On the role of local and global physical forcing to space-time dynamics of microbes: a case study from the Southern Seas

Author: Virginie van Dongen-Vogels

van Dongen-Vogels, Virginie, 2012 On the role of local and global physical forcing to space-time dynamics of microbes: a case study from the Southern Seas, Flinders University, School of Biological Sciences

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Global climate changes urge scientists to understand the effect of plankton communities on carbon cycle in aquatic systems. Although the microbial food web was for long overlooked, its potential importance to future oceanic and coastal systems has been stressed. Being less than 3 [mu]m diameter in size, picophytoplankton, heterotrophic bacteria and viruses are all major components of the microbial food web. These can reach abundances of respectively 105, 106, and 108 cells mL-1 and have a key role in marine carbon and energy transfer. However, the existence of diverse microbial populations may differently respond to changes in physical forcing, hence affecting the fate of organic matter and the efficiency of carbon and energy transfer of aquatic systems. The present research aimed to improve our understanding on the temporal dynamics in microbial community structure, with specific emphasis on physical forcing. Flow cytometry was used throughout this work to identify and enumerate distinct microbial populations. First, the responses of discrete heterotrophic bacterial populations to local short-term environmental fluctuations were investigated within the Polar Frontal Zone of the Southern Ocean. Secondly, the local temporal dynamics of distinct picophytoplankton populations in relation to local physical events (i.e. upwelling, downwelling) influenced by local (wind) and global (El Nino/La Nina) climatic forcing were examined for the first time within the South Australian continental shelf waters. Finally, the relative importance of local to global hydroclimatic forcing on the long-term dynamics in picophytoplankton, heterotrophic bacteria, and viruses abundances and their relationships were investigated at the national reference station of the Southern Australian shelves. Overall, results showed that the temporal variability in both picophytoplankton and heterotrophic bacterial community structure depended upon the sampled depth's properties, and, indicated that physical events of distinct nature differentially influenced various nodes of the microbial food web. The most abundant population or group often presented relatively little variability over time, but the least abundant population varied the most, suggesting that for different levels of organisation, microbes might present a constant vs sporadic behaviour over distinct time scales. In the South Australian continental shelf waters, the potential existence of distinct ecotypes of Prochlorococcus, Synechococcus and picoeukaryotes were reported with an unexpected southern extension of a High-Light and Low-Light adapted ecotypes of Prochlorococcus, likely due to advective transports. Upwelling and downwelling conditions associated to changes in the nature and intensities of stratification and mixing processes were found responsible for the local dominance of distinct picophytoplankton populations. The relationship between these dominant populations and upwelling conditions further showed the relative importance of local (wind field) and global (El Nino/La Nina) hydroclimatic forcing to picophytoplankton community structure. The distinct long-term temporal dynamics of picophytoplankton, heterotrophic bacteria, and viruses abundances reflected the temporal and vertical variability in salinity and temperature gradients associated to distinct upwelling and downwelling conditions. These revealed for the first time a vertical decoupling of viruses and bacteria during upwelling of an El Nino event. The present research has major implications to the functioning of distinct ecosystems with global changes.

Keywords: microbes,picoplankton,Prochlorococcus,Synechococcus,picoeukaryotes,upwelling,downwelling,physical forcing,El Nino,ENSO,du Couedic Canyon,South Australia,Australia
Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2012
School: School of Biological Sciences
Supervisor: Laurent Seuront, Jim Mitchell, Justin Seymour