## 1 Introduction  
The Ross Sea is regarded to be the most productive area of the Southern Ocean, playing a key role in the transfer of atmospheric carbon to the ocean's interior and in global climate regulation [1,2]. The high productivity of this region is linked essentially to its phytoplankton community, which is dominated primarily by various diatoms (Bacillusariophyceae) and the coccolithophyte _Phaeocystis antarctica_. These two functional groups exhibit different temporal and spatial patterns, with _P. antarctica_ dominating the early seasonal blooms in the polynysas [2, 3, 4, 5], accounting for more than 60% of the primary productivity in the southern Ross Sea [6], whereas diatoms dominate the coastal waters and the shallow upper mixed layer during the summer. Although the drivers regulating these blooms have been extensively investigated, the spatial and temporal differences between diatoms and _P. antarctica_ present several avenues for further research [7,8]. Studies conducted in the last decade have demonstrated that these blooms were subject to seasonal iron limitation and/or co-limitation with other micronutrients (such as vitamin B\({}_{12}\)) which could potentially influence the phytoplankton community structure [9, 10, 11, 12, 13]. The dominance of _P. antarctica_ in early spring blooms has been attributed to its ability to photosynthesize efficiently under relatively low light levels [14, 15, 16, 17] and to its remarkable tolerance to low iron concentrations as compared with the diatoms [18]. _P. antarctica_ usually occurs in colonies during the blooming phase [19, 20, 21], with cells embedded in a thin mucous matrix forming a balloon-like colony, which can measure several hundred micrometers in diameter. This has been suggested to be a defense mechanism against grazers [22] such as _Euphausia superba_ and, in turn, represents one of the most important prey items for several predators in the Antarctic waters. In the case of other less abundant groups, an increase in dinoflagellates, silicoflagellates, and heterotrophic protists has been reported in the phytoplankton communities later in the growing season [23, 24, 25, 26]. The first detailed research on Southern Ocean dinoflagellates was started by [PERSON] [27] who identified different species in 1976 [28], and the group has been later reported during both spring and summer in the Ross Sea ([[http://www.bco-dmo.org/dataset/2732](http://www.bco-dmo.org/dataset/2732)]([http://www.bco-dmo.org/dataset/2732](http://www.bco-dmo.org/dataset/2732)) and [[http://www.bco-dmo.org/dataset/3360](http://www.bco-dmo.org/dataset/3360)]([http://www.bco-dmo.org/dataset/3360](http://www.bco-dmo.org/dataset/3360))). Some dinoflagellates described in the Ross Sea are capable of kleptoplasty [29] and have been noted within _P. antarctica_ colonies, although the _P. antarctica_ morphotype that serves as the plastid source remains unknown [30,31]. Drivers regulating phytoplankton blooms in the Ross Sea have received significant attention in the last decade, which has led to several studies being conducted and new discoveries being made, including a new species of dinoflagellates belonging to the genus _Protoperidinium_ that was recently described by [PERSON] [31] in the south-central Ross Sea. Because the dominance of different functional groups channels the energy flow through different trophic patterns, thus, differentially influencing the export of carbon aggregates out of the photic zone [32,33], the discovery of new phytoplankton species could reflect the changes occurring in the Antarctic waters, inciting the re-evaluation of the role of minor functional groups within the phytoplankton community of the Ross Sea. In the present study, we isolated a new dinoflagellate species belonging to the genus Prorocentrum from colonial _P. antarctica_ collected from the central Ross Sea, and defined its growth rate and photophysiological responses to varying light intensities, as well as iron and vitamin B\({}_{12}\) limitation, in both monospecific and mixed cultures with _P.