Abstract
It is generally recognised that interactions between microalgae and bacteria play an important role in the functioning of marine ecosystems. In this context, increasing attention is paid to the processes that shape microalga-associated microbiomes. In recent years, conflicting evidence has been reported with respect to the relative importance of selective vs neutral processes in the assembly process. Whereas some studies report strong selection imposed by the host, others propose a more neutral, lottery-like assembly model according to which the chance of bacteria becoming part of the microbiome is proportional to their abundance in the environment and not driven by the selectional pressure created by the host. In the present study, we investigated to what degree selective vs neutral assembly processes constrain taxonomic, phylogenetic and functional variation within and between microbiomes associated with 69 isolates belonging to the Cylindrotheca closterium benthic marine diatom complex. The diatom cultures were initiated from non-axenic clonal isolates from different marine environments and geographic locations, and were then reared in a common garden (lab) environment. An important environmental imprint, likely due to in situ lottery dynamics, was apparent in the diatom microbiomes. However, microbiome assembly was also phylogenetically and functionally constrained through selective filtering related to the host microhabitat. Randomised microbiome assembly simulations revealed evidence for phylogenetic overdispersion in the observed microbiomes, reflecting an important role in the assembly process for competition between bacteria on the one hand and predominantly genetically driven differences between the hosts on the other hand. Our study thus shows that even between closely related diatom strains, host selection affects microbiome assembly, superimposing the predominantly stochastically driven recruitment process.
Highlights
Diatoms are an abundant and highly diverse group of eukaryotic microalgae [1]
Benthic C. closterium cells were isolated from sediment samples that were composed of silt to fine sand (median grain size 68.92 ± 96.75(SD) μm) with 2.41 ± 4.53% TOM
Since the resulting diversity in the C. closterium cultures was low, the number of bacteria attached in situ to C. closterium cells is likely to be low as well, which would be in line with the microscopic observations made on the pennate diatom Pseudo-nitzschia by Kaczmarska et al [28]
Summary
Diatoms are an abundant and highly diverse group of eukaryotic microalgae [1]. Close interactions with bacteria are likely one of the reasons behind their evolutionary success [2]. These interactions have major ecological implications as they affect food web structure and mediate biogeochemical cycling in both planktonic and benthic marine systems [3,4,5,6]. Changes in interacting partners are likely to have profound effects on the marine nutrient fluxes due to the highly specific nature of these interactions [10,11,12,13,14]. A good understanding of the processes governing the structure of diatom-bacteria associations is important
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