Abstract
Microbes occupy diverse ecological niches and only through recent advances in next generation sequencing technologies have the true microbial diversity been revealed. Furthermore, lack of perceivable marine barriers to genetic dispersal (i.e., mountains or islands) has allowed the speculation that organisms that can be easily transported by currents and therefore proliferate everywhere. That said, ocean currents are now commonly being recognized as barriers for microbial dispersal. Here we analyzed samples collected from a total of six stations, four located in the Indian Ocean, and two in the Southern Ocean. Amplicon sequencing was used to characterize both prokaryotic and eukaryotic plankton communities, while shotgun sequencing was used for the combined environmental DNA (eDNA), microbial eDNA (meDNA), and viral fractions. We found that Cyanobacteria dominated the prokaryotic component in the South-West Indian Ocean, while γ-Proteobacteria dominated the South-East Indian Ocean. A combination of γ- and α-Proteobacteria dominated the Southern Ocean. Alveolates dominated almost exclusively the eukaryotic component, with variation in the ratio of Protoalveolata and Dinoflagellata depending on station. However, an increase in haptophyte relative abundance was observed in the Southern Ocean. Similarly, the viral fraction was dominated by members of the order Caudovirales across all stations; however, a higher presence of nucleocytoplasmic large DNA viruses (mainly chloroviruses and mimiviruses) was observed in the Southern Ocean. To our knowledge, this is the first that a statistical difference in the microbiome (from viruses to protists) between the subtropical Indian and Southern Oceans. We also show that not all phylotypes can be found everywhere, and that meDNA is not a suitable resource for monitoring aquatic microbial diversity.
Highlights
Microbes constitute more than 90% of oceanic biomass (Suttle, 2007)
Measurements of absolute dynamic topography and sea surface temperature in the months during and prior to the sampling of the southern hemisphere samples indicate that stations S1 and S2 were not directly influenced by the Agulhas Return Current or Antarctic Circumpolar Currents (Rusciano et al, 2012) at the time of sampling (Figure 1). This confirms that stations S1 and S2 are representative of the greater south-western Indian Ocean gyre, while stations S3 and S4 are located south of the Antarctic Polar Front (APF) in the Southern Ocean (∼1000 km north of Antarctica) and stations S5
31.17% of the OTUs were unique to the SouthWest Indian Ocean, 23.30% present exclusively in the Southern Ocean, and 15.04% belonging to the South-East Indian Ocean (Table 3)
Summary
Microbes constitute more than 90% of oceanic biomass (Suttle, 2007). With more than 70% of the Earth’s surface covered by ocean, they drive almost half of the global net primary production (Azam et al, 1983; Cho and Azam, 1990; Field, 1998). We have yet to uncover their full significance in ocean processes. This is largely due to the fact that the majority of microorganisms cannot be grown in the laboratory (Handelsman, 2004) and are challenging to identify morphologically. Thanks to the developments in sequencing technologies, we are unveiling marine microbial diversity without the need for cultivation (Loman et al, 2012; Flaviani et al, 2017). The majority of sequences coming from marine (Sogin et al, 2006; Brum et al, 2013), soil (Roesch et al, 2007), or human gut (Turnbaugh et al, 2009) microbiome studies still remain largely uncharacterized
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