Slow microbial growth and recovery of microbial community structure after disturbances in the DISCOL experimental area

Investigations carried out during leg SO242/2 of RV SONNE to the DISCOL experimental area in Sep. 2015 show that the removal of the reactive surface layer by disturbances created in 1989 is still reflected in altered microbial standing stocks and activities. Based on measurements of microbial carbon assimilation rates (inorganic carbon uptake, oxygen uptake, leucine uptake) and the carbon and nitrogen content of the sediment, we estimated the time needed post-disturbance for microbial regrowth and community turnover to reach initial population sizes. According to these calculations, microbial cell numbers are expected to recover in less than a year. Surprisingly, 26 years after the disturbance, population sizes still do not reach the level found at reference sites. Also the community structure still differs at the disturbed sites. Various explanations for the lack of growth are possible: Grazing, viral lysis, removal of manganese nodules, and a lack of labile organic matter are some of the potential mechanisms which may limit microbial population growth, in spite of the relatively high carbon assimilation rates.