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Gross, Onno
Institute and Museum of Geology and Paleontology, University of Tubingen, Sigwartstr. 10, 72076 Tubingen, Germany.
Tel. ++49 (0) 7071-2974683
Fax. ++49 (0) 7071-2976990
Email. Onno.Gross@uni-tuebingen.de
Benthic deep-sea foraminifera are largely fueled by organic matter arriving from the sea-surface and they respond rapidly to incoming phytodetritus. In order to study such response of benthic deep-sea foraminifera, laboratory experiments with special microcosms have been conducted. Living foraminifera from down to 2880 m water depth were successfully maintained and observed. Microspheres were used to record the mode of foraminiferal movements - in the following termed migration - and the impact of foraminiferal bioturbation on the sediment.
Deep-sea foraminifera are not slower in their migration speeds than shallow water species. No differences in speed for epifaunal or infaunal foraminifera were observed. In contrast, factors such as temperature, food concentration and oxygen content clearly influenced the benthic foraminifera. For example, an increase of 5oC in temperature (from 10o- 15oC) resulted in an increase of 35 % in migration speed for Allogromia spp. Differences in food concentration within the substrate resulted in a difference of migration speed for Adercotryma glomerata (40 %). A low oxygen content within the sediment did not greatly affect migration speed significantly for Quinqueloculina seminula but generally resulted in migration of individuals to the sediment surface.
Particle displacement (e. g. tracer particles, dm = 10 µm) by benthic foraminifera was achieved either by burrowing or pseudopodial activity. Sediment ingesting species showed accelerated particle transport. Cementation by secondary tests removed grains from the bioturbation cycle. The bioturbation rate was elevated close to the sediment-water interface (0-0,5 cm) as compared to deeper sediment horizons (0,5-1,5 cm). A biological mixing coefficient (Db) of 0,4 cm2 d-1 was calculated.
Assuming a model population of 100 individuals/10cm2 of foraminifera (dm = 1 mm) a sediment displacement rate of 400 cm3/y was calculated. Oligotrophic conditions may result in higher bioturbation rates, whereas balanced food concentrations would show low sediment turnover. The rate of bioturbation thus seems to be directly related to the response of benthic foraminifera to the prevailing trophic conditions in the deep-sea.
Three categories could be recognized due to the migrational behavior. So called "lazy" foraminiferal species were characterized by mean migration speeds of maximal 2,5 um/min, "active" species exhibited mean migration speeds of up to 5,0 um/min and "steady-vagile" foraminiferal species moved with mean speeds greater than 5,0 um/min. By video documentation a maximal value of 87,76 µm/min was found for Hoeglundina elegans, followed by Cibicidoides floridanus (83,33 µm/min) and Cassidulina leavigata (82,76 µm/min). Migration speeds of the miliolids Pyrgo murrhina (66,67 µm/min) and Quinqueloculina laevigata (55,87µm/min) were also documented to have high values.
Starting from a model population of 100 individuals (10cm2) Allogromia spp. (dm: 1 mm) a rate of 400 cm3/y sediment displacement was calculated. Oligotrophic conditions result in higher bioturbation rates, whereas balanced food concentrations showed low sediment turnover. Thus the rate of bioturbation seems to be directly related to trophic conditions.
There are different modes of cyst production (also termed secondary tests). Growth of new chambers, feeding, reproduction and hypoxic conditions lead to formation of secondary tests. Sporadic encystment after feeding (Chlorella) were observed for some species (Gavelinopsis transluscens, Cibicidoides wuellerstorfi, Spiroplectinella wrightii). Stationary cysts were formed under hypoxic conditions (Glomospira gordialis, Rosalina spp., Trochammina squamata).
Miliolids (Quinqueloculina seminula, Pyrgo murrrhina) were observed to live more than 3 years and calcareous forms (Gavelinopsis transluscens) show an age of 2 1/2 years.
Reproduction takes place in special cysts, in which the juvenile schizonts may profit from the offered sediment particles (Textularia porrecta), or the juveniles are sheltered in a brood chamber (Gavelinopsis transluscens).
The juveniles grow up to adult size within several weeks (Textularia porrecta, Bulimina marginata) or some month (Rosalina spp.). The test of Bathysiphon spp. double their size within 1 month.
A feeding experiment under in situ original pressure conditions (12oC, 300 bar; removed from box core samples) showed that Uvigerina mediterranea increased the test size by adding a chamber after 2 weeks of incubation and feeding with algae (Chlorella).
Box core samples of deep-sea foraminifera showed higher enzymatic activities (Esterase) as the surrounding standing stock of sediment bacteria. Maximal FDA (Fluorescein-Diacetat) activity of 6,78 nmol/Ind./h of the total esterase pool was measured for Quinqueloculina sp.1. Specific FDA-activity reached a maximum level of 0,2 nmol/h/ug C. Hoeglundina elegans showed a positive correlation of enzymatic activity with test size and biomass.
Laboratory experiments showed that the activity of other enzyms (API-ZYM test) revealed a high content of phosphatase and hydrolase, which possibly corresponds to the active pseudopodial network in benthic foraminifera.
Migration speed of benthic foraminifera were neither linked to oceanic provinces nor water depth. No differences in speed for epifaunal or infaunal foraminifera were observed. Differences in bioturbation rate are obvious for varying trophic levels. A long lifespan, secondary tests, mode of reproduction and enzymatic activity in the observed foraminifera are indicators of a highly adaptativ strategy (K-strategy) to the deep-sea environment.
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