Survival strategy of benthic foraminifera in dysoxic environments: Distribution, cytological and genetic characteristics of Virgulinella fragilis

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Tsuchiya, Masashi۱; Toyofuku, Takashi۱; Heinz, Petra۲; Collen, John۳; Brüchert, Volker۴; Hemleben, Christoph۲; Hemleben, Vera۲; Yamamoto, Hiroyuki۱; Kitazato, Hiroshi۱
Holding Date 17 September 2008

Molecular phylogenetic analyses and ultrastructural observation were conducted in order to understand the survival strategy of Virgulinella fragilis, a benthic foraminifer restricted to dysoxic environments. Many benthic foraminifers can persist in sediment in dysoxic environments. Some of them probably live in symbiosis with bacteria that can denitrificate and also putatively oxidize sulfide. Observations of endosymbiotic bacteria and kleptoplasts in V. fragilis may allow the foraminifer to survive in low oxygen environments, which reveal the important role of speciation via endosymbiosis. Endosymbiotic activities, host-symbiont relationships, and their ambient environments are important for eukaryotic evolution.
Both modern and fossil records for the genus Virgulinella occurs exclusively in hypoxic environments. In modern oceanic environments, V. fragilis mainly arise in dysoxic conditions in relation to coastal upwelling, to the oxygen-minimum zone, or during strong stratification of the water column. Virgulinella fragilis was collected from Namako-ike, Japan, Walvis Bay, Namibia and Wellington Harbor, New Zealand. The lake water of Namako-ike is stratified throughout the year, and shows hypoxic conditions (<0.1mg/l) with hydrogen sulfide-enriched (50.7mg/l). A hypoxic environemnt are developed in Walvis Bay, although H2S concentrations are lower values than at Namako-ike (0.2~7.3mg/l). Well-oxygenated water conditions (7mg/l) are present in Wellington Harbor now but in the recent past bottom sediments were oxygen-poor and sulfidic.
Host foraminiferal ITS and SSU sequences are identical for all localities, although pore morphologies, pore densities and sizes are variable. DO and/or H2S concentrations affect these morphologies. We observed kleptoplasts inside the foraminiferal cytoplasm that descend from diatoms gathered by V. fragilis. Kleptoplasts in host individuals of different investigated areas differ in origin of diatom species. From the expected four membranes around single kleptoplasts, we can only find double membranes. The vacuole membrane formed by the ingesting host foraminifer and 2 outer membranes of the diatom chloroplasts must have been digested. This strategy may have a role in the interaction between the cellular substrates and the kleptoplasts.
In addition, we found vacuoled bacteria in the cell cortex. The most common in individuals from the dysoxic Namako-ike area as well as from the oxygenated Wellington Harbor is Desulfobacterium corrodens (δ-proteobacteria). This bacterium is not utilizing in ambient hydrogen sulfide. Desulfobacterium uses dissolved for the heterotrophic oxidation of organic matter. These bacteria may therefore use organic material provided by the host for carbon oxidation. We could not found from the surrounding sediments and in the water column. These characteristics may determine symbiotic or parasitic relationships in the past, with bacteria transmitting vertically.