Abstract
Sandy surface sediments of tidal flats exhibit high microbial activity due to the fast and deep-reaching transport of oxygen and nutrients by porewater advection. On the other hand during low tide, limited transport results in nutrient and oxygen depletion concomitant to the accumulation of microbial metabolites. This study represents the first attempt to use flow-through reactors to investigate virus production, virus transport and the impact of tides and season in permeable sediments. The reactors were filled with intertidal sands of two sites (North beach site and backbarrier sand flat of Spiekeroog island in the German Wadden Sea) to best simulate advective porewater transport through the sediments. Virus and cell release along with oxygen consumption were measured in the effluents of reactors during continuous flow of water through the sediments as well as in tidal simulation experiments where alternating cycles with and without water flow (each for 6 h) were operated. The results showed net rates of virus production (0.3–13.2 × 106 viruses cm−3 h−1) and prokaryotic cell production (0.3–10.0 × 105 cells cm−3 h−1) as well as oxygen consumption rates (56–737 μmol l−1 h−1) to be linearly correlated reflecting differences in activity, season and location of the sediments. Calculations show that total virus turnover was fast with 2 to 4 days, whereas virus-mediated cell turnover was calculated to range between 5–13 or 33–91 days depending on the assumed burst sizes (number of viruses released upon cell lysis) of 14 or 100 viruses, respectively. During the experiments, the homogenized sediments in the reactors became vertically structured with decreasing microbial activities and increasing impact of viruses on prokaryotic mortality with depth. Tidal simulation clearly showed a strong accumulation of viruses and cells in the top sections of the reactors when the flow was halted indicating a consistently high virus production during low tide. In conclusion, cell lysis products due to virus production may fuel microbial communities in the absence of advection-driven nutrient input, but are eventually washed off the surface sediment during high tide and being transported into deeper sediment layers or into the water column together with the produced viruses.
Highlights
20% of the primary production in the oceans takes place at the continental shelves (Jahnke, 2010)
For experiments with flow-through reactors, permeable sands were filled into the reactors, seawater from a reservoir was pumped through, and virus and prokaryotic cell numbers were counted in the effluents
The total number of viruses in sediments is a result of virus production and decay as well as import and export processes
Summary
20% of the primary production in the oceans takes place at the continental shelves (Jahnke, 2010). Advective transport is driven by hydraulic pressure gradients and is orders of magnitude faster than diffusion Thereby, it facilitates the deep penetration of particulate and dissolved organic carbon as well as oxygen and sulfate into the sediment fueling microbial degradation processes (Huettel et al, 2014). Intertidal sand flats and beaches fall dry, and in the absence of advection, oxygen is rapidly depleted and metabolic products accumulate (Jansen et al, 2009) During high tide, these degradation products can be flushed from the sediments by advective “skin circulation,” which pumps water through the surface layer with deep penetration of solutes like oxygen (Billerbeck et al, 2006). Nutrients are rapidly supplied from the bottom water into the surface of intertidal sands (Huettel et al, 2014)
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