| Summary: | Prochlorococcus, a unicellular cyanobacterium, is the smallest and the numerically dominant phytoplankton found in the oligotrophic ocean. Despite being one of the most important primary producers in the ocean, there are many questions regarding its lifestyle. Over recent years, aggregates have been regularly observed forming at the bottom of Prochlorococcus cultures, somewhat resembling macroscopic biofilms formed by related bacteria. This study aimed to investigate and quantify this aggregate formation in Prochlorococcus. Nine diverse strains from two clades, low-light IV (LLIV) and high-light II (HLII), were cultured for this study in order to compare the phenotype between and within ecotypes. The aggregate formation was tracked via hourly time-lapse imaging of Prochlorococcus grown under constant light axenically, and xenically with Alteromonas sp. EZ55, and after environmental stressors of dark incubation, heat shock, and EDTA- addition. An algorithm was developed to measure the size of aggregates over time, as well as to evaluate the density of both these aggregates and the surrounding liquid culture in the tube. While LLIVs generally formed more robust aggregates, this wasn’t universal and varied with strains and conditions. MIT9313 and MIT9301 had the most consistent and robust aggregate formation in these experiments. SB and MIT9215, in contrast, were the least disposed to aggregate formation. Notably, all strains formed aggregates under some conditions. The presence of a heterotroph increased the robusticity of the culture and aggregate formations in all strains. The heat shock, similarly, increased aggregation in all strains by triggering the stress response. Moreover, the size of aggregates grew independently from their density. This suggests that the aggregate formation is regulated by particular signals, which affect the interactions between cells when they join and expand an aggregate. In addition, the formation of a “hole” in the aggregate was observed and may be the result of changes in nutrients and light access for cells at the center of the aggregate. This study provides a preliminary analysis of Prochlorococcus aggregation and reveals many questions to be answered in the future regarding Prochlorococcus lifestyles in the open ocean.
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