Streamlined freshwater bacterioplankton Nanopelagicales (acI) and Ca. Fonsibacter (LD12) thrive in functional cohorts

While fastidious microbes can be abundant and ubiquitous in their natural communities, many of them fail to grow axenically in laboratories due to auxotrophies or other dependencies. To overcome auxotrophies these microbes rely on their surrounding cohort. A cohort may consist of kin (ecotypes) or more distantly related organisms and the cooperation may be reciprocal, expensive and non-reciprocal (Black Queen hypothesis), or costless (byproduct benefits). These metabolic partnerships (whether at single species population or community level) enable dominance by and coexistence of these lineages in nature. Here we examine the relevance of these cooperation models to explain the abundance and ubiquity of the dominant fastidious bacterioplankton of a dimictic mesotrophic freshwater lake. Using both culture dependent (minimalist mixed cultures) and culture independent (SSU rRNA gene time series and environmental metagenomics) methods we independently identified the primary cohorts of Actinobacterial genera Planktophila (acI-A) and "Ca. Nanopelagicus" (acI-B), and the Proteobacterial genus "Ca. Fonsibacter" (LD12). While Planktophila and "Ca. Fonsibacter" show no correlation to each other in their natural habitat, they have the potential to be complementary in laboratory settings. We also investigated the bi-functional catalase-peroxidase enzyme KatG (a common good which Planktophila is dependent upon) and its most likely providers in the lake. Further we found that while ecotype and community cooperation combined may explain Planktophila population abundance, the success of "Ca. Nanopelagicus" and "Ca. Fonsibacter" is better explained as a community byproduct. Ecotype differentiation of "Ca. Fonsibacter" as a means of escaping predation was supported but not for overcoming auxotrophies.