The genetic law of the minimum

How organisms are optimized in the face of environmental challenges remains one of the key unanswered questions in biology. The optimization of enzymes for changing nutrient concentrations in an organism's environment is well known (1). However, intricate genomics studies have revealed that optimization might affect the architecture of the entire genome (2). On page 683 of this issue, Shenhav and Zeevi (3) illuminate how selection driven by resource scarcity can affect the evolution of nucleotide and protein sequences in marine microbes.

In extremely nutrient-scarce regions of the ocean, microbial genomes are often small and streamlined, containing only the most essential genetic information (4). However, the essential selective factor often is not the absolute concentration of a single nutrient but rather its ratio with other required nutrients (5). For example, the sunlit ocean surface is typically limited in nitrogen but not organic carbon, because photosynthetic organisms require much of the former but produce copious amounts of the latter. When biomass from this surface layer dies and sinks, it is degraded by heterotrophic bacteria, which, because of the stoichiometry of elements in their food versus their cells, shift the balance toward carbon limitation (6, 7).

Recent work has shown that this transition from nitrogen to carbon limitation provides an explanation for an abrupt shift in the guanine-cytosine (GC) content in the genomes of marine microbes at the ocean surface versus those in the deep ocean (8, 9). Organisms living under consistent nitrogen limitation have a low GC content in their genomes, leading to a lower nitrogen demand for DNA synthesis. These organisms also use codons that favor a proteome with a comparatively low nitrogen and high carbon content (10, 11).