## 4 Discussion
In contrast, differences among treatments were less evident for Fv/Fm and \(\sigma\)PSII, with the former exhibiting a slight decrease during the experiment, whereas the latter remained almost constant (Figure 9). _P. antarctica_ presented an opposite trend, with a greater increase in both the Chl-a content and Fv/Fm in the DFB treatment. However, Fv/Fm did not exhibit considerable variation, and \(\sigma\)PSII was observed to be similar among the treatments, remaining almost constant throughout the experiment. The highest Chl-a/cell ratio was observed in the -B12 treatment, with smaller differences among the treatments than those detected for _Proocentrum_ sp. These results suggest that both these species are more sensitive to changes in the dissolved iron availability than to B\({}_{12}\). The increase in Chl-a observed in _P. antarctica_ could reflect the dissolution of colonies (data not shown) under iron limitation, thus \"conserving\" the energy required for colony formation and resulting in an increase in the single cell morphotype. The lower iron threshold and ability of _P. antarctica_ colonies to chelate iron has been well documented, and similar trends of photosynthetic parameters observed in the DFB treatment have suggested that this species was not affected by iron limitation, likely due to mucus storage. It has been estimated that approximately 50%-80% of the _Phaeocystis_ carbon content was present in the extracellular mucilaginous matrix [67; 21; 68; 68], which improved the ability of the colony to sequester micronutrients (e.g., iron) [14; 69] and function as a microbiome and vitamin B\({}_{12}\) source [9]. The Perid/Hex ratio, used as a proxy to determine the dominant phytoplankton in mixed cultures, was lower in the DFB treatment, indicating the ability of colonial _Phaeocystis_ to grow well even under iron limitation. A change was noted in the color of dissociating colonies in the DFB treatment (data not shown). This phenomenon warrants further investigation, considering the poor digestibly of the  
Figure 9: Fv/Fm plotted against relative functional absorption cross-section of PSII (\(\sigma\)PSII) for _Proocentrum_ sp. (symbol X) and _Phaeocystis antarctica_ (symbol \(\bullet\)) with color indicating different treatments. CTRL (black), -B12 (orange), DFB (green). The figure includes all data points with trend lines. mucus and low grazing pressure on colonies, which allow for efficient export of carbon to the bottom of the ocean. The presence of dinoflagellates in the marine environment has been correlated to water column stability or nutrient deficiency [70; 71]. Of the mixotrophic species, food vacuoles or prey ingestion have been observed in Prorocentrales, particularly in _Prorocentrum minimum_[72], and the host species of dinoflagellates within the mucilaginous colonies of _P. antarctica_ have been previously reported [30; 73]. In addition, a novel dinoflagellate, related to the ichthyotoxic genera _Karenia_ and _Karlodinium_, discovered in the Ross Sea by [74] was closely related to the free-living, unicellular _P. antarctica_. Although it is not clear if kleptoplastic dinoflagellates prefer colonial or unicellular stages of _P. antarctica_ as the plastid source [30; 31], the important ecological role of mixotrophy in providing access to limiting nutrients has long been recognized [75; 76; 77], as it has significantly impacted prey populations in marine microbial food webs. The Ross Sea is considered to be the most productive region in the Southern Ocean, playing a key role in the transfer of atmospheric carbon to the ocean's interior and in global time regulation. The phytoplankton bloom dynamics in the region have been well documented, with communities dominated mainly by diatoms and the _P. antarctica_, which can form almost monospecific blooms with distinctive biogeochemical imprints due to their differential metabolism and trophic fate.