HVEM of Gromia (Protista) from the Abyss

SS Bowser, AJ Gooday, J Fahrni, J Pawlowski
2010 Microscopy and Microanalysis  
Gromia is a genus of large (0.3 mm -5 cm diameter), benthic protists belonging to the supergroup Rhizaria. Frequently overlooked as "invertebrate eggs" or "species incertus" in biodiversity surveys, protistologists characterize Gromia by the presence of an organic shell with one or more apertures through which branching pseudopodia are extended. These organisms are abundant in certain marine settings; e.g., in eastern McMurdo Sound, Antarctica, populations may exceed 80,000/m 2 [1]. Recent
more » ... ular studies, and deep-sea sampling, show that their diversity is much greater than previously estimated. Although Gromia is now known to be an abundant and highly diverse group [1] [2] [3] [4] , the role of these organisms in marine ecosystems remains poorly defined. One factor confounding biological studies of Gromia is the presence of numerous accretions of mineral grains, termed stercomata, within its cytoplasm. Stercomata and surface-bound debris render the sectioning of conventional silver/gold sections for EM nearly impossible. To define structural characters useful for species identification, we overcome this limitation by cutting thick (>1-µm) sections using relatively inexpensive "histology grade" diamond knives and view them at 1 MeV using the Albany HVEM [e.g., 1,4]. In the present study, an unusually large Gromia-like organism (Fig. 1) was collected from abyssal stations off the eastern margin of South America during the DIVA 3 expedition (July -August, 2009) aboard RV METEOR. By SEM (Fig. 2) , the surface of these organisms appeared vested by a finely agglutinated layer that masks the presence of diagnostic pores in the shell [1] [2] [3] [4] [5] . Characteristic stercomata were seen by LM (Fig. 3) and HVEM (Fig. 4) of thick sections. The shell wall was found to be perforated by pores that follow tortuous paths through the thickened fibrous interior regions, and terminate in stacks of "honeycomb" structures ( Fig. 5 ) that are unique to Gromia. Oriented normal to the beam, these structures appeared as electron-opaque bands that correspond to the Type I patterns seen in Gromia oviformis [5] . These findings establish the deepest record of Gromia, and extend its range to the abyssal South Atlantic [6] .
doi:10.1017/s1431927610057946 fatcat:lpmrsalbabckvopo7sxvhnz354