Abstract:
Heterotrophic marine bacterioplankton affect nutrients stoichiometry and cycling, utilizing and determining the fate of dissolved organic carbon pool. In oligotrophic waters, heterotrophic bacteria can play a relevant role in the transfer of carbon to higher trophic levels in the food chain when bacteria are ingested by protozoa, through the microbial loop. In addition, different bacteria carbon metabolisms as dark CO2 ifixation tend to have a special importance in oligotrophic ecosystems and can be an important carbon flux pathway for bacteria metabolic imbalances compensation. While photo-heterotorphic bacteria has been considered to contribute a large proportion of bacterioplankton carbon production in oligotrophic waters. In the Red Sea warm and oligotrophic waters, heterotrophic bacteria metabolisms and carbon transfer via bacteria could be an important pathway for carbon flux into higher trophic levels. Moreover, the Red Sea is warming fast with increasing sea-surface water temperature which could have a remarkable effect on the biogeochemical cycle and carbon processes. However, planktonic species such as diatoms can develop some adaptation mechanisms in response to increasing temperature. Metabolomics approaches have been considered cruicial methods to evaluate the response of microalgae changes in responses to stressors such as warming. The evolutionary responses of microalgae without doubt may influence their metabolic acitvites and profiles. In addition, the evolutionary changes may alter their relaitonship with their microbiome, therefore, influencing the microbiome richness. The PhD thesis aimed to quantify bacterial production (BP) with aiming to assess the contribution of BP to the carbon flow by the microbial loop to higher trophic levels in the Red Sea. The bacteria production and the carbon transfer by heterotrophic bacteria were quantified by using 13C-labeling experiments and CRDS-Picarro analytical technique. The thesis in addition aimed to quantify the relevance of dark CO2 incorporation (dark213C-bicarbonate) and its contribution to the total photosynthetic production in the Red Sea coastal, open and deep waters. Furthermore, the thesis quantified the relevant role of photo- heterotrophic metabolism (light 13C-glucose uptake) to the Red Sea bacterioplankton relative to the dark heterotorphic production (dark 13C-glucose). As temperature is a relevant environmental condition of the Red Sea, the thesis analyzed the effect of increasing temperature associated with ocean warming on the production of metabolites products in Red Sea phytoplankton species using different analytical robust tools as NMR and GC-MS. In addition, the thesis addressed the effect of long term adaptation to warming temperature on diatom species Chaetoceros tenuissimus metabolic activity and their microbiom richness. The results of this thesis confirmed that carbon transfer by bacteria via the microbial loop to higher trophic levels could be a significantly important pathway in the oligotrophic and warm waters of the Red Sea. In addition, our results revealed that the contribution of dark CO2 fixation by bacteria to the total CO2 fixation was found to increase significantly when primary production decreased in the photic zone or was absent in the deep water. Despite the expected significant contribution of photo- heterotrophic production to the carbon dynamic in oligotrophic waters, light was shown to have an inconsistent and minor role in stimulating heterotrophic production in the oligotrophic Red Sea. Yet some significant increase of HP was found under high light conditions in some stations.
We succeeded in optimizing the extraction procedure using combined metabolomics approaches of NMR and GC-MS to evaluate the metabolome diversity of the diatom Chaetoceros tenuissimus isolated from the tropical Red Sea. This optimization allowed us to overcome possible challenges, such as the different detected ranges and limits of metabolites by NMR and GC-MS, in addition to the low biomass harvested due to the diatom small cell size and the insufficient amount of both water-soluble and lipid layers from the same extraction. Using these metabolomics approaches and the extraction procedure, we found that the diatom Chaetoceros tenuissimus accumulated amino acids under long-warm adaption while storing fewer carbohydrates. In addition, our results found that the diatom Chaetoceros tenuissimus changed their thermal capacities as a response to long-term warming. Furthermore, the microbial community associated with the diatom species C. tenuissimus was subjected to significant changes in their richness and microbiome under long-term adaptation.

Bio:
Afrah Alothman hold a bachelor degree in Biology from King Faisal University, Al-Hassa, Saudi Arabia, Master degree in Biology from Dalhousie University, Halifax, Canada and she is now a current PhD candidate in Marine Science program working in the Biological Oceanography lab under Prof. Susana Agusti supervision.