The ecophysiology of iron and vanadium accumulation by North Atlantic ascidians

Stacey, Joy E. (2009) The ecophysiology of iron and vanadium accumulation by North Atlantic ascidians. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Some ascidians (Tunicata, Ascidiacea) accumulate high levels of iron and/or vanadium in their tissues, the latter being very unusual among animals. Previous work has investigated aspects of the vanadium accumulation by Phlebobranch ascidians, including the identification of the vanadocytes, the oxidation state of the vanadium and the pH in the vacuole in which it is held, and a proposed reduction mechanism for vanadium. The iron dynamics of these animals have received comparatively little attention. There is also a lack of data on the ecophysiological aspects of ascidian metal accumulation. This thesis examines the seasonal variation in vanadium and iron concentrations of two Phlebobranch ascidians, Ciona intestinalis and Ascidia callosa, which accumulate vanadium and iron, and a Stolidobranch ascidian, Halocynthia pyriformis, which accumulates iron only. Experiments examined whether these metal concentrations are responsive to increased food availability and dissolved vanadium concentration. -- An HPLC method was developed to simultaneously measure vanadium and iron in large numbers of biological samples. Using this method, a study of the metal concentrations of some tissues of C. intestinalis from Woods Hole, USA, found that the vanadium and iron concentrations in the hemocytes and other tissues was higher in March than in the fall or summer sampling periods, corresponding with high activity of a key enzyme of a proposed reductive pathway (G6PDH). A feeding experiment investigated whether food availability/particle concentration affects the vanadium and iron concentrations of C. intestinalis. While there were few differences in metal concentrations among high and low food groups, vanadium levels were maintained and iron levels rose compared to animals sampled fresh from Woods Hole. The former may be indicative of low vanadium loss rates and the latter of the importance of dietary sources of iron. -- This pattern was confirmed by a year long study of the vanadium and iron concentrations of tissues of two ascidians from Newfoundland, Canada. A. callosa, a high level vanadium accumulator, exhibited a peak in the vanadium and iron concentrations of the hemocytes and other tissues in spring prior to the spring plankton bloom. This corresponded with an increase in both the total number of circulating hemocytes and the proportion of cells containing reduced vanadium. H pyriformis, a non vanadium accumulator, exhibited a peak in the iron concentration of some tissues at the spring bloom. A vanadium enrichment experiment indicated that A. callosa can respond to increased environmental vanadium concentration. Vanadium concentration of the hemocytes and other tissues initially increased, followed by a marked decline, although remaining elevated above that of control animals. The vanadium exposed animals also displayed an increase in the total number of circulating hemocytes and the number of vanadocytes in all sampling periods. The vanadium levels used in this experiment did not cause an increase in the vanadium concentration of H. pyriformis.

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