Depicting Temporal, Functional, and Phylogenetic Patterns in Estuarine Diazotrophic Communities from Environmental DNA and RNA

Seasonally nitrogen-limited and phosphorus-replete temperate coastal waters generally host dense and diverse diazotrophic communities. Despite numerous studies in marine systems, little is known about diazotrophs and their functioning in oligohaline estuarine environments. Here we applied a combination ofnifHtranscript and metagenomic shotgun sequencing approaches to investigate temporal shifts in taxonomic composition andnifHactivity of size-fractionated diazotrophic communities in a shallow and mostly freshwater coastal lagoon. Patterns in activenifHphylotypes exhibited a clear seasonal succession, which reflected their different tolerances to temperature change and nitrogen (N) availability. Thus, in spring, heterotrophic diazotrophs (Proteobacteria) dominated thenifHphylotypes, while increasing water temperature and depletion of inorganic N fostered heterocystous Cyanobacteria in summer. Metagenomic data demonstrated four main N-cycling pathways and three of them with a clear seasonal pattern: denitrification (spring) -> N(2)fixation (summer) -> assimilative NO(3)(-)reduction (fall), with NH(4)(+)uptake into cells occurring across all seasons. Although a substantial denitrification signal was observed in spring, it could have originated from the re-suspended benthic rather than planktonic community. Our results contribute to a better understanding of the realized genetic potential of pelagic N(2)fixation and its seasonal dynamics in oligohaline estuarine ecosystems, which are natural coastal biogeochemical reactors.

Automated summary

This study investigated the seasonal shifts of diazotrophic communities in a freshwater coastal lagoon using nifH transcript and metagenomic sequencing approaches. The results revealed a clear seasonal succession of active nifH phylotypes, reflecting different tolerances to temperature change and nitrogen availability. Metagenomic data showed seasonal patterns in nitrogen-cycling pathways, contributing to a better understanding of pelagic N2 fixation dynamics in oligohaline estuarine ecosystems.