Search

Microorganisms play a fundamental role in global biogeochemical cycles, which are increasingly being altered by the current rapid changes in the environment, threatening biodiversity and the provisioning of key ecosystem services. This thesis addresses key uncertainties surrounding the assessment of ecological resilience and the role of previous disturbances in shaping the stability especially of microbial communities to future disturbances. Firstly, a quantitative framework was developed for the assessment of ecological resilience, which combines current knowledge into a reiterative approach that gradually reduces uncertainty about the level of resilience in a system. Following this, I conducted laboratory experiments in which bacterioplankton communities were exposed to repeated disturbances, to investigate the role of disturbance history in shaping functional and structural stability under subsequent disturbances. I found that disturbance history can affect functional stability in response to both new and familiar disturbances, providing evidence of an “ecological memory” effect which then declined over time. In contrast, microbial community composition appeared to be dominated by a stochastic process, i.e. drift. This suggests the more deterministic responses of the microbial functions arose from sub-lethal effects of the disturbance on microbial activities rather than changes in community composition. These results highlight potential uncertainties associated with application of resilience frameworks developed for multicellular communities to microbial organisms, especially where non-deterministic drift dominates microbial community assembly, and variation in community structure is not strongly connected with variation in functioning.

Bacterial communities play a central role in ecosystems and studying their resistance and resilience provides an important step towards predicting the response of microbial ecosystems to environmental disturbances. Since natural communities undergo regular disturbances and incidences of extremes in environmental conditions, communities may ‘accumulate’ a disturbance history over time. It is generally assumed that recurring disturbances increase resistance and resilience through selection processes, but on the other hand too many disturbances may result in more vulnerable communities and slow down community recovery. Moreover, any change in resistance and resilience due to adaptation might be specific to the type of disturbance that caused the selection pressure, or confer a more general advantage in the face of a broad range of disturbances. Here we hypothesize that (a) resistance and resilience of bacterial communities increases over time when communities are exposed to recurring disturbances and (b) that communities with a history of frequent exposure to strong disturbances of one type, will be less sensitive and more resilient to a novel type of disturbance. In order to test these hypotheses, natural lake bacterioplankton communities were subjected to weekly temperature shocks of two different intensities for four weeks, and subsequently exposed to an acidification event. Throughout the experiment bacterial abundance and bacterial production were measured daily, whereas enzyme activities and bacterial community composition were determined every week. The results from these experiments indicated that exposure to repeated temperature disturbances can increase the resistance and resilience of microbial communities, but it depends on the parameter that is measured. Additionally, the history of temperature disturbances had a clear effect on the ability of the communities to deal with an acidification event, but again it depended on the parameter that was measured whether it had a positive or negative effect on the measured parameter. The community composition was mainly affected by the temperature disturbances and not by the acidification, suggesting possible importance of time lags as well as similarity in the stress tolerance mechanisms that were selected for by the disturbances.