Resilience of microbial mats in Antarctic ponds to climate-relevant environmental disturbance

Abstract

Continental Antarctica is a cold desert, where the hydrologic system is dependent on melting of snow and ice. In a warming climate it is projected that there will be a significant change in precipitation, evaporation, cloud formation, all affecting the ice-water dynamic. Hydrology is considered vulnerable to climatic change. Hydrological change cascades through the environment affecting Antarctic ponds which are important centres for inland microbial biodiversity. An understanding of community vulnerability to anticipated change can be developed through assessing organism and functional resilience to climate-related and other forms of disturbance. The aim of this study is to identify the effects of disturbance on microbial communities, specifically cyanobacterial mats, with a particular focus on changes that may occur resultant to anthropogenic climate change.

This study undertook three experiments, which identified impacts of disturbance on three key cyanobacterial mat functions – nitrogen fixation, photosynthesis/respiration, and recovery after a physical disturbance. Sampling was undertaken in the McMurdo Ice Shelf (MIS) meltwater ponds in late January 2019. The design for the nitrogen fixation experiment used a natural gradient of conductivity across five ponds (Fresh, Casten, P70, Brack and Salt) in a space-for-time approach that compared microbial composition and nitrogen fixation rates. The light disturbance study completed in New and P70 ponds tested mat ability to retain photosynthesis and respiration under a pulsed disturbance – covering microbial mats with shade cloth for 12 months. In this study the light-photosynthesis response and mat composition were analysed when shaded and not shaded (control plots). The third experiment examined short-term response to physical disturbance by observing the recovery of mat structure and community composition for 2 years was also completed in New and P70 ponds.

It was identified that there was no significant change in acetylene reduction (as a proxy for nitrogen fixation) over the salinity range. Acetylene reduction ranged between 22.4 ± 3.4 µMol/m²/h in Brack Pond (conductivity = 10.5 mS/cm) and 49.6 ± 17.1 µMol/m²/h in Casten Pond (conductivity = 2.3 mS/cm). The acetylene reduction difference was statistically significant between Brack Pond and Casten Pond (F = 0.006 (Bonferroni Correction significance of F ≤ 0.01)). Microbial mat composition changed across the conductivity gradient, but significant proportions of different N-fixing and mat-forming genera were evident along the salt gradient, suggesting functional resilience through species turnover.

It was also identified that when receiving a 95-96% decrease in ambient light photosynthetic bacteria were able to maintain photosynthesis and respiration at similar rates within the microbial mats. However, in shaded plots the maximum net oxygen production occurred at an average irradiance between 350 – 450 µMol photons m-²s-¹ and was inhibited at higher levels. In control plots the maximum net oxygen production was ≥900 µMol photons m-²s-¹. In control samples net photosynthesis exceeds zero (photosynthesis exceeds respiration) at ~275 µMol photons m-²s-¹, in shaded samples this occurred at ~100 µMol photons m-²s-¹. Rapid shifts in the appearance of mats under, and after shading led to the conclusion that the acclimation may be based on vertical migration of cyanobacteria within the mats, which showed no significant changes in relative abundance of taxa.

After physical removal of microbial mat, communities were shown to reform to similar relative abundances as control samples within two years of the disturbance, within this experiment a successional change in species abundance was observed.

This research highlights the resilience of microbial mat populations on the MIS to the types of change that are anticipated to accompany climate change. It supports conclusions previously identified that cyanobacterial mats are functionally resilient to short-term and long-term disturbances. Ongoing research will improve the knowledge on the resilience of other functions required in these environments.