A. Sabbatini

Deep-Sea Biodiversity in the Mediterranean Sea: The Known, the Unknown, and the Unknowable

Deep-sea ecosystems represent the largest biome of the global biosphere, but knowledge of their biodiversity is still scant. The Mediterranean basin has been proposed as a hot spot of terrestrial and coastal marine biodiversity but has been supposed to be impoverished of deep-sea species richness. We summarized all available information on benthic biodiversity (Prokaryotes, Foraminifera, Meiofauna, Macrofauna, and Megafauna) in different deep-sea ecosystems of the Mediterranean Sea (200 to more than 4,000 m depth), including open slopes, deep basins, canyons, cold seeps, seamounts, deep-water corals and deep-hypersaline anoxic basins and analyzed overall longitudinal and bathymetric patterns. We show that in contrast to what was expected from the sharp decrease in organic carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of both the eastern and the western basins of the Mediterranean Sea is similarly high. All of the biodiversity components, except Bacteria and Archaea, displayed a decreasing pattern with increasing water depth, but to a different extent for each component. Unlike patterns observed for faunal abundance, highest negative values of the slopes of the biodiversity patterns were observed for Meiofauna, followed by Macrofauna and Megafauna. Comparison of the biodiversity associated with open slopes, deep basins, canyons, and deep-water corals showed that the deep basins were the least diverse. Rarefaction curves allowed us to estimate the expected number of species for each benthic component in different bathymetric ranges. A large fraction of exclusive species was associated with each specific habitat or ecosystem. Thus, each deep-sea ecosystem contributes significantly to overall biodiversity. From theoretical extrapolations we estimate that the overall deep-sea Mediterranean biodiversity (excluding prokaryotes) reaches approximately 2805 species of which about 66% is still undiscovered. Among the biotic components investigated (Prokaryotes excluded), most of the unknown species are within the phylum Nematoda, followed by Foraminifera, but an important fraction of macrofaunal and megafaunal species also remains unknown. Data reported here provide new insights into the patterns of biodiversity in the deep-sea Mediterranean and new clues for future investigations aimed at identifying the factors controlling and threatening deep-sea biodiversity.

Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy)

We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (∼10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

Molecular analyses reveal high levels of eukaryotic richness associated with enigmatic deep-sea protists (Komokiacea)

Komokiaceans are testate agglutinated protists, extremely diverse and abundant in the deep sea. About 40 species are described and share the same main morphological feature: a test consisting of narrow branching tubules forming a complex system. In some species, the interstices between the tubules are filled by sediment, creating a mudball structure. Because of their unusual and sometimes featureless appearance, komokiaceans were frequently ignored or overlooked until they formal description in 1977. The most recent taxonomy places the Komokiacea within the Foraminifera based on general morphological features. To examine their taxonomic position at the molecular level, we analysed the SSU rDNA sequences of two species, Normanina conferta and Septuma ocotillo, obtained either with specific foraminiferal or universal eukaryotic primers. Many different sequences resulted from this investigation but none of them could clearly be attributed to komokiaceans. Although our study failed to confirm univocally that Komokiacea are foraminiferans, it revealed a huge eukaryotic richness associated with these organisms, comparable with the richness in the overall surrounding sediment. These observations suggest strongly that komokiaceans, and probably many other large testate protists, provide a habitat structure for a large spectrum of eukaryotes, significantly contributing to maintaining the biodiversity of micro- and meiofaunal communities in the deep sea.

Survival of benthic foraminifera under hypoxic conditions: results of an experimental study using the CellTracker Green method.

We present results of an experimental study, in which benthic foraminiferal faunas have been kept under strongly hypoxic conditions. Sixteen short sediment cores from a 35m deep site in the Adriatic Sea were incubated for a maximum of 69days. Some of the cores were air-bubbled and remained well oxygenated throughout the experiment. The other cores were bubbled with nitrogen; the overlying waters of these cores became strongly hypoxic, whereas the sediment remained virtually without oxygen. Live foraminifera have been inventoried with the CellTracker Green method. Our results show that all dominant taxa survive strongly hypoxic conditions. Nouria polymorphinoides and Nonionella turgida show a clear tendency to move to the sediment surface in the nitrogen-bubbled cores, whereas Bulimina spp. and Eggerella scabra do not show such a migrational response. We suggest that this is a response to the concentration of nutritional resources at the sediment-water interface.

Soft-shelled benthic foraminifera from a hadal site (7800 m water depth) in the Atacama Trench (SE Pacific): preliminary observations

Soft-shelled foraminifera (organic-walled allogromiids and agglutinated saccamminids) are an important component of the deep-sea meiofauna. Although these largely monothalamous taxa are common at bathyal and abyssal sites in almost all oceans, there are only two records from hadal depths. Here we report the occurrence of numerous allogromiids and saccamminids in a sample collected at 7800 m water depth in the Atacama Trench. The >20 μm fraction of the core sample (0–6 cm layer) yielded a total of 546 soft-walled specimens, the vast majority of them Rose Bengal stained, belonging to 20 morphospecies. Most specimens were allogromiids (82.0%), followed by saccamminids (11.0%) and psammosphaerids (6.0%). Allogromiids, particularly Nodellum- and Resigella-like forms, were responsible for a distinct peak around 120–160 μm in the size distribution, while the spherical Allogromiid sp. 1 dominated the larger-size classes. This sample provides further evidence for the widespread occurrence of soft-walled monothalamous foraminifera in marine habitats. A form resembling Resigella is common in the Atacama Trench sample but has not been observed at abyssal sites in the Pacific Ocean or Atlantic Ocean.

