Nitrogen availability drives gene length of dominant prokaryotes and diversity of genes acquiring Nitrogen-species in oceanic systems

Nitrogen (N) is a key element for prokaryotes in the oceans and often limits phytoplankton primary production. An untested option to reduce prokaryotic N-demand under N-limitation is to reduce gene length. Here we show that in the sunlit Atlantic Ocean genes of the prokaryotic microbial communities in the permanently stratified N-limited (sub)tropics are up to 20% shorter than in N-replete regions further south and north. Average gene length (AGL) of major pelagic prokaryotic genera and two virus families correlated positively with nitrate concentrations. Further, the genomic G+C content of 60% of the taxa was lower and the gene repertoire to acquire inorganic and organic N-species higher in N-limited than in N-replete regions. A comparison of the N-demand by reducing gene length or G+C content showed that the former is much more efficient to save N. Our findings introduce a novel and most effective mode of evolutionary adaptation of prokaryotes to save resources including N and energy. They further show an enhanced diversification of genes acquiring N-species and -compounds in N-deplete relative to N-replete regions and thus add important information for a better understanding of the evolutionary adaptation of prokaryotes to N-availability in oceanic systems.