Original articleNitrogen fixation and diurnal changes of photosynthetic activity in Arctic soil crusts at different development stage

- Community structure and the development stage of soil crusts may affect the response to the environment.
- Stage of soil crust development influences the response of photosynthesis to diel PAR changes.
- Nitrogenase activity follows the gradient of soil crust development.

Nitrogen fixation and photosynthesis provided by microbial phototrophs (cyanobacteria and eukaryotic microalgae) are important processes occurring in Arctic soil crusts. Here, we describe and compare these processes in biological soil crusts from Central Svalbard at different stages of development. The gradient from poorly-developed to well-developed soil crusts was accompanied by the changes in biovolume of microbial phototrophs, nitrogenase and photosynthetic activity. The lowest biovolume of microbial phototrophs was detected in poorly-developed soil crusts as a consequence of the initial stage of soil colonization. The biovolume initially increased during the soil crust development but decreased in well-developed lichenized soil crusts. However, nitrogenase activity decreased from poorly to more developed soil crusts. Diurnal courses of photosynthetic activity differed among the soil crust types showing shifts in diurnal minima and maxima; the poorly-developed soil crust reacted faster to changes in temperature and PAR. In spite of different microclimatic conditions during the measurements, temperature was the main factor influencing photosynthetic activity while the effect of PAR was not significant. Higher temperatures led to inhibition of photosynthetic activity and increased energy dissipation, indicating acclimation/adaptation of the soil crust photosynthetic microorganisms to a cold environment.