Abstract
Synechococcus are picocyanobacteria with a cosmopolitan distribution. They are capable of surviving in a wide variety of environmental conditions. Synechococcus have been isolated from the Chesapeake Bay during winter months, and they show an impressive tolerance to cold temperatures. Cold-adapted Synechococcus are unique and diverse, as they have complex phylogenetic lineages closely related to subalpine cluster II, Bornholm Sea cluster, CB7 cluster, and some novel lineages which are independent from summer estuarine strains in subcluster 5.2. CBW1002 and CBW1006 are the first complete genomes to represent Bornholm Sea cluster Synechococcus strains. They have some of the largest genomes among the Synechococcus (3.8 Mb) and share many unique and cryptic homologs which could give insight into their ability to tolerate such cold and dynamic conditions in the Chesapeake Bay estuary.