Benthic protists and fungi of Mediterranean deep hypsersaline anoxic basin redoxcline sediments

Abstract

Some of the most extreme marine habitats known are the Mediterranean deep hypersaline anoxic basins (DHABs; water depth ∼3500 m). Brines of DHABs are nearly saturated with salt, leading many to suspect they are uninhabitable for eukaryotes. While diverse bacterial and protistan communities are reported from some DHAB water-column haloclines and brines, the existence and activity of benthic DHAB protists have rarely been explored. Here, we report findings regarding protists and fungi recovered from sediments of three DHAB (Discovery, Urania, L’ Atalante) haloclines, and compare these to communities from sediments underlying normoxic waters of typical Mediterranean salinity. Halocline sediments, where the redoxcline impinges the seafloor, were studied from all three DHABs. Microscopic cell counts suggested that halocline sediments supported denser protist populations than those in adjacent control sediments. Pyrosequencing analysis based on ribosomal RNA detected eukaryotic ribotypes in the halocline sediments from each of the three DHABs, most of which were fungi. Sequences affiliated with Ustilaginomycotina Basidiomycota were the most abundant eukaryotic signatures detected. Benthic communities in these DHABs appeared to differ, as expected, due to differing brine chemistries. Microscopy indicated that only a low proportion of protists appeared to bear associated putative symbionts. In a considerable number of cases, when prokaryotes were associated with a protist, DAPI staining did not reveal presence of any nuclei, suggesting that at least some protists were carcasses inhabited by prokaryotic scavengers.

Inter-comparison of the potentially active prokaryotic communities in the halocline sediments of Mediterranean deep-sea hypersaline basins

“…These differences are most likely shaping prokaryotic community structure in the waters of DHABs (Kerhervé et al 1999). Differing DHAB chemistries have been shown to shape the protistan assemblages of DHAB haloclines, which share only a few common eukaryotic phylotypes (Bernhard et al 2014;Stoeck et al 2014). This leads us to speculate that the geochemical features of DHAB sediments (Table S2) would shape prokaryotic communities as well.…”

“…The surface of the halocline was easily discernable by the schlieren effect produced by the density gradient as dense halocline water encountered normal salinity seawater. Sediment samples were collected with Alvin--type push cores (6.35--cm diameter; hereafter referred to as "cores") obtained from the Deep Submergence Lab at Woods Hole Oceanographic Institution (www.whoi.edu/groups/DSL/) and configured with a seal to prevent contamination during ascent from the seafloor of L' Atalante, Urania and Discovery basins (Figure 2 in Bernhard et al 2014;Table S1). Control or normoxic, i.e.…”

“…sediment underlying the normal salinity water column outside the DHAB, and from under the lower halocline; UC, ULH, respectively) and five from Discovery (two from under the upper halocline, one from under the middle halocline and two from under the lower halocline; DUH1, DUH2, DMH, DLH1, DLH2, respectively). Salinities of the overlying water were measured using a refractometer, and oxygen profiles were performed on replicate cores from the same location (Table S1; Bernhard et al, 2014). Subsamples from each core were taken from the core center using a sterile 20--ml syringe (inner diameter of ca.…”

“…One study examined diversity of Bacteria and Archaea in sediments of the Medee basin (Akoumianaki et al 2012) and another study presented information on bacterial isolates generated from Mediterranean Sea DHAB sediments (Sass et al 2008). A more recent study, conducted in parallel with the investigation presented in this manuscript, focused on protist and fungal assemblages present in the sediments underlying the haloclines of Discovery, Urania and L'Atalante basins (Bernhard et al 2014). Here we show data on active bacterial and archaeal assemblages present in the surface sediments underlying the halocline water column of Urania, L' Atalante and Discovery basins based on analysis of reverse transcribed rRNA gene sequences.…”

Recent Advances in Halophilic Protozoa Research

“…In fact, the eukaryotic communities of the stagnant waters of the DHABs and the underlying sediments are inferred to be more productive and have higher cell abundances than the deep oligotrophic waters and the benthos of the Mediterranean Sea (Bernhard et al. ; Pachiadaki et al. ).…”