Further constraints on the diagenetic influences and salinity effect on Globigerinoides ruber (white) Mg/Ca thermometry: Implications in the Mediterranean Sea

We analyzed Mg/Ca ratios of the planktonic species Globigerinoides ruber (white) picked from 49 box core samples covering the whole Mediterranean Sea and 2 core tops from the Atlantic Ocean. Over the entire data set, we found no significant correlation between Mg/Ca and δ18O-derived calcification temperatures. This lack of correlation is chiefly due to the presence of an early diagenetic, Mg-rich calcite coating, which can constitute up to 20% of the total shell calcite in the central and eastern Mediterranean basin and result in anomalously high Mg/Ca values and a high scattering. In the western Mediterranean Sea, however, G. ruber Mg/Ca scattering shows smaller amplitude and Mg-rich calcite remains under the XRD detection limit. SEM observations indicate that only a few samples are affected by trace amounts of post-mortem calcite overgrowths (most of this calcite being likely removed during the chemical cleaning for Mg/Ca analyses). Using core top sediments from the western Mediterranean Sea, we performed an empirical calibration exercise, which confirms that G. ruber Mg/Ca is not only related to temperature but it is also significantly affected by sea surface salinity. This salinity effect is not specific to high salinity environments such as the Mediterranean Sea, since it appears to be coherent with recent results obtained on Indo-Pacific and Atlantic surface sediments, which suggest that a +1 (psu) change in SSS results in a +1.7°C Mg/Ca-temperature bias. This sensitivity to salinity is significantly higher than those deduced from culture experiments.

Vellaria zucchellii sp. nov. a new monothalamous foraminifer from Terra Nova Bay, Antarctica

Vellaria zucchellii sp. nov. is described from coastal sediment samples from Terra Nova Bay (Ross Sea, Antarctica, 74°40′28.1″S, 164°04′11.6″E, Tethys Bay, 25 m depth). This organic-walled monothalamous (single chambered) foraminifer is characterized by a wide, prominent aperture that facilitates attachment to larger particles (small sand grains or other foraminiferal shells). It shares this feature with the two other known species of Vellaria, both of which were described from an Indian estuary. Phylogenetic analysis of small subunit rRNA gene sequences suggest that V. zucchellii is related to the genus Psammophaga. However, the new species lacks the mineral grain inclusions that are characteristic of Psammophaga. The description of this new organic-walled monothalamous foraminiferal species further documents the high taxonomic diversity of these delicate and abundant protists in the polar benthic communities.

Contribution of soft-shelled monothalamous taxa to foraminiferal assemblages in the Adriatic Sea

Monothalamous foraminifera with organic and agglutinated test walls (“allogromiids” sensu lato) deserve attention because of their importance in deep-sea and shallow-water soft-bottom communities and their crucial phylogenetic position at the base of the foraminiferal evolutionary tree. However, our knowledge of the biodiversity and ecology of monothalamous foraminifera is very incomplete and geographically patchy. Here, we present a short review based on the available data on monothalamous, soft-walled foraminiferal taxa from the Adriatic Sea in response to several environmental parameters (i.e. organic matter, oxygen, grain size, depth). The main results of the studies provide evidence of the importance of these foraminiferal taxa in this shallow, temperate latitude area; they represent a start for the identification of soft-shelled monothalamous morphotypes that could be potential bioindicators of environments influenced by inputs of fresh waters, increasing eutrophication and consequent fluctuations in bottom-water oxygenation. The contribution of this soft-shelled component to living benthic foraminiferal assemblages appears not negligible and excluding it from foraminiferal studies can potentially lead to a loss of ecological information. The study, therefore, provide an atlas of the Adriatic soft-shelled foraminiferal taxa in order to: 1) encourage the species-level description, if possible, or alternatively a basic morphotype characterization; 2) facilitate future comparisons of taxa from similar settings; 3) promote their potential use in future biomonitoring investigations together with the hard-shelled foraminifera.

Abyssal benthic foraminifera in the Polar Front region (Pacific sector): Faunal composition, standing stock and size structure

We have evaluated the quantitative composition of Rose Bengal-stained benthic foraminiferal assemblages of surface samples from two box cores (ANTA98-28 bc; ANTA01-01 bc) collected at the Polar Front in the Pacific sector of the Southern Ocean. Size structure, faunal composition and standing stock were analysed for living (Rose Bengal-stained) and dead foraminifera. The size fractions among 63–150, 150–250 and >250 µm were counted separately in order to study the foraminiferal size distribution and to compare our results with earlier Antarctic studies. The low organic carbon content of the sediment, and hence the limited food availability, is reflected by very low standing stocks and low diversity values. Calcareous taxa dominated dead foraminiferal assemblages that were more diverse than the stained assemblages. In the sediment samples of ANTA01-01 bc, the living fauna contained agglutinated taxa and soft-shelled monothalamous forms. Our data represent the southernmost record of benthic foraminiferal community (63° S) from the Polar Front region in the Pacific sector. These samples provide further evidence for the occurrence of meiofaunal foraminifera in extreme environments, like the Antarctic region, where the uncoupled annual variations in temperature and production cause strong effects on the structure of benthic communities.

Biostratigraphy as a Tool to Validate High-Resolution OSL, CRN, and Sequence-Stratigraphic Data

We present a biostratigraphic study of the Enza section that provides robust constraints on the timing of deposition in a case where the lack of magnetic polarity reversals in the marine part of the section does not allow palaeomagnetic age constraints to be obtained.