EVIDENCE OF BIOLOGICAL ACTIVITY RELATED TO ANOXIC HYPERSALINE LAKES AND MUD VOLCANOES
C. Corselli*, D. Basso*, E. Erba*, G.Rodondi°, F.Brusa^ *Dept. Of Earth Sciences, University of Milano °Biology Dept., University of Milano ^Dept. Of Food Science and Microbiology, University of Milano

Anoxic basins: microbiology. Biological studies on the anoxic basins of the Eastern Mediterranean started after the discovery of gelatinous matter of organic origin. The laminar gelatinous matter was observed within the cores collected in the Bannock and Tyro basins, containing anoxic sediments obtained during oceanographic expeditions for the geological exploration of the Mediterranean Ridge (R/V Bannock, cruises from 1984 to 1990). Microbiological and ultrastructural investigations were carried out on core sediment samples and on overlying water and brine in the Urania, Discovery and L’Atalante basins (R/V Urania, cruises 1993, 1994). The organic nature of the mucilaginous pellicles found in the cores and their relation with numerous microbic forms present in all the samples were demonstrated. Viable microorganisms, prevalently Gram negative and aerobic as well as facultative anaerobes, were found in the samples collected above, within and below the water/brine interface. Different microbic forms were isolated in pure culture: a vibrio (Nitrosovibrio spp.) a coccus (Staphylococcus spp.) and some rods of the family Pseudomonadaceae. In addition, laminar formations were observed in growth medium of mixed cultures that could be interpreted as the first stages of the mucilaginous pellicles seen in the cores.



Gelatinous pellicle structure with increasing magnifications. Submillimetric, anastomosing microlaminae forming an alveolar structure that entraps several biogenic remains.
E.Erba,G.Rodondi,E.Parisi,H.L.Ten Haven, M.Nip and J.W. De Leeuw (1987) -Gelatinous pellicles in deep anoxic hypersaline basins from the eastern Mediterranean. Marine Geology, 75: 165-183.

Formation and sedimentation model for pellicles. Bacterial mats grow at the interface where they trap biogenic and inorganic particles. When pelagic sedimentation prevails, pellicles sink to the basin floor after load increase at the interface (right). Turbidity currents and sediment slumping disrupting the interface destroy the mats and trasport fragments of pellicles to the basin bottom (left).
E.Erba (1991) -Deep mid-water bacterial mats from eastern Mediterranean anoxic basins. Marine Geology, 100: 83-101.
Corselli C.And McCoy F.W. (1989) -Sedimentation of organic matter, Bacino Bannock. In: M.B.Cita, A.Camerlenghi and C.Corselli (Editors), Anoxic Basins of the Eastern Mediterranean. Ric.Sci., Suppl., 72: 50-53.

Urania Basin water samples. 1-2 Microorganisms and pellicles are present. The laminar portions (L) show fibrils variously oriented and no connections with bacteria are visible ( <--> )(ruthenium red)(x25,000;x12,000). 3-4 Most bacteria are Gram-negative and display,externally the outher membrane, radiating fibrils ( -> ) (x55,000; x133,000).

T.Brusa, E.Del Puppo,A.Ferrari,G.Rodondi,C.Andreis, S.Pellegrini (1997) -Microbes in deep-sea anoxic basins. Microbiol.Res., 152: 45-56.
Sediments of the Bannock Basin 1-2 The fibrillar pellicles (L) are formed by packs of microfibrillae pressed togheter Diatom, frustule (D) and bacteria are recurrent (x20,000; x5,000. 3-4 The morphotypes of bacteria are bacilli or vibrios. Following ruthenium red reaction, the vibrios show a thick capsula ( C )strictly connected (->) with laminar pellicles (L) (x90,000; x30,000).

T.Brusa,E.Del Puppo,A.Ferrari,G.Rodondi,C.Andreis, S.Pellegrini (1997) -Microbes in deep-sea anoxic basins. Microbiol.Res., 152: 45-56.

Anoxic basins: macrobiology Lack of suitable tools for the accurate biological exploration of the Mediterranean Ridge, like submersible and ROV justify the paucity of direct evidence of macrobenthic life along the "beaches" around the anoxic lakes or close to the seeps in aerobic/disaerobic environments. An unusual density of polychaete (worm) calcareous tubes have been observed on the top layer of 06 GC (gravity core, R/V Bannock cruise 1984) collected in the steep wall of Bannock basin, close to the water/brine interface. These tubes belong to Filogranula stellata, which normally lives on rocky bottoms, dead corals or other hard substrata in deep waters of the Atlantic/Mediterranean region. On the contrary, several dozens of specimens collected on the top of 06 GC were cemented to planktonic foraminiferal tests, that is a very unusual substrate for them. They probably were filter-feeding on bacteria which exploit the H2S diffusing from the brine (sulfide oxidizing bacteria) and the accumulation of organic matter at the water/brine interface due to the high density of the brine.
Mud volcanoes: macrobiology.
Mud diapirs and mud volcanoes are present at different sites along the crestal area of the Mediterranean Ridge. The geological deformation of this accretionary prism pressurizes the circulating fluids which are expelled from sediments. Relatively dense macrobenthic communities, mainly composed of mollusks were observed in cores and video images recorded at the top of the Napoli Dome, S of Crete. A box-corer taken (fig. 1-2) near a “black spot”, showed 3 cm of fluid, surficial, pteropod mud containing some lucinids (fig.3). Other lucinids and vesicomyids (fig.4) were recognized on the top of other bottom samples collected on the Napoli Dome. All these taxa are known to host bacterial gill symbionts, which exploit the reduced gases emitted from the mud volcano for the synthesis of organic compounds. The chemosynthetic organic compounds provide energy to the molluscan hosts, allowing them to thrive in these otherwise inhospitable, extreme environments. The precise type of chemotrophy which characterizes the bacteria linked to the anoxic lakes and to the mud volcanoes and the different type of producers involved in these trophic webs are still far to be completely understood.

C.Corselli, D.Basso (1996) -First evidence of benthic communities based on chemosynthesis on the Napoli mud volcano (eastern Mediterranean). Marine Geology, 132: 227-239
ACKNOWLEDGEMENTS
We are grateful to the Italian Consiglio Nazionale delle Ricerche (CNR) for the ship time of the R/V Bannock and R/V Urania. Funds from Consiglio Nazionale delle Ricerche (CNR), and EU MAST Programme Projects: MARFLUX (MAST I), PALEOFLUX (MAST II), SAP (MAST III).

EU-Workshop on Extreme Marine Environments, Kiel 19-22 November 1998