Fig. 4. Pili avoid sinking in planktonic marine picocyanobacteria.

a Non-shaken cultures of wild-type (WT) and pili mutant (Δpili) Synechococcus sp. WH7803. The pili mutation and phenotype has proven very stable with the mutant strain unable to remain in a planktonic form when not shaken. The appearance of the mutant in the image is consistently achieved after 3–4 days of no shaking. b Example of a tracked suspended cell from a wild-type and pili mutant culture of Synechococcus sp. WH7803. The movement was tracked over 100 s and movement is indicated from dark red (start) to yellow (end). Full data of all tracked cells can be found in Supplementary Data 2 (n = 2614 WT and n = 439 pili mutant cells) and examples in Supplementary Movies 1–4. c Non-shaken Prochlorococcus sp. MIT9313 cultures grown in Pro99 medium (left) and PCR-S11 medium (right). The appearance of cells in the image is consistently achieved after 2–3 days of no shaking. PilA1 (PMT_0263) was only detected in the exoproteome of suspended cultures (left; Supplementary Data 4). d Nutrient step gradient column where wild-type and pili mutant cells of Synechococcus sp. WH7803 were placed at the top. Nutrient deplete (SW) and nutrient enriched layers (ESW) are indicated. Cells were harvested by centrifugation from late-exponential cultures grown under optimal conditions and resuspended in SW before placing them on top of the column. The arrow indicates the trajectory made by the cells, and where they accumulated after four days. Images represent one of three culture replicates. Figure S6 shows the quantification of the sinking and accumulation of cells in the different layers over time.