PhD Dissertation Defense - Najwa Al-Otaibi

Sep 22 2020 10:00 AM - Sep 22 2020 11:00 AM

TITLE: Seasonal and diel variability of autotrophic and heterotrophic picoplankton in the central Red Sea: Effects of nutrients and temperature

PRESENTER: Najwa Al-Otaibi

ADVISOR: Professor Xosé Anxelu G. Morán

DATE: Tuesday, September 22, 2020

TIME: 10 am - 11 am


ABSTRACT: Picoplankton, composed of cells between 0.2 and 2 μm in size, play a vital role in the carbon flow and nutrient cycling in marine food webs. Auto- and heterotrophic picoplankton dominate the biomass of oligotrophic tropical and subtropical oceans. However, little is known about their vertical distribution, changes in space and time, and relationships with environmental variables driving their responses in the central Red Sea. The goal of this Ph.D. dissertation is to obtain baseline knowledge about their abundance, cellular characteristics (cell size, relative pigment, and nucleic acid content), and biomass at seasonal and high-frequency temporal resolution (every 2 hours during 4 diel cycles). This dissertation also aims at assessing picoplankton responses to separate and joint effects of nutrients additions (inorganic, organic, and mixed) and temperature in order to be able to predict the relative contribution of eutrophication and warming in the future standing stocks of this planktonic size fraction in the Red Sea. I conducted a total of 63 vertical profiles (15 at around noon plus 48 more from the high-frequency diel samplings) from the surface down to the bottom (ca. 700 m) at a station situated 6 km off the coast of King Abdullah Economic City (KAEC) in the central Red Sea and performed 4 nutrient and temperature experiments lasting every 6 days with surface waters from the harbor of King Abdullah University of Science and Technology (KAUST). Flow cytometry allowed me to consistently identify five groups of autotrophs (Prochlorococcus, two populations of Synechococcus separated by their relative phycoerythrin fluorescence, and two differently-sized groups of picoeukaryotes) and two groups of heterotrophic prokaryotes characterized by their different relative nucleic acid content. One of the most surprising findings is the relatively lower abundances and to a lesser extent also growth rates of picoplankton compared with other tropical and subtropical oceans. The results also show that seasonality in environmental conditions, higher than anticipated for a tropical site, emerged as an important factor in the response of picoplankton to nutrient additions and temperature. Picoplankton mostly responded to inorganic and mixed nutrient additions rather than warming. Overall, the information provided in this dissertation fills the gap of a critical component of Red Sea pelagic ecosystems and expands the information available on picoplankton communities in tropical waters, allowing future studies to examine in further detail anthropogenic impacts on tropical microbial plankton.

BIO: Najwa Al-Otaibi is a Ph.D. candidate in Microbial Oceanography and Biogeochemistry Lab under the supervision of Prof. Xosé Anxelu G. Morán. Najwa obtained her bachelor's degree in the Biotechnology department at Taif University in 2011 and a master's degree in Bioscience at Water Desalination and Reuse Center (WDRC), KAUST, in 2014. Najwa Ph.D. dissertation addresses the temporal variability of autotrophic and heterotrophic picoplankton in the central Red Sea and how temperature and nutrient additions affect the response of planktonic microbial communities in surface waters from KAUST harbor.