Climate change effects on planktonic bacterial communities in the ocean: from structure and function to long-term and large scale observations.

Planktonic bacterial communities of marine environments can be considered to be affected by climate change through a set of direct and indirect effects. As direct effects of climate change, elevated temperature and ambient CO2 levels have to be taken into account. As indirect effects a large spectrum of impacts ranging from increased stratification of surface water, deoxygenation of subsurface water, increased occurrence of extreme weather events, and a changed food chain and nutrient regime resulting in a changed top down and bottom up control for bacterioplankton. All these direct and indirect effects will affect bacterial communities in a multifaceted way. Changes in the bacterial communities due to climate change impacts can be expected on all different taxonomic levels, i.e. from the clonal intraspecies level to the phylum level. The focus of the article will be on the evaluation of bacterioplankton observations over time and space and along climate gradients in the frame of environmental parameters allowing modelling with respect to climate change scenarios. A specific emphasis will be on bacterioplankton analysis based on analysis of samples of the Continuous Plankton Recorder Archive. This sample archive allowed insights into particle associated bacteria of coastal environments over more than the last 60 years. Beside effects of climate change effectors on bacterial growth and community composition, effects of climate change on bacteria-mediated marine biogeochemical cycling and potential hazards by increased abundance of pathogenic bacteria such as Vibrio cholerae will be considered.