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SHADE ADAPTED BENTHIC DIATOMS BENEATH ANTARCTIC SEA ICE 1

A dense community of shade adapted microalgae dominated by the diatom Trachyneis aspera is associated with a siliceous sponge spicule mat in McMurdo Sound, Antarctica. Diatoms at a depth of 20 to 30 m were found attached to spicule surfaces and in the interstitial water between spicules. Ambient irradiance was less than 0.6 μE · m−2· s−1 due to light attenuation by surface snow, sea ice, ice algae, and the water column. Photosynthesis‐irradiance relationships determined by the uptake of NaH14CO3 revealed that benthic diatoms beneath annual sea ice were light‐saturated at only 11 μE·m−2·s−1, putting them among the most shade adapted microalgae reported. Unlike most shade adapted microalgae, however, they were not photoinhibited even at irradiances of 300 μE·m−2·s−1. Although in situ primary production by benthic diatoms was low, it may provide a source of fixed carbon to the abundant benthic invertebrates when phytoplankton or ice algal carbon is unavailable.