Protist, Vol. 156, 113—126, June 2005 http://www.elsevier.de/protis Published online 22 April 2005 ORIGINAL PAPER The Second Species of Gromia (Protista) from the Deep Sea: its Natural History and Association with the Pakistan Margin Oxygen Minimum Zone Andrew J. Gooday a,1 , and Samuel S. Bowser b,c a Southampton Oceanography Centre, Empress Dock, European Way, Southampton SO14 3ZH, UK b New York State Department of Health, Wadsworth Center, P.O. Box 509, Albany, New York 12201, USA c Department of Biomedical Sciences, University at Albany, Albany, NY 12201, USA Submitted August 24, 2004; Accepted November 21, 2004 Monitoring Editor: Bland J. Finlay We describe a gromiid protist Gromia pyriformis sp. nov., from bathyal depths on the Pakistan margin (NE Arabian Sea), an area characterised by a well-developed Oxygen Minimum Zone (OMZ). The new species is smaller (length usually o1 mm) than the only other described deep- sea gromiid species (Gromia sphaerica) or the well-known coastal species Gromia oviformis. Its identification as a gromiid is based on the test-wall ultrastructure. This includes (i) an outer wall (165—300 nm thick) limited by an electron-opaque layer and perforated by pore structures which typically extend through its entire thickness, and (ii) inner ‘‘honeycomb membrane’’ structures which form a discontinuous sheet (18—20 nm thick) lying parallel to the outer wall. An outermost glycocalyx (75 nm thick), not observed in other gromiid species, is also present and imparts a finely granular appearance to the outer test surface, as seen by Scanning Electron Microscopy (SEM). Numerous rod-shaped prokaryotes are attached to the exterior of the glycocalyx. Gromia pyriformis sp. nov. typically occurs above the sediment—water interface, attached to the large arborescent foraminiferan Pelosina sp. It is confined to a very narrow bathymetric zone (~1000- m water depth) in the lower portion of the OMZ, where bottom-water oxygen concentrations are 0.2 ml l 1 . & 2005 Elsevier GmbH. All rights reserved. Key words: prokaryotic epibionts; deep sea; gromiid; oxygen minimum zone; Pelosina. ARTICLE IN PRESS Abbreviations: CTD: Conductivity—Tempera- ture—Depth instrument, HVEM: high-voltage electron microscopy, OMZ: oxygen minimum zone, SEM: scanning electron microscopy 1 Corresponding author; fax 44 23 80596247 e-mail ang@soc.soton (A.J. Gooday) & 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.protis.2004.11.002