Abstract
Hadal trenches represent the deepest part of the ocean and are dynamic depocenters with intensified prokaryotic activity. Here, we explored the distribution and drivers of prokaryotic and viral abundance from the ocean surface and 40 cm into sediments in two hadal trench regions with contrasting surface productivity. In the water column, prokaryotic and viral abundance decreased with water depth before reaching a rather stable level at ~ 4000 m depth at both trench systems, while virus to prokaryote ratios were increasing with depth, presumably reflecting the declining availability of organic material. Prokaryotic and viral abundances in sediments were lower at the adjacent abyssal sites than at the hadal sites and declined exponentially with sediment depth, closely tracking the attenuation of total organic carbon (TOC) content. In contrast, hadal sediment exhibited erratic depth profiles of prokaryotes and viruses with many subsurface peaks. The prokaryotic abundance correlated well to extensive fluctuations in TOC content at centimeter scale, which were likely caused by recurring mass wasting events. Yet while prokaryotic and viral abundances cross correlated well in the abyssal sediments, there was no clear correlation in the hadal sites. The results suggested that dynamic depositional conditions and higher substrate availability result in a high spatial heterogeneity in viral and prokaryotic abundances in hadal sediments in comparison to more stable abyssal settings. We argue that these conditions enhance the relatively importance of viruses for prokaryotic mortality and carbon recycling in hadal settings.
Highlights
× 107 prokaryotes mL−1 in eutrophic surface waters to less than 1 × 104 prokaryotes mL−1 at abyssal water depths (e.g., Wigington et al 2016)
Deepocean prokaryotes appear adapted to their in situ conditions (Tamburini et al 2013) and it has been shown that decompression and transient heating during sample recovery from great oceanic depth may induce artifacts that affect solute concentrations and prokaryotic activities (Chastain and Yayanos 1991; Glud et al 1994; Hall et al 2007), but it remains unknown if sample recovery from hadal depths affects the quantification of prokaryotes and virus abundances
The difference between samples fixed in situ vs. onboard was in this case statistically not significant (p > 0.05; paired two-tailed Welch’s t-test), with an average value of 1.2 × 104 ± 2.5 × 103 more prokaryotes (72%) per mL−1 in samples fixed in situ compared to onboard
Summary
× 107 prokaryotes mL−1 in eutrophic surface waters to less than 1 × 104 prokaryotes mL−1 at abyssal water depths (e.g., Wigington et al 2016). Prokaryotic and viral abundances in hadal trenches abundances and intensified diagenetic activities as compared to nearby abyssal settings (Glud et al 2013; Wenzhöfer et al 2016) This apparent increase at hadal depths is presumed to be related to intensified deposition dynamics in trench systems, facilitated by mass wasting, downslope funneling, and fluid dynamics that enrich labile organic material in the central basins (Itou et al 2000; Turnewitsch et al 2014; Ichino et al 2015). VP ratios generally increase with decreasing cell density (Wigington et al 2016) and are elevated in deep waters (Parada et al 2007) These patterns, in combination with the decrease in the relative contribution of eukaryotic grazing to prokaryotic mortality with oceanic depth (Rocke et al 2015), hints an increase in importance of viruses below epipelagic depths (> 1000 m). We evaluate potential sample recovery artifacts and use our data to discuss key drivers of prokaryotic and viral distribution, and assess the overall importance of viruses in the hadal realm
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