Fluidized muds: a novel setting for the generation of biosphere diversity through geologic time* J. Y. ALLER, 1 R. C. ALLER, 1 P. F. KEMP, 2 A. Y. CHISTOSERDOV 3 AND V. M. MADRID 3 1 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA 2 Center for Microbial Oceanography: Research and Education (C-MORE), School of Ocean and Earth Science, University of Hawaii, Honolulu, HI, USA 3 Department of Biology, University of Louisiana at Lafayette, Lafayette, LA, USA ABSTRACT Reworked and fluidized fine-grained deposits in energetic settings are a major modern-day feature of river deltas and estuaries. Similar environments were probably settings for microbial evolution on the early Earth. These sedimentary systems act as efficient biogeochemical reactors with high bacterial phylogenetic diversity and func- tional redundancy. They are temporally rather than spatially structured, with repeated cycling of redox conditions and successive stages of microbial metabolic processes. Intense reworking of the fluidized bed entrains bacteria from varied habitats providing new, diverse genetic materials to contribute to horizontal gene transfer events and the creation of new bacterial ecotypes. These vast mud environments may act as exporters and promoters of biosphere diversity and novel adaptations, potentially on a globally important scale. Received 6 October 2009; accepted 12 February 2010 Corresponding author: J. Y. Aller. Tel.: +1 631 632 8655; fax: +1 631 632 3066; e-mail: jyaller@notes.cc. sunysb.edu INTRODUCTION Geologic and isotopic evidence suggests that microbial life may have been present on Earth for up to 3.7 billion years (e.g. Schopf et al., 2007). Regardless of the exact time of origin, the environmental and biogeochemical contexts in which life evolved are often depicted as some form of stable biomat, suspended large aggregates, and ⁄ or hydrothermally influenced system (e.g. Brasier et al., 2006). On the early Earth, and in fact throughout geologic time, there were prob- ably a wide variety of sedimentary habitats for colonization (Westall, 2003; Veizer & Mackenzie, 2004). In the present contribution, we describe the biogeochemical and microbial community properties of a major class of sedimentary environ- ment: fluidized muds that, despite its common occurrence on the modern Earth, has been largely neglected as a significant microbial habitat. We contrast the geomicrobiological proper- ties of these highly disturbed muds with more structured biomats, which are used as one of the most common models of early Earth microbial communities (Konhauser, 2007). We argue that energetic sedimentary systems probably have played a major role throughout geologic history as major sources of microbial productivity and evolutionary innova- tion, and that they continue to do so. The relative importance of these habitats and processes probably varied in their impor- tance for the diversification of life depending on environmen- tal conditions at particular times. On the present-day Earth, vast expanses of fluidized and reworked muds dominated by microbes characterize energetic coastlines downdrift of major river deltas, particularly in the tropics, and include large rivers such as the Amazon, Fly and Ganges–Brahmaputra (Fig. 1). Fluid muds are largely fluid-supported nearbed suspensions with concentrations of fine-grained particles >10 and <400 g L )1 (see Wells, 1983; Smith & Kirby, 1984; Faas, 1991; Mehta, 1991; Kineke et al., 1996). They have the consistency of cream to soft custard sauce, reach thicknesses of 0.5–2 m, and often overlie a more stable muddy seabed, which itself can be subject to relatively frequent physical reworking and extend 0.1–3 m (e.g. remobilized over weeks to year timescales, Kuehl et al., 1986). We refer to this fluid mud and underlying frequently reworked seabed with which it exchanges as fluidized mud. In the tropical deltaic expression of these environments, reworked sediments are often rich in iron oxides delivered from highly weathered source terrains. Episodic physical disturbance mixes mobile surface sediment layers as much as 2–3 m thick, and in combination with overlying oxygen-rich * Fluidized mud microbial communities provide clues into both past and future evolutionary processes. Ó 2010 Blackwell Publishing Ltd 169 Geobiology (2010), 8, 169–178 DOI: 10.1111/j.1472-4669.2010.00234.x