Radiolaria occur abundantly in surface water with low silicate concentrations (ca. 1 mu M), yet they construct remarkably robust siliceous skeletons. This indicates they compete very effectively for the available silicate. To examine the effects of variation in silicate concentration on skeletal growth and vitality, two spongiose radiolarians Spongaster tetras and Dictyocoryne truncatum, collected in surface plankton tows near Barbados, West Indies, were maintained in plastic tissue culture dishes, each well of the dishes containing ca. 15 ml seawater emended with additional sodium silicate of 0 mu M (control), 50 mu M, 100 mu M and 150 mu M. Their longevity and skeletal weight gain, the latter calculated from equations relating skeletal weight to a linear dimension of the skeleton, are reported with descriptions of their living features. The addition of silicate up to 100 mu M had no statistically significant effect on the weight of silicate deposited per shell for either S. tetras or D. truncatum. However, noticeable detrimental effects resulted in earlier death in both S. tetras and D. truncatum at 150 mu M. Early death was more frequent in D. truncatum. Spongaster tetras that grew skeletons may survive longer when cultured in 50 mu M silicate-enriched seawater compared to 150 mu M enriched seawater, but not longer than those cultured in unsupplemented seawater. D. truncatum has the longest longevity in unsupplemented seawater. In addition, scanning electron microscopy of shells collected from individuals that grew in culture revealed no evidence of enhanced skeletal growth or structures with silicate enrichment of the culture medium. These data add further evidence that S. tetras and D. truncatum are capable of competing very effectively for silica in ambient seawater concentrations of approximately 1.0 mu M, and also that changes in seawater silicate may not have a dramatic effect on longevity, skeletal size, or weight of some radiolarians. Thus other environmental variables, such as variations in temperature and salinity, may more likely impress a relatively more salient signal in the microfossil record. Daily mean weight gain values at the four treatments ranged from 4.3 ng to 6.3 ng in S. tetras and from 19.7 ng to 27.1 ng in D. truncatum. For the control group, mean total weight gain was 0.035 mu g in S. tetras and 0.083 mu g in D. truncatum. Compared to the previously published results for S. tetras, using larger glass vials, the weight gain values seem to be significantly smaller. However, estimated silicate consumption ratios with respect to the available amount in the culture vessel by S. tetras agree substantially with each other. This suggests that observed radiolarian skeletal growth in the previous and present experiments was rather substantially affected by other environmental factors, such as available micronutrients, related to the water volume used in the culture vessel.
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