Nitrogen fixation in endolithic cyanobacterialcommunities of the McMurdo Dry Valley, Antarctica

摘要

Nitrogen is scarce in the sandstones of McMurdo Dry Valley, Antarctica, and the possibility for the input fromprecipitation is minimal. In endolithic communities dominated by phototrophs, nitrogen availability by nitrogen fixation maytherefore be very important. To investigate this, nitrogenase activity of the whole communities (rock plus microorganisms)of Linneus Terrace, McMurdo Dry Valley, was measured by acetylene reduction assay and environmental factors affectingthe enzyme activity were studied. The activity obtained from these studies presumably came from the dominant phototrophidentified, the ancient unicellular cyanobacterium Chroococcidiopsis. We describe a comparative study of nitrogen fixationwhere one set of experiments was conducted under conditions likely to be present in the Antarctic from where the sampleswere obtained i.e., 275 lux and 5 °C, and the other under laboratory conditions with 2500 lux and 20 °C. Our studies revealedthat there was a distinct diurnal pattern of nitrogenase activity in these endoliths. Nitrogen fixation thus seems to be a veryimportant activity in the Antarctic endoliths where there is a permanently immobilized layer of phototrophic cells exposedto extreme environmental conditions. It is probable that, although there is a drastic reduction in the photosynthesis activeradiation of about 1-3% photons reaching the phototrophic region of the endolith for photosynthesis during the day, it isthis that provides the energy for nitrogen fixation during the night. Also a heterotrophic mechanism involved in the nitrogenfixation process cannot be ruled out, as osmolytes are abundant in these environments to protect the cells from desiccation.