Spatial variability of prokaryotic and viral abundances in the Kermadec and Atacama Trench regions

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. Pelagic carbon fluxes generally decrease with increasing oceanic depth and can be approximated by simple power functions known as Martin curves (Martin et al. 1987). The reduction in organic carbon availability with increasing water depth leads to a decrease in pelagic prokaryotic abundances (Azam 1998; Parada et al. 2007

Anammox bacteria drive fixed nitrogen loss in hadal trench sediments

“…The relative abundance peaked at 1.7 to 4.9% of all reads in the nitrogenous zone in the Atacama Trench and 0.2 to 0.4% in the Kermadec Trench. Since total cell counts in the two trenches were similar (20), this is consistent with the relative difference in qPCRbased counts, though lower depth resolution of the profiles in the Kermadec Trench might have smeared peak values (5-or 15-cm sections versus 1-cm sections in Atacama at the depths of interest).…”

“…Thus, the hadal sediments studied here were characterized by broad nitrogenous zones with nitrate and nitrite in the micromolar range and low ammonium concentrations and essentially free of reduced iron (SI Appendix, Fig. S3) and of bioturbation, as indicated by visual layering (20). These redox conditions are similar to those found in the anoxic core of OMZs (34), while previous studies of benthic anammox bacteria have mainly focused on coastal and continental shelf sites (e.g., ref.…”

Plankton respiration in the Atacama Trench region: Implications for particulate organic carbon flux into the hadal realm

“…The findings here, combined with results from complementary studies in the Atacama Trench, suggest a close link between surface productivity and hadal ecosystem processes. Despite the lack of respiratory data from other trenches to compare with, there have been found higher prokaryotic abundances in the hadopelagic zone of the Atacama Trench than in hadal realms underlying less productive water columns (Schauberger et al 2021 ). Given that prokaryotes are an important component of the plankton < 100 μ m, and that these were responsible for most of the respiratory carbon demands measured here (Fig.…”

Bacterial and eukaryotic intact polar lipids point to in situ production as a key source of labile organic matter in hadal surface sediment of the Atacama Trench

“…Among bacteria, those membranes reported to contain PC belong to the alpha and gamma subgroups of the Proteobacteria . Given that these bacterial groups are abundant in trench samples from Puerto Rico (Eloe et al, 2011), the Mariana Trench (Nunoura et al, 2015), and recently in the Atacama Trench (Schauberger et al, 2021), it is possible that PCs present in high abundance in the Atacama Trench are consistent with high abundance of Proteobacteria in these regions. Given their general known association and abundance in Atacama Trench sediments (Fig.…”

“…Recent metagenomic data have revealed the presence of abundant heterotrophic microorganisms in sediments of the Challenger Deep (Nunoura et al, 2018), which are likely fueled by the endogenous recycling of available OM (Nunoura et al, 2015;Tarn et al, 2016). Furthermore, the abundance of prokaryotes in hadal depths can be influenced by dynamic depositional conditions (Schauberger et al, 2021), which in turn may be influenced by the intensity of propagating internal tides (Turnewitsch et al, 2014). All these factors likely alter the deposition, distribution, and composition of OM present in trench sediments.…”