Our primary field site, at Explorers Cove, Antarctica, is essentially a very conveniently-placed piece of the deep ocean floor. Because the area is bypassed by ocean currents and has a permanent ice cover, it is always still, cold, and dark, even though it's only about 60 feet down to the bottom.

The resemblance does not stop there. Deep-sea environments have relatively few species of the more famous hard-shelled, "calcareous" foraminifera; rather, they are dominated by single-chambered, soft-walled forams called "allogromiids*". In our studies of Explorers Cove forams, we have found that this pattern holds there as well. Further, our research has shown that some forams are a lot harder to find than others.

Some foraminifera in Explorers Cove, such as this Astrammina rara cell (arrow), are easy to spot. (This cell is about the size of a BB, not including the "arms", which are called stolons.) Less conspicuous species are more difficult to detect.

Initial studies by Ward et al. (1987) and Bernhard (1987) revealed a low diversity foram assemblage dominated by small calcareous taxa; no soft-bodied allogromiids appear in their reports, and only a few species of single-chambered forams of any kind. They studied the forams in the Cove by placing the sediment in carbon tetrachloride; lighter objects, like foram shells, float to the top. We think allogromiids may not be as buoyant in this system, causing them to be undercounted.

Using refined "wet-picking" methods, subsequent work by our group identified a much richer diversity of agglutinated and soft-bodied forams at Explorers Cove (Gooday et al. 1996). This method involves passing the sediment through a series of sieves and then carefully picking through the size fractions to identify the forams. Many of the newly identified morphotypes comprised a bewildering array of "quartz spheres" and "mudballs" that were difficult to identify beyond the genus or, in some cases, the family level. The method is also very labor-intensive. We and our collaborators next adopted molecular methods to help identify these problematic members of the assemblage. By purifying and testing the DNA of the individual organisms that were isolated, we were able to tell apart different species that looked nearly identical to the eye (Pawlowski et al. 2002, Bowser et al. 2002).

The new field of "molecular ecology" has provided a powerful new way of looking at living communities. Essentially, organisms are identified by their DNA rather than by their morphology. Previous studies (such as this one) screened all of the DNA purified from a volume of water or sediment. While known species were identified, most of the signal was from previously unknown organisms.

Foraminifera are large and well-studied protists, so we were not sure how important this "cryptic diversity" problem would be for us. We modified the environmental-DNA screening technique to detect only foraminiferans, and analyzed sediment samples from Explorers Cove and from the jetty near McMurdo Station. Much to our surprise, we found that at least 75% of the allogromiid diversity at Explorers Cove remains uncharacterized (Habura et al. 2003).

Total number of allogromiids detected at Explorers Cove (dot on map) depends on sampling method. Carbon tetrachloride float method (A and B) favors detection of non-allogromiids. Hand-picking of sieve fractions (C) detected five times as many allogromiid species. This method combined with DNA-based detection (D) revealed even more of these small, soft-bodied taxa. The estimated total diversity at this site (E) is based on the percentage of moltypes found in a whole-sediment DNA screen that were also observed in (D). Dark gray bars indicate total morphotypes, light gray total molecular types.

What is the significance of this finding? It could mean any one of several things. First, many undescribed forams may be small or fragile, which would mean they would be difficult to identify morphologically. Some of the signal could also be from juveniles or gametes of forams that don't survive to adulthood in the Cove. (If this were the case, it would be interesting to find out how they got into Explorers Cove; the currents in that area are basically nonexistent.) Finally, this could be a case where forams that look alike are actually different species.

An important current project in the lab is determining the overlap between morphological and molecular data from the same site, and figuring out new ways of detecting cells morphologically. Both of these avenues of study are needed to give us the best possible picture of the foram assemblage in this area. The breathtaking diversity of softbodied forams here--we think of it as the "Amazon Basin of allogromiids"--is already giving us a window into the evolution and cell biology of this important and neglected group of foraminifera.


* If you are familiar with foram taxonomy, this nomenclature is based on the Tjarno (1999) definition of allogromiids. The definition excludes groups such as the Textulariida, Rotalida, Miliolida, and the other multilocular taxa, but includes members of the traditional groups Astrorhizida, Allogromiida, and Athalamida. For the most part, if an organism is a member of the Granuloreticulosea and never makes more than one chamber in its test, it's an allogromiid.

Bibliography

Bernhard, J.M. (1987) Foraminiferal biotopes in Explorers Cove, McMurdo Sound, Antarctica. Journal of Foraminiferal Research 17:286-297.

Bowser, S.S., Bernhard, J.M., Habura, A., and Gooday, A.J. (2002) Structure, taxonomy and ecology of Astrammina triangularis (Earland), an allogromiid-like agglutinated foraminifer from Explorers Cove, Antarctica. Journal of Foraminiferal Research 32:364-374.

Gooday, A.J., Bowser, S.S. and Bernhard, J.M. (1996) Benthic foraminiferal assemblages in Explorers Cove, Antarctica: A shallow-water site with deep-sea characteristics. Progress in Oceanography 37:117-166.

Habura, A., Pawlowski, J., Hanes, S.D., and Bowser, S.S. (in press, J. Euk.
Micro.) Unexpected foraminiferal diversity revealed by small-subunit rDNA analysis of Antarctic sediment.

Pawlowski, J., Fahrni, J., Brykczynska, U., Habura, A. and Bowser, S. S. 2002. Molecular data reveal high taxonomic diversity of allogromid foraminifera in Explorers Cove (McMurdo Sound, Antarctica). Polar Biology 25:96-105.

Ward, B.L., Barrett, P.J. and Vella, P. 1987. Distribution and ecology of benthic foraminifera in McMurdo Sound, Antarctica. Paleogeography, Paleoclimatology, and Paleoecology 58:139-153.


About the image: This is a top view of a sediment core--we use these to bring sections of the seafloor to the surface without disturbing them. If you look carefully, you can see two juvenile brittle-stars on the surface. There's a lot of informative DNA in the sample too--we've used a bit of artistic license so you can see it.