Sea ice microbial communities. IV. The effect of light perturbation on microalgae at the ice-seawater interface in McMurdo Sound, Antarctica

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Abstract
In McMurdo Sound, Antarctica, an in situ light perturbation study was conducted on 1.5 m thick annual sea ice during the 1981 austral spring ice microalgal bloom. A 100 m2 quadrat was covered with snow to a depth of 70 cm reducing under ice irradiance by 97% relative to a control quadrat with 7 cm of natural snow cover. Samples were collected from the ice-sea water interface within each quadrat by SCUBA divers. After the first 3 wk of light perturbation (Oct. 24-Nov. 18, 1981), net ice growth was about 20 cm; there was no significant difference in C:chl a [chlorophyll a] ratios in the 2 quadrats during this period. However, during the following 3 wk when net ice growth was zero (Nov.18-Dec.7, 1981), C:chl a ratios dropped to 38 in control while ratios in the light perturbed quadrat remained high (> 200). Photosynthetic rate in the control reached a peak of 0.35 mg C (mg chl a)-1 h-1 in early Dec. but was not detectable in the light perturbed quadrat. Facultative heterotrophy was found in natural populations of sea ice diatoms at substrate concentrations close to ambient. Respiration and incorporation of 14C-serine into acid-insoluble particles in the .gtoreq. 1.0 .mu.m size fractions were found in both control and light perturbed quadrats. Variation in uptake among samples was very high, and a light effect was not detected. Microautoradiographs revealed that ice diatoms including Nitzschia sublineata Hasle, a common sea ice alga lacking epiphytic bacteria, were labeled with 14C-serine. Although heterotrophic C uptake from serine represented < 0.3% of photosynthetic carbon fixation in the control, heterotrophy may be a more important part of the carbon budget in certain species such as N. sublineata. Low rates of heterotrophic uptake may provide sufficient C and energy for maintenance metabolism during the dark winter months.