Adaptation to pH stress by Vibrio fischeri can affect its symbiosis with the Hawaiian bobtail squid (Euprymna scolopes)

Abstract

Many microorganisms engaged in host-microbe interactions pendulate between a free-living phase and a host-affiliated stage. How adaptation to stress during the free-living phase affects host-microbe associations is unclear and understudied. To explore this topic, the symbiosis between Hawaiian bobtail squid (Euprymna scolopes) and the luminous bacterium Vibrio fischeri was leveraged for a microbial experimental evolution study. V. fischeri experienced adaptation to extreme pH while apart from the squid host. V. fischeri was serially passaged for 2000 generations to the lower and upper pH growth limits for this microorganism, which were pH 6.0 and 10.0, respectively. V. fischeri was also serially passaged for 2000 generations to vacillating pH 6.0 and 10.0. Evolution to pH stress both facilitated and impaired symbiosis. Microbial evolution to acid stress promoted squid colonization and increased bioluminescence for V. fischeri, while symbiont adaptation to alkaline stress diminished these two traits. Oscillatory selection to acid and alkaline stress also improved symbiosis for V. fischeri, but the facilitating effects were less than that provided by microbial adaptation to acid stress. In summary, microbial adaptation to harsh environments amid the free-living phase may impact the evolution of host-microbe interactions in ways that were not formerly considered.

Nocturnal Acidification: A Coordinating Cue in the Euprymna scolopes–Vibrio fischeri Symbiosis

“…Vibrionaceae are found world-wide, in brackish environments with disparate and fluctuating pH conditions reflecting changing organic nutrient load, dissolved organic carbon levels, and active geologic activity [ 40 , 41 ]. While even small pH changes can dramatically impact microbial communities such as the Vibrionaceae, V. fischeri has evolved to thrive in the microenvironment of the light organ, where the matrix pH changes dramatically during the diel cycle, gradually decreasing from a pH ~7.0 after venting at dawn, to a pH ~5.5 just prior to the next venting event [ 42 , 43 ]. This is in agreement with experimental work exhibiting several V. fischeri strains able to grow in culture between pH 5–10.6 [ 44 ].…”

“…In a study investigating the impact that global climate change driven acidification of ocean waters (increasing atmospheric CO 2 leading to increased levels of dissolved CO 2 that forms carbonic acid) will have upon the E. scolopes–V. fischeri symbiosis, it was found that the V. fischeri that was experimentally evolved under low pH (6.0 vs. 7.4) growth conditions for multiple generations outcompeted non-evolved ancestor V. fischeri strains in colonizing juvenile squid [ 43 ]. Additional studies used a similar experimental evolution approach to assess how adaptation to a low pH environment affects V. fischeri growth rates and bioluminescence production, and squid colonization demonstrated significant increases in colonization rates, faster generation times, and increased bioluminescence compared to ancestral wild-type strains [ 45 ].…”

Symbiotic organs: the nexus of host–microbe evolution

“…The light organ of bobtail squid is one of the best characterized, comprising both internal crypts that house symbionts and external structures that are lost after symbiont colonization [23,29]. The crypts are acidic and enriched with host antimicrobials, selecting for light-producing Vibrio fischeri, creating an environment that is inhospitable to most other microbes, and increasing the availability of oxygen for light production [23,29,30].…”

Environmental Stress Selects for Innovations That Drive Vibrio Symbiont Diversity

“…Moreover, how these stressors can drive increased bacterial diversity in the free-living state and hosts is addressed. Three interesting empirical case studies have been completed with temperature and pH quite recently, which readers are encouraged to read (Cohen et al, 2019(Cohen et al, , 2020Nourabadi and Nishiguchi, 2021).…”