To provide insights into the biogeochemistry of environments where steep chemical gradients place anaerobes, microaerophiles, and aerobes in close proximity, it might be necessary to survey biotic distributions on scales that are not possible using conventional ecological approaches. To overcome such limitations, we adapted sedimentological and cell biological methods to examine the life positions of microbes within sediments. This fluorescently labeled embedded core (FLEC) technique was used to survey the submillimeter distributions of eukaryotic nanobiota and meiofauna, plus co-occurring prokaryotes, inhabiting laminated sediments of the bathyal Santa Barbara Basin (SBB; 34Њ13ЈN, 120Њ02ЈW). Although SBB sediments were vertically structured on the scale of millimeters (i.e., as laminae), and microelectrode profiles suggested vertically distinct oxygenated and anoxic-sulfidic layers, the distributions of aerobes, microaerophiles, and sulfide-tolerant anaerobes were not concomitantly structured. Unprecedented associations were observed among microorganisms. For example, relatively deep in the sediments, where high sulfide concentrations were expected, flagellates were intimately associated with Beggiatoa. Ciliates were typically solitary, whereas flagellates were often aggregated in nearly monomorphotypic swarms of Ͼ3 ϫ 10 4 mm Ϫ3 . Such aggregations could significantly affect geochemical pore-water processes at scales Ͻ1 l. Our observations indicate that a mosaic of chemically heterogeneous microhabitats exist in both vertical and horizontal dimensions, suggesting that biogeochemical processes in the SBB are more complex than predictions based on standard biotic assessments and microelectrode profiling.Conventional methods used to study nano-, meio-and macrofauna, such as sieving or density gradient extractions, obliterate the life positions of benthic organisms. Consequently, little is known about their subcentimeter-scale spatial distributions. This information is crucial to understanding the biotic and physicochemical interactions in sediments, particularly those with steep pore-water gradients. Within the surface centimeter in such settings, for example, conditions can range from oxygen depletion to anoxia, with or without sulfide enrichment (e.g., Jørgensen 1988).1 Corresponding author (joan.bernhard@sc.edu).
AcknowledgmentsAssistance from the following is gratefully acknowledged: the captain and crew of the RV Robert Gordon Sproul, as well as the resident technicians of Scripps Institution of Oceanography; all members of the 1998 and 1999 cruises' scientific parties (especially Ellen Braun-Howland, Dennis Bazylinski, Brad Dubbels, Annette Dean, Shelly Hoeft, and Dan Rogers); Sam McGee-Russell for introducing us to hot knife microtomy, Sara Meyers, Brenda Famolaro, Kat Benson, and Adam Meyer for help in adapting the technique; Monterey Bay Aquarium Research Institute for bathymetric data; Jim Barry for construction of the baseline map; Tom Chandler for LSCM access, and two anonymous reviewers for th...