Bacterial Biogeography across the Amazon River-Ocean Continuum.

Mary Doherty,Patricia M Medeiros,Victoria J Coles,Patricia L Yager,Byron C Crump,Mary Ann Moran,Jérôme P Payet,Alex V Krusche,Henrique O Sawakuchi,Jeffrey E Richey,Brandon M Satinsky,Caroline S Fortunato,Nicholas D Ward

doi:10.3389/fmicb.2017.00882

Abstract

Spatial and temporal patterns in microbial biodiversity across the Amazon river-ocean continuum were investigated along ∼675 km of the lower Amazon River mainstem, in the Tapajós River tributary, and in the plume and coastal ocean during low and high river discharge using amplicon sequencing of 16S rRNA genes in whole water and size-fractionated samples (0.2–2.0 μm and >2.0 μm). River communities varied among tributaries, but mainstem communities were spatially homogeneous and tracked seasonal changes in river discharge and co-varying factors. Co-occurrence network analysis identified strongly interconnected river assemblages during high (May) and low (December) discharge periods, and weakly interconnected transitional assemblages in September, suggesting that this system supports two seasonal microbial communities linked to river discharge. In contrast, plume communities showed little seasonal differences and instead varied spatially tracking salinity. However, salinity explained only a small fraction of community variability, and plume communities in blooms of diatom-diazotroph assemblages were strikingly different than those in other high salinity plume samples. This suggests that while salinity physically structures plumes through buoyancy and mixing, the composition of plume-specific communities is controlled by other factors including nutrients, phytoplankton community composition, and dissolved organic matter chemistry. Co-occurrence networks identified interconnected assemblages associated with the highly productive low salinity near-shore region, diatom-diazotroph blooms, and the plume edge region, and weakly interconnected assemblages in high salinity regions. This suggests that the plume supports a transitional community influenced by immigration of ocean bacteria from the plume edge, and by species sorting as these communities adapt to local environmental conditions. Few studies have explored patterns of microbial diversity in tropical rivers and coastal oceans. Comparison of Amazon continuum microbial communities to those from temperate and arctic systems suggest that river discharge and salinity are master variables structuring a range of environmental conditions that control bacterial communities across the river-ocean continuum.

Highlights

The phylogenetic composition of microbial communities in freshwater and marine water predicts, to some degree, the genomic potential of communities in each of these environments (Zaneveld et al, 2011; Langille et al, 2013; Staley et al, 2014), and shapes the ecosystem services they provide in river and coastal ecosystems (Judd et al, 2006; They et al, 2013; Wear et al, 2014)
During each sampling period temperature was similar at all sites, but conductivity, Dissolved inorganic carbon (DIC), chlorine, sulfate, sodium, potassium, and magnesium were higher in the mainstem (Óbidos and Macapá) compared to the Tapajós River. pH was similar at Óbidos and Macapá, and was different at Belém and in the Tapajós River on most sampling dates
This first survey of microbial communities along the riverocean continuum of the largest river in the world revealed that seasonal and spatial patterns were not fundamentally different than those previously observed in smaller rivers (Levine and Crump, 2002; Sekiguchi et al, 2002; Crump and Hobbie, 2005; Winter et al, 2007; Crump et al, 2009), river plumes (King et al, 2013; Mason et al, 2016), and riverocean continuums (Fortunato et al, 2012; Ortega-Retuerta et al, 2013; Ma et al, 2016), suggesting globally consistent patterns in microbial community composition across river-ocean gradients

Summary

Introduction

The phylogenetic composition of microbial communities in freshwater and marine water predicts, to some degree, the genomic potential of communities in each of these environments (Zaneveld et al, 2011; Langille et al, 2013; Staley et al, 2014), and shapes the ecosystem services they provide in river and coastal ecosystems (Judd et al, 2006; They et al, 2013; Wear et al, 2014). This large mass of water is associated with intense biological activity, delivering nutrients to support enhanced primary production (Demaster and Pope, 1996), including diatom blooms in lower salinity plume water, and large blooms of diatom-diazotroph assemblages (DDAs) at higher salinities (Carpenter et al, 1999; Subramaniam et al, 2008; Yeung et al, 2012) These blooms are associated with carbon export to the deeper ocean as they die off and sink to the bottom (Subramaniam et al, 2008; Yeung et al, 2012)

Methods

Results

Conclusion