Novel Coccolithophores from the Lower Deep Photic Zone Off Bermuda

Coccolithophores are a ubiquitous oceanic phytoplankton group. Their unique ability to acquire carbon from different environmental sources and to produce calcareous body scales (coccoliths) make them an integral functional group in the biogeochemical cycling of carbon. Despite this, their vertical distribution, particularly in the lower photic zone (LPZ), species composition, and life cycles, are still poorly understood. Discrete water samples were examined from the LPZ during the 2020 fall overturn event occurring from October to November at hydrostation S of the Bermuda Atlantic Time-Series (BATS). This provided an opportunity to compare our results with a previous BATS survey of coccolithophore population dynamics taken 28 to 26 years earlier (1992-1994). This latter study demonstrated that coccolithophores exhibit seasonal changes in their vertical and horizontal distribution and showed that the coccolithophore population transition of the LPZ occurs primarily at overturn events. Here, we place particular emphasis on those LPZ coccolithophore species adapted to live between the deep chlorophyll maximum and the upper mesopelagic zone because of their potential for mixotrophic activity. We discovered numerous unidentified taxa in this region, which may be either new to science or alternate phases of already described species. Some of the holococcolithophores appear to be associated with the Papposphaeraceae, with similarities to the Turrisphaera-phase. In addition, we provide the first unquestionable evidence of Florisphaera profunda combination coccospheres, featuring both heterococcolith and holococcolith phases in the same sample.