Abstract
Microbial community dynamics are influenced not only by biological but also physical and chemical phenomena (e.g., temperature, sunlight, pH, wave energy) that vary on both short and long-time scales. In this study, samples of continental shelf waters of the northwest Atlantic Ocean were periodically collected from pre-sunrise to post-sunset and at multiple depths over summers of 2016 and 2017. Metatranscriptomic analyses revealed expression of photosynthetic genes in surface water samples corresponding to a diel relationship with sunlight. Photosynthetic genes originated from known phototrophs including Aureococcus, Ostreococcous, Synechocococus, and Prochlorococcus. Photosynthetic gene expression occurred pre-sunrise, suggesting the community initiates transcription before sunlight exposure, ostensibly to harvest energy more efficiently when the anticipated increase in light occurs. Transcripts from photoheterotrophic members of the SAR11 clade were also documented in surface samples, with rhodopsin expression being more abundant pre-sunrise and post-sunrise. Conversely, samples taken from the aphotic layer exhibited expression of transcripts related to nitrification that did not vary over the diel cycle. Nitrification gene transcripts, specifically amoA, nirK, hao, and norAB, were taxonomically related to well-known genera of ammonia oxidizers, such as Nitrospira, Candidatus Nitrosomarinus, Nitrosospira, and Nitrosopumilus. Overall, this study documents the role of light (varying with time and depth) in shaping the photosynthetic microbial community activity in the surface ocean, and further demonstrates that this diel regulation of photosynthesis is decoupled from the activity of the nitrifying microbial community in deeper and darker waters.
Highlights
Microbial communities change over short-term time scales to occupy ecological niches within marine ecosystems
As light intensity varies over the course of a day, the dynamics of photosynthesis, dissolved organic carbon (DOC) production and utilization, nutrient uptake and regeneration and community metabolism should all be reflected in the corresponding dynamics of the metatranscriptome
The 5 m samples of the 2016 cruise and the 3 m samples of the 2017 cruise were within surface sunlit layers (“photic”), the 16 m sample of the 2016 cruise was at the peak of the deep chlorophyll max (“DCM”) layer, and the 30 m samples from both the 2016 and 2017 cruises and the 40 m samples from the 2017 cruise were collected at depths where the photosynthetically active radiation (PAR) was 1% or lower
Summary
Microbial communities change over short-term time scales to occupy ecological niches within marine ecosystems. Examining shifts in community gene expression over short-time periods can allow for insight into tightly coordinated ecosystem-level dynamics that orchestrate daily biogeochemical fluxes and into how they may respond in the face of rapid natural and anthropogenic environmental change. The relatively short time scales of diel changes in light intensity affect a multitude of different physical variables in the environment. Viruses have been shown to have diel oscillations in surface ocean microbial communities, resulting additional recourse competition within this ecosystem (Aylward et al, 2017). Vislova et al (2019) investigated depthdependent changes in diel patterns for primary and secondary production, metabolic activity, and gene expression in marine microbial communities in the NPSG and found diel transcript oscillations in microbial communities decreased in abundance and regularity with depth and light levels
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