Temporal variability in the growth-enhancing effects of different bacteria within the microbiome of the diatom Actinocyclus sp.

Reciprocal metabolite exchanges between diatoms and bacteria can enhance the growth of both partners and therefore fundamentally influence aquatic ecosystem productivity. Here, we examined the growth-promoting capabilities of 15 different bacterial isolates from the bacterial community associated with the marine diatom Actinocyclus sp. and investigated the magnitude and timing of their effect on the growth of this diatom. In the presence of its microbiome, Actinocyclus sp. growth was significantly enhanced relative to axenic cultures. Co-culture with each of the 15 bacterial isolates examined here (seven Rhodobacteraceae, four Vibrionaceae, two Pseudoalteromonadaceae, one Oceanospirillaceae and one Alteromonadaceae) increased the growth of the diatom host, with four isolates inducing rates of growth that were similar to those delivered by the diatom's full microbiome. However, the timing and duration of this effect differed between the different bacteria tested. Indeed, one Rhodobacteraceae and one Alteromonadaceae enhanced Actinocyclus sp. cell numbers between days 0-6 after co-incubation, five other Rhodobacteraceae promoted diatom cell numbers the most between days 8-12, whilst four Vibrionaceae, one Oceanospirillaceae and one Rhodobacteraceae enhanced Actinocyclus sp. cell abundance between days 14-16. These results are indicative of a succession of the growth-enhancing effects delivered by diverse bacteria throughout the Actinocyclus sp. life cycle, which will likely deliver sustained growth benefits to the diatom when its full microbiome is present.

Coral endosymbiont growth is enhanced by metabolic interactions with bacteria.

Bacteria are key contributors to microalgae resource acquisition, competitive performance, and functional diversity, but their potential metabolic interactions with coral microalgal endosymbionts (Symbiodiniaceae) have been largely overlooked. Here, we show that altering the bacterial composition of two widespread Symbiodiniaceae species, during their free-living stage, results in a significant shift in their cellular metabolism. Indeed, the abundance of monosaccharides and the key phytohormone indole-3-acetic acid (IAA) were correlated with the presence of specific bacteria, including members of the Labrenzia (Roseibium) and Marinobacter genera. Single-cell stable isotope tracking revealed that these two bacterial genera are involved in reciprocal exchanges of carbon and nitrogen with Symbiodiniaceae. We identified the provision of IAA by Labrenzia and Marinobacter, and this metabolite caused a significant growth enhancement of Symbiodiniaceae. By unravelling these interkingdom interactions, our work demonstrates how specific bacterial associates fundamentally govern Symbiodiniaceae fitness.