Abstract
The diel cycle is of enormous biological importance in that it imposes temporal structure on ecosystem productivity. In the world’s oceans, microorganisms form complex communities that carry out about half of photosynthesis and the bulk of life-sustaining nutrient cycling. How the functioning of microbial communities is impacted by day and night periods in surface seawater remains to be elucidated. In this study, we compared the day and night metaproteomes of the free-living and the particle-attached bacterial fractions from picoplanktonic communities sampled from the northwest Mediterranean Sea surface. Our results showed similar taxonomic distribution of free-living and particle-attached bacterial populations, with Alphaproteobacteria, Gammaproteobacteria and Cyanobacteria being the most active members. Comparison of the day and night metaproteomes revealed that free-living and particle-attached bacteria were more active during the day and the night, respectively. Interestingly, protein diel variations were observed in the photoautotroph Synechococcales and in (photo)-heterotrophic bacteria such as Flavobacteriales, Pelagibacterales and Rhodobacterales. Moreover, our data demonstrated that diel cycle impacts light-dependent processes such as photosynthesis and UV-stress response in Synechococcales and Rhodobacterales, respectively, while the protein regulation from the ubiquitous Pelagibacterales remained stable over time. This study unravels, for the first time, the diel variation in the protein expression of major free-living and particle-attached microbial players at the sea surface, totaling an analysis of eight metaproteomes.
Highlights
Introduction iationsMicroorganisms in marine ecosystems are extremely diverse, dominate biomass and play key roles in biogeochemical processes [1,2]
The reads encoding for the 16S rRNA were extracted from the Ocean Sampling Day 2014 (OSD14) metagenome and reflected the abundance of each operational taxonomic unit (OTU) in the studied bacterial communities
The metagenome taxonomic structure showed that Proteobacteria was the most abundant phylum, with 66.89% of the total detected 16S rRNA bacterial reads, followed by Bacteroidetes (15.51%) and Cyanobacteria (12.22%) (Table 1)
Summary
Microorganisms in marine ecosystems are extremely diverse, dominate biomass and play key roles in biogeochemical processes [1,2]. Marine picoplanktonic communities are composed of both free-living and particle-attached microorganisms. A comparison of these bacterial fractions in coastal environments showed differences in cell abundance [4], morphology and metabolic activity [5]. In terms of phylogenetic diversity, studies suggested that free-living and particle-attached communities were fundamentally different [6,7,8], while others reported high similarities between both fractions [9,10,11]. The diel oscillation of solar radiation reaching the Earth’s surface temporally structures biological events, activities, and physiological processes across all kingdoms of life [12]
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