The bacterial quorum sensing molecule 2-heptyl-4-quinolone physiological mode of action in Emiliania huxleyi controls critical pathways fundamental to algal metabolism and cell division

Coccolithophores contribute half of the total calcium carbonate production in the pelagic zone via biomineralization of their carbonate skeletons. Elucidating the mechanisms that promote the growth and mortality of these phytoplankton are critical to better model oceanic nutrient cycling and impacts on global climate. The model cosmopolitan coccolithophore, Emiliania huxleyi, was previously found to be highly susceptible to the bacterial quorum sensing molecule, 2-heptyl-4-quinolone (HHQ), which arrests E. huxleyi cell division without inducing mortality by manipulating critical pathways fundamental in eukaryotic metabolism and cell division. We also show that HHQ may protect E. huxleyi from viral-induced mortality, suggesting intricate co-evolutionary interactions between viruses, bacteria, and algae. Biochemical assays indicate exposure to nanomolar concentrations of HHQ leads to an accumulation of DNA strand breaks, blocking DNA replication, and stalling cell division. Using ‘omic approaches to identify intracellular targets, we determined HHQ significantly alters proteins involved in DNA synthesis and repair, including DNA remodeling enzymes, polymerases and enzymes necessary for pyrimidine biosynthesis. Moreover, HHQ exposed cells exhibited significant changes in pathways involved in metabolism including oxidative respiration, TCA cycle proteins, gluconeogenesis, along with components of photosynthesis, vitamin B6 synthesis, and protein degradation. A detailed exploration of cellular architecture by TEM indicated HHQ-exposed alga had enlarged chloroplasts and mitochondria, as well as a highly vacuolized cytoplasm, indicating massive cellular restructuring after only 24 hours of exposure. This work aims to continue to build an ecological framework for how chemical messages influence host-symbiont interactions by elucidating the molecular mechanisms by which HHQ potentially enables algal-host protection against viral attack.