Abstract
Quorum sensing (QS) describes a process by which bacteria can sense the local cell density of their own species, thus enabling them to coordinate gene expression and physiological processes on a community-wide scale. Small molecules called autoinducers or QS signals, which act as intraspecies signals, mediate quorum sensing. As our knowledge of QS has progressed, so too has our understanding of the structural diversity of QS signals, along with the diversity of bacteria conducting QS and the range of ecosystems in which QS takes place. It is now also clear that QS signals are more than just intraspecies signals. QS signals mediate interactions between species of prokaryotes, and between prokaryotes and eukaryotes. In recent years, our understanding of QS signals as mediators of algae–bacteria interactions has advanced such that we are beginning to develop a mechanistic understanding of their effects. This review will summarize the recent efforts to understand how different classes of QS signals contribute to the interactions between planktonic microalgae and bacteria in our oceans, primarily N-acyl-homoserine lactones, their degradation products of tetramic acids, and 2-alkyl-4-quinolones. In particular, this review will discuss the ways in which QS signals alter microalgae growth and metabolism, namely as direct effectors of photosynthesis, regulators of the cell cycle, and as modulators of other algicidal mechanisms. Furthermore, the contribution of QS signals to nutrient acquisition is discussed, and finally, how microalgae can modulate these small molecules to dampen their effects.
Highlights
Bacteria in aquatic systems respire between 25 and 50% of the carbon fixed by algae, and this carbon is often metabolized by bacteria mere hours after it has been fixed [5,6]
3 of how certain Quorum sensing (QS) signals directly affect the growth of planktonic microalgae, as well as how quorum sensing can dictate other microbial processes indirectly dictating algae–bacteria interactions, and the microbial loop as a whole
This review has focused on how QS signals act as a direct signal between bacteria and microalgae
Summary
Marine microalgae are a highly diverse array of photosynthetic organisms, comprised of prokaryotic (i.e., cyanobacteria) and eukaryotic taxa. Planktonic microalgae are the foremost primary producers in the ocean, and are principal components of the earth’s biogeochemistry [2,3,4] They live among marine bacteria whose effects on microalgae are both diverse and pervasive. Bacteria in aquatic systems respire between 25 and 50% of the carbon fixed by algae, and this carbon is often metabolized by bacteria mere hours after it has been fixed [5,6] Some of these bacteria are ‘passive’ players, surviving off nutrients exuded by microalgae [7], while other bacteria are ‘algicidal’ and have evolved methods to inhibit the growth or even kill microalgae, thereby leading to cell lysis and a spike in the nutrients in their immediate vicinity [8]. It is within the phycosphere that the majority of interactions between algae and bacteria take place [14]
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