The Influence of Bacteria on the Growth, Lipid Production, and Extracellular Metabolite Accumulation by Phaeodactylum tricornutum (Bacillariophyceae)

Abstract

To examine the impact of heterotrophic bacteria on microalgal physiology, we co‐cultured the diatom Phaeodactylum tricornutum with six bacterial strains to quantify bacteria‐mediated differences in algal biomass, total intracellular lipids, and for a subset, extracellular metabolite accumulation. A Marinobacter isolate significantly increased algal cell concentrations, dry biomass, and lipid content compared to axenic algal cultures. Two other bacterial strains from the Bacteroidetes order, of the genera Algoriphagus and Muricauda, significantly lowered P. tricornutum biomass, leading to overall decreased lipid accumulation. These three bacterial co‐cultures (one mutualistic, two competitive) were analyzed for extracellular metabolites via untargeted liquid chromatography mass spectrometry to compare against bacteria‐free cultures. Over 80% of the extracellular metabolites differentially abundant in at least one treatment were in higher concentrations in the axenic cultures, in agreement with the hypothesis that the co‐cultured bacteria incorporated algal‐derived organic compounds for growth. Furthermore, the extracellular metabolite profiles of the two growth‐inhibiting cultures were more similar to one another than the growth‐promoting co‐culture, linking metabolite patterns to ecological role. Our results show that algal–bacterial interactions can influence the accumulation of intracellular lipids and extracellular metabolites, and suggest that utilization and accumulation of compounds outside the cell play a role in regulating microbial interactions.

Single cell carbon and nitrogen incorporation and remineralization profiles are uncoupled from phylogenetic groupings of diatom-associated bacteria

“…The majority of the isolates did not impact P. tricornutum growth (Table 1, Fig. S2), with the exception of Marinobacter, Devosia , and Alcanivorax , the presence of which increased algal chlorophyll fluorescence in early stationary growth phase compared to axenic cultures, as previously reported (Chorazyczewski et al 2021). Four bacterial strains were detected attached to P. tricornutum cells ( Marinobacter, Oceanicaulis, Roseibium , and Yoonia ), all of which originated from phycosphere enrichments where free-living bacteria were washed away (Samo et al 2018).…”

“…We did not find that exchange of N was associated with mutualism: none of the three mutualistic strains showed evidence of N mineralization. Furthermore, the highest remineralizers of DON ( Muricauda, Oceanicaulis, Arenibacter , and Algoriphagus ) were not mutualistic, and two of those strains ( Muricauda and Algoriphagus ) were recently shown to be growth-inhibiting in co-culture with P. tricornutum (Chorazyczewski et al 2021). In fact, most of the 15 tested strains, including the three mutualists, provided little to no remineralized N to the algal cells.…”

Dynamic Phaeodactylum tricornutum Exometabolites Shape Surrounding Bacterial Communities

“…We used the model diatom P. tricornutum strain CCMP 2561. Bacterial isolates were isolated in our lab and have been described elsewhere (19, 20). Information on their origins and identifications is summarized in Table 1.…”

“…We conducted a time resolved metabolomic analysis to characterize exudates from axenic P. tricornutum and identify specific exometabolites of interest. Using selected identified metabolites, we conducted growth assays with bacterial isolates previously characterized for their associations with P. tricornutum (19, 20). Based on results from these assays, we again used microbial community analysis to investigate the role of specific identified exometabolites in the context of a complex microbial community with and without algae and algal exudates, allowing us to investigate the interacting roles of exometabolites, algae, and bacteria.…”

Bacterial response to spatial gradients of algal-derived nutrients in a porous microplate

“…Then we co-cultured P. tricornutum with two commonly algal-associated bacterial strains (genera Algoriphagus and Marinobacter ), where the bacteria were incubated at different distances from the alga. The strains were previously isolated from P. tricornutum mesocosms [10, 13] and have been previously shown to affect algal growth [28]. Finally, we incubated P. tricornutum in the porous microplate with mixed bacterial communities to observe how community structure responds to physical distance from the algal host.…”