Bacterial Communities of Diatoms Display Strong Conservation Across Strains and Time

Abstract

Interactions between phytoplankton and bacteria play important roles in shaping the microenvironment surrounding these organisms and in turn influence global biogeochemical cycles. This microenvironment, known as the phycosphere, is presumed to shape the bacterial diversity around phytoplankton and thus stimulate a diverse array of interactions between both groups. Although many studies have attempted to characterize bacterial communities that associate and interact with phytoplankton, bias in bacterial cultivation and consistency and persistence of bacterial communities across phytoplankton isolates likely impede the understanding of these microbial associations. Here, we isolate four strains of the diatom Asterionellopsis glacialis and three strains of the diatom Nitzschia longissima and show through metabarcoding of the bacterial 16S rDNA gene that though each species possesses a unique bacterial community, the bacterial composition across strains from the same species are highly conserved at the genus level. Cultivation of all seven strains in the laboratory for longer than 1 year resulted in only small changes to the bacterial composition, suggesting that despite strong pressures from laboratory culturing conditions associations between these diatoms and their bacterial communities are robust. Specific operational taxonomic units (OTUs) belonging to the Roseobacter-clade appear to be conserved across all strains and time, suggesting their importance to diatoms. In addition, we isolate a range of cultivable bacteria from one of these cultures, A. glacialis strain A3, including several strains of Shimia marina and Nautella sp. that appear closely related to OTUs conserved across all strains and times. Coculturing of A3 with some of its cultivable bacteria as well as other diatom-associated bacteria shows a wide range of responses that include enhancing diatom growth. Cumulatively, these findings suggest that phytoplankton possess unique microbiomes that are consistent across strains and temporal scales.

Consistency in microbiomes in cultures of Alexandrium species isolated from brackish and marine waters

“…To investigate the effect of long term culturing on the associated bacterial communities, A. ostenfeldii (strains 13-20) and A. minutum/ tamarense (strains 21-28) isolates were monitored after 6 and 4 months of culturing respectively. Similar to studies of diatoms in culture (Schäfer et al, 2002;Behringer et al, 2018), the bacterial communities of the Alexandrium isolates is this study were shown to be consistent over time (Supporting Information Table S1 and Fig. S8).…”

“…There are several considerations that should be made before drawing conclusions about in situ associations from in vitro conditions (Sala et al, 2005;Garcés et al, 2007;Hattenrath-Lehmann and Gobler, 2017;Behringer et al, 2018). For example, culture conditions and time in culture can have significant impacts as culturing can alter the bacterial community composition within hours (Sapp et al, 2007;Weissbach et al, 2010).…”

“…The diversity, functioning and efficiency of bacteria associating with phytoplankton cells (the microbiome) are thought to be determined by the biochemical composition of phytoplankton and their released organic matter (Azam and Malfatti, 2007;Buchan et al, 2014;Seymour et al, 2017). The composition of the microbiome has been shown to be host specific in association with, for example, diatoms and dinoflagellates (Schäfer et al, 2002;Guannel et al, 2011;Lawson et al, 2017;Behringer et al, 2018).…”

Algae−bacteria interactions that balance the planktonic microbiome

“…Besides short‐term associations, different diatoms establish long‐term specific relationships with selected bacteria, creating a stable community unique for each species across strains. This situation suggests control of the associated microbiome based on chemical signalling (Behringer et al ., ). The involvement of chemical compounds in this process was further demonstrated by documenting community shifts in bacterioplankton after addition of organic matter derived from different phytoplankton species (Tada et al ., ).…”

Identification of Bacterial Genes Expressed During Diatom-Bacteria Interactions Using an in Vivo Expression Technology Approach

“…In this way, different types of interactions were discovered, which were either microbial population driven or dictated by individual bacterial organisms. Recently, Behringer et al (2018) demonstrated that bacterial communities on diatom surfaces displayed strong conservation across diatom strains. Seymour et al (2017) elegantly summarized the main corresponding mechanisms and processes, which show an impressive array of diversity and complexity.…”