Large-Scale Distribution and Activity of Prokaryotes in Deep-Sea Surface Sediments of the Mediterranean Sea and the Adjacent Atlantic Ocean

Abstract

The deep-sea represents a substantial portion of the biosphere and has a major influence on carbon cycling and global biogeochemistry. Benthic deep-sea prokaryotes have crucial roles in this ecosystem, with their recycling of organic matter from the photic zone. Despite this, little is known about the large-scale distribution of prokaryotes in the surface deep-sea sediments. To assess the influence of environmental and trophic variables on the large-scale distribution of prokaryotes, we investigated the prokaryotic assemblage composition (Bacteria to Archaea and Euryarchaeota to Crenarchaeota ratio) and activity in the surface deep-sea sediments of the Mediterranean Sea and the adjacent North Atlantic Ocean. Prokaryotic abundance and biomass did not vary significantly across the Mediterranean Sea; however, there were depth-related trends in all areas. The abundance of prokaryotes was positively correlated with the sedimentary concentration of protein, an indicator of the quality and bioavailability of organic matter. Moving eastwards, the Bacteria contribution to the total prokaryotes decreased, which appears to be linked to the more oligotrophic conditions of the Eastern Mediterranean basins. Despite the increased importance of Archaea, the contributions of Crenarchaeota Marine Group I to the total pool was relatively constant across the investigated stations, with the exception of Matapan-Vavilov Deep, in which Euryarchaeota Marine Group II dominated. Overall, our data suggest that deeper areas of the Mediterranean Sea share more similar communities with each other than with shallower sites. Freshness and quality of sedimentary organic matter were identified through Generalized Additive Model analysis as the major factors for describing the variation in the prokaryotic community structure and activity in the surface deep-sea sediments. Longitude was also important in explaining the observed variability, which suggests that the overlying water masses might have a critical role in shaping the benthic communities.

Environmental Drivers Controlling Bacterial and Archaeal Abundance in the Sediments of a Mediterranean Lagoon Ecosystem

“…chemical potential energy) in marine sediments is recognised as another important factor that affects the metabolism, distribution, and dynamics of benthic microorganisms [7, 15, 19]. Specifically, under trophic resource-limiting conditions, heterotrophic Bacteria and Archaea are primarily controlled by BPC and POC-flux [7, 15].…”

“…9, 13, 19, 31, 33, 44, 57] and a variety of environmental factors have been reported to influence their abundance, such as sediment depth [20], salinity [47], phosphorus [27], organic matter [26], and pH [23]. Only recently, the role of Archaea in biogeochemical processes started to be elucidated, showing that they are involved in organic matter transformation such as methanogenesis [18], anaerobic methane oxidation (AMO, [8]), protein degradation [25], and aerobic ammonium oxidation [22].…”

Exploring the Relationship between Macrofaunal Biodiversity and Ecosystem Functioning in the Deep Sea

“…The environmental effects were strong in the c-eM basin, where most relationships were context-dependent. Indeed, environmental conditions, such as food depletion or current regime have been reported to be major factors influencing and structuring benthic populations in the Eastern Mediterranean (Tselepides et al, 2000;Króncke et al, 2003;Tecchio et al, 2011;Giovannelli et al, 2013). Furthermore, Zeppilli et al (2016) showed that the high sea floor heterogeneity along an open slope system may shape BEF relationships, even on a very small spatial scale.…”

“…The deep Mediterranean Sea is a highly oligotrophic environment, where nutrient depletion in surface waters combines with high water temperature, promoting the degradation of sinking organic matter (Sardà et al, 2004). The Mediterranean basin is characterized by a trophic gradient between the less productive eastern basin and the more productive western basin (D'Ortenzio and Ribera d 'Alcalà, 2009;Giovannelli et al, 2013; Table S1), where the greater nutrient inputs are due to river runoff, the inflow of Atlantic surface water, and the outflow of relatively nutrient-rich Levantine Intermediate Water through the Strait of Gibraltar (Bergamasco and Malanotte-Rizzoli, 2010). The Mediterranean open-slope systems selected for the present study are characterized by a high percentage of silt sediment at all depths.…”

“…The top 1 cm of a sub-sample from each boxcorer was collected and frozen at −20 • C to analyze chlorophyll-a, phaeopigment, and organic matter content. Replicates of about 1 ml wet sediment were fixed using buffered formaldehyde and stored at +4 • C until processing for total prokaryotic abundance and biomass determination (Giovannelli et al, 2013). The top 20 cm was preserved at +4 • C for grain size analysis.…”