PhD Dissertation Defense - Luis Ribeiro da Silva

Oct 17 2019 03:30 PM - Oct 17 2019 04:30 PM

TITLE: Dynamics of heterotrophic bacterioplankton and their role in epipelagic carbon fluxes in the Red Sea

ADVISOR: Prof. Xelu Moran 

DATE: Thursday, October 17, 2019

TIME: 3:30 pm – 4:30 pm

LOCATION: Building 3, Level 5, Room 5220

Abstract: Heterotrophic bacterioplankton dynamics have seldom been assessed in the Red Sea, an exceptionally warm oligotrophic basin, which could be used as a model for the future ocean. To understand the function of heterotrophic bacteria in biogeochemical cycles and the flows of matter and energy to higher trophic levels, it is peremptory to understand how bacterial growth is controlled. Bottom-up (resources availability), top-down (mortality by predators and viruses) and temperature controls are the main hypotheses of control of bacterial activity and stocks. This dissertation aims to assess the spatial-temporal variability of heterotrophic bacteria and their interactions with diverse sources of dissolved organic matter (DOM) through the observed effects on bacterial growth rates and productivity in coastal environments of the central Red Sea. To that end we conducted a total of 66 short-term incubations (4-6 days) concurrently with the whole microbial community and with only the bacterial component after removing protistan grazers by filtration in various shallow ecosystems (an embayment, a coastal lagoon, open water) characterized by different dominant sources of DOM. Frequent sampling during the seawater estimations and combined flow cytometry and biogeochemical analysis allowed us to measure bacterial standing stocks, including the maximum abundance attainable or carrying capacity, growth rates, characterize DOM concentrations and lability, assess bacterial DOM consumption rates and biomass production and ultimately quantify bacterial growth efficiencies. Our findings suggest that although bacteria seemed to thrive in nutrient-sufficient waters, the central coastal Red Sea is characterized by unusually low bacterial standing stocks (4.05 ± 0.31 x105 cells ml-1), probably controlled by protistan grazing. At the same time, bacterioplankton showed high potential to grow (0.35-1.75 d-1, reaching 4.16 d-1 when dilution and pre-filtration were performed). Even though the highest specific growth rates were observed during the warmer periods, we did not find any consistent relationship with temperature. While temperature seemed not to constrain bacterial specific growth rates, we observed a tight link between bacterial growth and resource availability in terms of both quantity and quality.  Overall, by surveying one of the warmest marine regions on Earth, this dissertation provides detailed insights into heterotrophic bacterioplankton dynamics and how bottom-up, top-down and temperature regulate them in tropical waters, a vast geographical extension of the world oceans that had remained strongly undersampled to date.