Detection of live magnetotactic bacteria in South Atlantic deep-sea sediments

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Abstract

Since they were first descrbed in 1975 [1], magnetotactic bacteria have been observed in a wide variety of environments, including soils [2], rivers, freshwater lakes and coastal marine sediments [3,4], and in one case at a water depth of about 600 m in a hemipelagic sediment [5]. Here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern South Atlantic. Different morphologies were identified (cocci, spirilla, vibrionic and rod-shaped forms) at water depths to about 3000 m on the African continental margin between the equator and 30°S, and on the Walvis Ridge in a pelagic environment about 1400 km off the coast. No actively swimming magnetotactic bacteria could be detected in deeper waters. Magnetotactic bacteria were observed only in the upper 10 cm of the sediments, with the highest concentrations always found in a well-defined narrow subsurface layer. The depth of this layer is dependent on the input and turnover rates of organic matter in the sediment. Measurements of oxygen and nitrate concentrations [6,7] provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment. The data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone, in contrast to former assumptions that they may inhabit microaerobic layers [8–11].

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