SPECIES SPECIFIC ECOLOGY

Stained tests were mentioned in the typus description of Virgulinella fragilis only from the former dumping area off Wellington, but unstained tests were reported from all over the harbour area (Grindell and Collen 1976). Regardless of their own results derived from Rose Bengal staining, these authors suggested that the new species thrived in oxygen- saturated environments. Consequently, most subsequent publications regarded V. fragilis as a normal marine shelf foraminifer (references in Revets 1991). Reduced oxygen availability and large amounts of organic debris in decomposition were noted for the first time for habitats of modern virgulinellids by Sellier de Civrieux (1977, referred to Virgulinella pertusa), and vital populations of V. fragilis were subsequently described by Bernhard (2002) and Altenbach et al. (2002) from anoxic and sulfidic environments. All following studies that included live stains and oxygen measures corroborated these findings. V. fragilis was found in sulfur-oxidizing bacterial mats in the Cariaco Basin and the Arabian Sea (Bernhard 2003, Erbacher and Nelskamp 2006), or attributed to microxic and sulfidic conditions off Japan and India (Takata et al. 2005, Nigam et al. 2007). It occurs in the shallow Ria de Vigo estuary in Spain (Diz and Frances 2008, referred to V. pertusa), a habitat that is characterized by seasonal oxygen depletion and benthic methanogenesis (Kitidis et al. 2007). Multiple survival strategies have been discussed for this heterotrophic eukaryote, such as symbiotic sulfur oxidizing bacteria, functional kleptoplasts, and peroxisome proliferation (Bernhard 2003, Horak et al. 2004, Bernhard and Bowser 2008). V. fragilis meanwhile can be considered a characteristic species for microhabitats near redox boundary conditions.

Nonionella stella, Fursenkoina fusiformis, and Bolivina pacifica occur under normal marine conditions on the upper continental slope off Namibia (Leiter 2008), as well as in other areas with enhanced organic carbon flux rates to the sea floor off western Africa (Lutze 1980, Altenbach et al. 2003, Licari and Mackensen 2005). However, these species have also been reported from anoxic or dysoxic environments (Bernhard and Sen Gupta 1999). Highest standing stock was recorded for F. fusiformis from dumping areas (Alve 2003). Laboratory experiments revealed an increased survival rate of F. fusiformis under anoxia (Bernhard and Alve 1996) that was further supported by the availability of large amounts of organic matter (Ernst et al. 2005). B. pacifica co-occurs with V. fragilis in sediments covered with the sulfur bacteria Thioploca and Beggiatoa in the oxygen minimum zone off Pakistan (Erbacher and Nelskamp 2006).

Laboratory experiments of Risgaard-Peterson et al. (2006) suggest that Nonionella cf. stella and Stainforthia sp. are capable of complete denitrification near the chemocline. The results were later corroborated by field investigations off Chile (Høgslund et al. 2008). Similar to Virgulinella fragilis, the function of kleptoplasts and peroxisome proliferation has also been discussed for Nonionella stella and Fursenkoina fusiformis (Bernhard and Alve 1996, Grzymski et al. 2002, Horak et al. 2004, Bernhard and Bowser 1999, 2008). For the central part of the DMB, the sediment column below zero to five cm has to be considered anoxic and sulfidic (Brüchert et al. 2003). The deep infaunal occurrence of V. fragilis, F. fusiformis, N. stella, and Bolivina pacifica (Figure 4) may indicate long-term adaptability to such conditions.

For the occurrence of Discammina compressa, amplifying towards the outer edges of the DMB, and for the single occurrence of Hyperammina sp. and Crithionina pisum under microxic conditions species specific ecological data remain conjectural. These taxa have been reported from oxic environments of the NW-African upwelling area and the Gulf of Guinea (Lutze 1980, Altenbach et al. 2003). Within the DMB, stained D. compressa specimens display a detritic, coarse overlay around the test that is glued together by organic matter intensively colored by Rose Bengal stain. This overlay is either missing on empty tests, or occurs in the form of an unstained, patchy residue. Coarsely agglutinated larger textulariid taxa, such as Hyperammina sp., may perform enormous hydrolytic activities, exceeding values reported for symbiotic bacterial-macrofaunal interactions (Koester et al. 1991, Meyer-Reil and Koester 1991). Richardson and Cedhagen (2001) noted extracellular digestion for Crithionina granum, and, quoting the work of Meyer-Reil and Koester, they mentioned that bacteria will benefit from exoenzymes produced by foraminifera. However, we can only speculate about gardening, or commensal, or symbiotic interactions at present.