All ETDs from UAB

Advisory Committee Chair

Jeffrey Morris

Advisory Committee Members

Asim Bej

Karolina Mukhtar

Document Type

Thesis

Date of Award

2020

Degree Name by School

Master of Science (MS) College of Arts and Sciences

Abstract

Prochlorococcus is an unusually small and numerous genus of marine cyanobacteria which is thought to contribute 8.5% of marine primary production. Prochlorococcus lacks a gene for catalase, resulting in significant sensitivity to stress from reactive oxygen species (ROS), particularly hydrogen peroxide. Prochlorococcus is known to rely on other species of microbes in the ocean to degrade ROS, such as Alteromonas, a heterotrophic “helper” bacterium with which it is frequently co-located. However, it has been observed that Y2100 predicted ocean acidification conditions (800 ppm atmospheric CO2) reduce catalase production by Alteromonas strain EZ55, increasing oxidative stress on Prochlorococcus strain MIT 9312. The Long-Term Phytoplankton Evolution (LTPE) experiment, which preceded the experiments described here, found that 500 generations of Prochlorococcus – Alteromonas co-culture co-evolution at 800 ppm CO2 was sufficient to rescue Prochlorococcus growth rates in 800 ppm CO2 growth conditions.Here we sought to determine whether evolved Prochlorococcus remain dependent on assistance from Alteromonas, and if so, which of co-evolved, ancestral, or evolved Alteromonas was the best helper for ancestral Prochlorococcus, Prochlorococcus evolved at 400 ppm CO2, and Prochlorococcus evolved at 800 ppm CO2. Axenic cultures of the three Prochlorococcus types were reinoculated into co-culture with ancestral or evolved Alteromonas, or were left axenic with no Alteromonas, and were grown at 400 or 800 ppm CO2 for a total of 18 culture conditions. These were grown in semi-continuous culture and assessed for viability, exponential growth rate (EGR), and Malthusian growth rate (MGR). We conclude that Prochlorococcus does remain dependent on Alteromonas even after evolution at 800 ppm CO2, and that the evolutionary and co-evolutionary backgrounds of the helper bacteria did not affect Prochlorococcus growth metrics. Unlike in the LTPE experiment, co-cultures evolved at 800 ppm did not rescue growth rates from impairment by elevated CO2. Differences in these results suggest that population diversity of Alteromonas or other “helper” organisms has a notable effect on the degree of helpfulness provided.

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