Long-Term Survival of Synechococcus and Heterotrophic Bacteria without External Nutrient Supply after Changes in Their Relationship from Antagonism to Mutualism

Marine phytoplankton and heterotrophic bacteria share a very close but usually changeable relationship. However, the ultimate fate of their unstable relationship on a long-term scale is unclear. Here, the relationship between Synechococcus and heterotrophic bacterial communities underwent a dramatic shift from antagonism to commensalism and eventually to mutualism during long-term cocultivation. The relationship change is attributed to the different (even opposite) effects of diverse bacterial members on Synechococcus and the ratio of beneficial to harmful bacteria. Different bacterial members also interact with each other (e.g., quorum-sensing communication, hostility, or mutual promotion) and drive a dynamic succession in the entire community structure that corresponds exactly to the shift in its relationship with Synechococcus. In the final mutualism stage, a self-sufficient nitrogen cycle, including nitrogen fixation, denitrification, and organic nitrogen degradation, contributed to the healthy survival of Synechococcus for 2 years without an exogenous nutrient supply. This natural selective trait of Synechococcus and heterotrophic bacteria toward mutualism under long-term coexistence provides a novel clue for understanding the ubiquity and competitive advantage of Synechococcus in global oceans.

Pseudoroseicyclus tamaricis sp. nov., isolated from seashore sediment of a Tamarix chinensis forest and emended descriptions of the genus Pseudoroseicyclus Park et al. 2016

An aerobic, Gram-stain-negative, bacterium, designated CLL3-39^T was isolated from seashore sediment collected at a Tamarix chinensis forest in the Marine Ecology Special Reserve of Changyi, Shandong Province, PR China. Cells of strain CLL3-39^T were olive-shaped and no flagellum was observed. Strain CLL3-39^T grew optimally at 33 °C, pH 7.5 and salinity (sea salts) of 40 g l^-1. The main fatty acids in the cell membrane of strain CLL3-39^T comprised anteiso-C_15 : 0 (22.3 %), iso-C_15 : 0 (14.0 %), C_16 : 0 (9.2 %) and summed feature 8 (contains C_18 : 1  ω7c/C_18 : 1  ω6c. 26. 7 %). The main polar lipids of CLL3-39^T were phosphatidylglycerol, diphosphatidylglycerol and phosphatidylcholine. The respiratory quinone was Q10. The G+C content of the genomic DNA of strain CLL3-39^T was 69.6 mol%. The average nucleotide identity between CLL3-39^T and Pseudoroseicyclus aestuarii DB-4^T was 74.7 % and the in silico DNA-DNA hybridization value was 20.1 %. Phylogenetically, strain CLL3-39^T belonged to the genus Pseudoroseicyclus, branching with only one type strain P. aestuarii DB-4^T with 96.3 % 16S rRNA gene similarity, followed by Limimaricola cinnabarinus LL-001^T (95.2 %). Based on its phenotypic, phylogenetic and chemotaxonomic characteristics, we propose strain CLL3-39^T (=MCCC 1A14815^T =KCTC 72665^T) as a representative of a novel species in the genus Pseudoroseicyclus, for which the name Pseudoroseicyclus tamaricis sp. nov. is proposed.

Influence of photobioreactor set-up on the survival of microalgae inoculum

Cultivation of specific microalgae is still difficult in an industrial setup as contamination and balancing the economic cost are not always possible. Understanding the ecology of cultivation of microalgae is therefore necessary to implement stable production. The aim of the study was to understand how different types of photobioreactors and types of culture medium influenced the survival of a specific microalgae inoculum, S. almeriensis. The bacterial and microalgae community were studied using Illumina sequencing. Only the closed configuration was able to maintain the inoculated species while all the other systems developed a different eukaryotic community due to contamination and the higher fitness of contaminants. Photobioreactor configuration was more important than medium in shaping the eukaryotes community, while the bacterial community was influenced strongly by both. Results showed that even a well-adapted strain is maintained only in the closed reactor while the open reactors are colonized by a multispecies consortium.