From Sea to Society - Climate Change, Microbial Community Interactions and Assessing Climate Risk on Society

摘要

Unicellular organisms, microalgae and bacteria, less than one millimeter in size make theworld go round. Phytoplankton and cyanobacteria, for example, plays an inevitable rolecontributing 50-85% to the world’s oxygen via photosynthesis. Bacteria, one of the first lifeforms to appear on earth, are vital for nutrient cycling both in the terrestrial, limnetic andmarine ecosystems. Both phytoplankton and marine bacteria in addition form an importantfoundation of marine food webs. The adversities of climate change loom large on the two, asimpacts at the level of primary production will most likely have a bottom up effect across thedifferent trophic levels. Warming of the earth system is proceeding at a rate that isunprecedented, yet its effects on the world’s largest ecosystem, the marine pelagic realm, arelargely unknown. Using experimental approaches including mesocosm settings and geneexpressionanalysis, this thesis aims to increase our understanding of climate-inducedprocesses in a costal pelagic tropical ecosystem in southwest coastal Arabian Sea. The resultsshow that sea surface warming and freshening is likely to cause an increase in the abundanceof phytoplankton in winter season. While in the summer, freshening of sea surface water canbe a leading cause for the increase in phytoplankton growth. The findings presented in thisthesis also highlight the tight coupling between phytoplankton and bacterial community. Anyimpact of warming and freshening is subsequently propagated to bacteria as increasedphytoplankton biomass supports increased biomass of bacteria. To understand the underlyingbases of this phytoplankton-bacterial interrelationship, it is important to recognize thecommunication over species borders. We looked for signs of existing cross signallingbetween a predominant bacterial species, Vibrio harveyi, and a dominant diatom species,Skeletonema marinoi. In V. harveyi, quorum sensing (QS), cell-to-cell signalling amongbacteria, indirectly regulates bacterial virulence gene expression. Previous studies have shownthis communication being interrupted by inhibitory compounds produced by some species ofmacro- and microalgae. Our results indicate activation of QS in V. harveyi strains during theexperiment period, but QS did not regulate the expression levels of virulence factors. Thissuggests that the QS activity of V. harveyi was not interrupted by the presence of S. marinoistrain, implying that this diatom-bacteria relationship may in this respect be considered acommensalistic relationship. In the face of climate change, our results make importantpredictions for the future scenarios suggesting faster growth and development of microbialblooms, both phytoplankton and bacteria including pathogenic bacteria and toxin-producingphytoplankton. These can accumulate in commercially important bivalves, subsequentlypassing to higher trophic levels. This may have considerable implications for seafood safety,putting human health at high risk. We propose an integrated science-social scienceframework for assessment of human vulnerability to climate change effects in marineecosystems. This can help to weigh, more explicitly and systematically, human vulnerabilityinfluenced by complex and simultaneous environmental and socio-economic changes.