diatoms

These are benthic diatoms collected at the McMurdo Station jetty [3]. Benthic diatoms under the annual sea ice are among the most shade-adapted microalgae and play a role in supplying fixed carbon and energy to the McMurdo Sound benthic fauna [6]. Benthic diatom distribution is affected by grazing, substrate, turbulence, ice, and availability of sunlight [5]. Many benthic diatoms live within the sponge spicule mat on the seafloor; the sponge spicules create a dimensional matrix in the sediment in which the diatoms live, as well as act as natural optical fibers channeling light a short distance (five centimeters) into the mat, thus enhancing benthic diatom photosynthesis [12,13].

Benthic diatoms collected in the sponge spicule mat at Cape Armitage at twenty meters depth are dominated by the pennate diatom Trachyneis aspera [6]. Benthic diatoms collected at Explorers Cove at New Harbor at twenty meters depth are dominated by Amphora antarctica with these diatoms in lower abundance: Trachyneis aspera, Amphiprora kufferathii, Fragilariopsis sp. and Nitzschia sp. [4].

The fish Pagothenia borchgrevinki is perched on a grounded iceberg just south of Cape Evans on Ross Island. The surface of the ice is covered with a sea ice microbial community comprised primarily of diatoms and also other microalgae, bacteria and protozoans; the brown coloration is due to photosynthetic pigments [2,8]. Crustaceans and molluscan pteropods graze on the microalgae and fish prey on the crustaceans. McMurdo Sound is continuously dark for four months in the winter and continually light for four months in the summer; these are separated by two-month transition periods in which the light increases or decreases by twenty minutes per day [1]. To the human eye, lighting can be dim under the sea ice. The sea ice reduces light as does its overlying snow cover and the organisms living on its underside. At noon during McMurdo Sound summer, the sea ice undersurface receives less than 1% of the sun's irradiance [2]. This isn't much light for diatoms and algae to use for their photosynthesis yet they have adapted. Called cryophiles for their ice lifestyle but more properly named epicryotic, they live in low light intensity and make a significant contribution to primary productivity under the ice [2,7,8].

Here's a closer look at a diatom mat on the underside of sea ice in McMurdo Sound. A study of the diatoms under the sea ice at Cape Armitage found 32 species of which eleven were numerically dominant: Amphiprora kjellmanii, Amphiprora oestrupii, Biddulphia weissflogii, Coscinodiscus subtilis, Eucampia balaustium, Fragilaria linearis, Nitzschia martiana, Nitzschia seriata, Pleurosigma antarcticum, Rhizosolenia alata, and Rhizosolenia rostrata [8]. Another study noted the Cape Armitage ice diatom community encompassed Amphiprora spp., Berkeleya spp., Nitzschia spp., and Pleurosigma spp. [6]. These epicryotic diatoms living in association with the underside of the sea ice are a community distinct from plankton diatoms, though with the deterioration of the annual sea ice during summer, the epicryotic diatoms join the plankton [8].



This is the triangular diatom Triceratium sp. [3].

Antarctic planktonic diatoms are grazed by phagotrophic protists (loricate and naked ciliates, radiolaria, foraminifera, flagellates, choanoflagellates) and large zooplankton (salps, copepods, krill) [9]. Antarctic benthic diatoms have been found in the gut contents of a wide variety of Antarctic organisms [11].

These are centric diatoms, possibly Podosira sp. or Hyalodiscus sp. [3].

Diatoms are hosted by several Antarctic sponges within the food-capturing cells lining the passages through which the sponge circulates water; these endobiont diatoms live by consuming carbohydrates produced by the sponge and also by photosynthesis [10,14,15]. Diatoms produce large amounts of polysaccharids, thus giving the sponge an alternative food source during food-scarce periods [14,15]. This symbiotic adaptation by the diatoms enhances their survival in the low light levels found down deep under the ice (as well as the dark months of winter) [10]. Diatoms that have been observed in this association are usually the pennate diatoms Fragilariopsis curta, Fragilariopsis sp., and Pseudogomphonema sp. and less commonly, centric diatoms belonging to the genera Porosira, Coscinodiscus, and Rhizosolenia [10,14,15].

1: Science 238:1285-1288, 1987; 2: Polar Biology 2:171-177, 1983; 3: Kathleen Conlan, personal communication, 1999; 4: Marine Ecology Progress Series 64(1-2):129-136, 1990; 5: Antarctic Journal of the United States 8(5):307-309, 1973; 6: Journal of Phycology 21(4):664-667, 1985; 7: Oceanography and Marine Biology, an Annual Review 9:83-139, 1971; 8: Nature 199(4900):1254-1255, 1963; 9: Polar Marine Diatoms. LK Medlin and J Priddle, eds. Cambridge: British Antarctic Survey, Natural Environment Research Council, 1990; 10: Biological Bulletin 198:29-33, 2000; 11: Citations too numerous to list; diatoms as a food item are noted throughout this Field Guide for relevant organisms; 12: Scientia Marina 63(Supplement 1):113- 121, 1999; 13: Nature 383(6599):397-398, 1996; 14: Ross Sea Ecology : Italiantartide Expeditions (1987-1995). FM Faranda, L Guglielmo, A Ianora, eds. Berlin : Springer, 2000. pp. 551-561; 15: Polar Biology 14:55-58, 1994

Text ©Peter Brueggeman. Photographs ©Canadian Museum of Nature (Kathleen Conlan) & Norbert Wu. Photographs may not be used in any form without the express written permission of Canadian Museum of Nature (Kathleen Conlan) and Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?