Interactions between autotrophic and heterotrophic bacteria are fundamental for marine biogeochemical cycling but the impact of global change on these essential microbial players is not well understood. This PhD dissertation aims to assess the responses of two representative Red Sea cyanobacteria (Prochlorococcus RSP50 and Synechococcus RS9907) and their major heterotrophic bacteria to temperature. After acclimating both co-cultures at different temperatures and determining the 3 dominant heterotrophic groups by 16S rRNA gene amplicon, the cellular properties were monitored by flow cytometry. The 3 dominant heterotrophic bacteria present in RS9907 were affiliated to the genera Paracoccus, Marinobacter and Muricauda while 2 of the heterotrophic compositions in RSP50 belonged to the family Rhodobacteraceae and 1 to Flavobacteriaceae. The activation energy of Prochlorococcus (0.58 eV) was higher than the hypothesized metabolic theory of ecology value for autotrophs (0.32 eV), but lower than any of their 3 heterotrophic bacteria (1.02 to 1.58 eV), exceeding the expected MTE value for heterotrophs (0.65 eV). Conversely, Synechococcus activation energy was very similar to the expected value (0.34 eV) while the values of their heterotrophic bacteria varied notably (0.16-1.15 eV). Although both Cyanobacteria became larger with increasing temperature, exactly the opposite of the temperature-size rule expectation while their associated heterotrophic bacteria did not change in a coherent way. In order to allow a deeper understanding at the gene expression level, the transcriptomic responses of RSP50 to temperatures and salinities naturally found in the Red Sea. Only at 40 of salinity, 2 genes related to the synthesis of the osmolyte glucosylglycerol were found expressed. At 30˚C, genes involved in carbon fixation, such as CsoS2 and CsoS3, and photosynthetic electron transport, such as PTOX, appeared to be downregulated, indicating a cellular homeostasis and energy-saving mechanism response. Heat shock proteins including DnaK, 10 kDa and 60 kDa chaperonins were likewise downregulated, implying a lack of effective adaptation. This suggests RSP50 accumulates glucosylglycerol as a coping mechanism to the high salinity and might be unable to cope effectively at 30˚C or higher. Altogether, the responses to future ocean warming of autotrophic and heterotrophic bacteria might not be well described by theoretical universal rules.
|Date made available
|KAUST Research Repository