genome from which the mRNA is transcribed) as point mutations here could give rise to more "expensive" amino acids, higher in either nitrogen or carbon content compared with the original. Thus, the genetic code is parsimonious in terms of nitrogen or carbon usage.
To illustrate, the amino acid threonine can be encoded by 4 triplet codes on DNA- ACC, ACA, ACG and ACT. C to G transversion (where a purine is replaced by pyrimidine or vice versa, rather than purine to purine, etc-the latter is called transition) at the second position of the triplet code will give rise to serine for ACT and ACC, but will produce arginine if the transversion occurs in ACG or ACA. Arginine is higher in both nitrogen and carbon content than threonine while serine and threonine only differ in the position of the oxygen atom. In a ground-breaking paper published in Science, Shenav & Zeevi found that in 187 microbial oceanic species, ACT was far more likely to be favoured than ACA, while ACC was similarly preferred to ACG. This demonstrates that the genetic code has evolved to favour lower usage of nitrogen and carbon, as these two elements are likely to be in nutritional deficit in the environment. The same constraints were not found for oxygen, which is abundant.
Once again, due to the inverse relationship between nitrogen and carbon availability, mutations that lead to lower N usage are inversely related to those that lead to lower C usage.
References:
1. L Shenav, D Zeevi. Science 370, 683 (2020).
2. JJ Gryzmski, AM Dussaq. ISME J. 6, 71 (2012).
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