Abstract
Phytoplankton strongly interact with their associated bacteria, both attached (PA), and free-living (FL), and bacterial community structures can be specific to phytoplankton species. Similarly, responses to environmental stressors can vary by taxon, as exemplified by observed shifts in phytoplankton community structure from diatoms to phytoflagellates after the Deepwater Horizon (DWH) oil spill. Here, we assess the extent to which associated bacteria influence the phytoplankton taxon-specific oil response by exposing xenic and axenic strains of three phytoplankton species to oil and/or dispersant. The dinoflagellates Amphidinium carterae and Peridinium sociale, and the diatom Skeletonema sp., all harbored significantly distinct bacterial communities that reflected their host oil response. Oil degrading bacteria were detected in both PA and FL communities of the oil resistant dinoflagellates, but their FL bacteria were more efficient in lipid hydrolysis, a proxy for oil degradation capability. Inversely, the growth rate and photosynthetic parameters of the diatom Skeletonema sp. was the most impacted by dispersed oil compared to the dinoflagellates, and oil-degrading bacteria were not significantly associated to its microbiome, even in the dispersed oil treatment. Moreover, the FL bacteria of Skeletonema did not show significant oil degradation. Yet, the lack of consistent significant differences in growth or photosynthetic parameters between the xenic and axenic cultures after oil exposure suggest that, physiologically, the associated bacteria do not modify the phytoplankton oil response. Instead, both oil resistance and phycosphere composition appear to be species-specific characteristics that are not causally linked. This study explores one aspect of what is undoubtedly a complex suite of interactions between phytoplankton and their associated bacteria; future analyses would benefit from studies of genes and metabolites that mediate algal-bacterial exchanges.
Highlights
Phytoplankton are important contributors to carbon dioxide fixation (Field, 1998), biogeochemical cycling, and aquatic food webs
Growth rate of PeriAX showed no significant difference from control, and photosystem II cross section (Sigma PSII) of SkelX significantly increased relative to control
The strong phytoplankton-bacteria interactions highlighted in numerous studies (Cole, 1982; Amin et al, 2012; Durham et al, 2017), and the detection of oil-degrading bacteria in phytoplankton phycospheres (Severin et al, 2016; Thompson et al, 2017) suggests that dispersed and undispersed oil would be more deleterious to axenic phytoplankton
Summary
Phytoplankton are important contributors to carbon dioxide fixation (Field, 1998), biogeochemical cycling, and aquatic food webs. Most studies noted a decline in diatom biomass and an increase in phytoflagellates after exposure to oil in both environmental (Fiori et al, 2016) and laboratory settings (Harrison et al, 1986; Mishamandani et al, 2015), but some observed the opposite (González et al, 2009; Gilde and Pinckney, 2012; Parsons et al, 2015) These equivocal results highlight that the taxon-specific phytoplankton oil response is complex and depends on several factors: oil type and concentration (Dunstan et al, 1975; Huang et al, 2011; Ozhan et al, 2014a), nutrient concentrations (Ozhan and Bargu, 2014), initial phytoplankton community structure (Prouse et al, 1976), zooplankton oil tolerance (González et al, 2009), and phytoplankton-associated bacteria (Mishamandani et al, 2015; Severin et al, 2016; Thompson et al, 2017)
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