## 4 Discussion  
Considering the role of dissolved iron in modulating the primary productivity of the Antarctic waters, the main objective of this work was to describe the responses and relationships between a new species of _Prorocentrum_ sp. and _Phaeocystis antarctica_ under iron and vitamin B\({}_{12}\) limitation, and the consequent ecological implications for Antarctic waters. The redistribution of dissolved iron in the Ross Sea is one of the most debated concepts in Antarctic ecology, and the ability of the phytoplankton community to respond rapidly to environmental changes renders it crucial in the monitoring of marine environment and the transfer of carbon to higher trophic levels. The spatial and temporal differences among diatoms and haptophyte blooms in the Ross Sea have been correlated to the differential ability of species to grow under varying environmental conditions, such as light availability or iron and vitamin B\({}_{12}\) co-limitation, which could potentially shape the phytoplankton community structure [9; 17; 60; 61; 62]. The dominance of _P. antarctica_ in early spring, in the presence of a deep mixed layer and nonlimiting iron concentrations, has been attributed to its ability to photosynthesize efficiently under low irradiance by increasing the Chl-a content and iron uptake. Furthermore, [18] demonstrated that the iron threshold of _P. antarctica_ was 100 times lower than that of the diatoms (e.g., _Chaotecros_ sp. ), and that high light and iron availability could trigger colony formation. In spite of the numerous studies on the processes involved in colony formation, the trophic fate of mucilaginous colonies remains poorly known [14; 18; 63; 64; 65]. In view of the significant ecological implications of these, the isolation and identification of a new _Prorocentrum_ sp. from colonies of _P. antarctica_ prompted us to investigate the relationship between the two species under iron and vitamin B\({}_{12}\) limitation. We further compared the responses of the two species with different levels of irradiance without nutrient limitation. Laboratory studies conducted under controlled conditions have demonstrated the effect of light, iron, and iron and vitamin B\({}_{12}\) co-limitation on _P. antarctica_, including mechanisms involved in colony formation, in cells preadapted to different growing conditions for weeks [9; 18; 61]. In this study, isolates were initially grown under nutrient availability, and then exposed to limitation conditions at the maximum growth rate. Overall, the results illustrated the differential photosynthesizing ability of the two species under high levels of irradiance (Figure 8). _Prorocentrum_ sp. was unable to photosynthesize efficiently under light exceeding 60 mmol photons m\({}^{-2}\) s\({}^{-1}\), as indicated by Fv/Fm \(\sim\) 0.01 and slight changes in \(\sigma\)PSII, with the Dt/(Dd + Dt) \(\times\) 100 value  
Figure 8: Fv/Fm plotted against relative functional absorption cross-section of PSII (\(\sigma\)PSII) for _Prorocentrum_ sp. (symbol X) and _Phaeocystis antarctica_ (symbol \(\bullet\)) with color indicating different levels of irradiance. Red (HL), light green (L1), light blue (L2), and blue (L3). The figure includes all data points with trend lines. 2.5 times higher in HL than in the L3 treatment. In contrast, _P. antarctica_ was able to regulate the functional optical cross-section of \(\sigma\)PSII in relation to different irradiance levels, maintaining high Fv/Fm under varying light intensity, with the Dt/(Dd + Dt) \(\times\) 100 value 9.5 times higher in HL than in the L3 treatment. We noted that a Viola-like pigment increased at high light intensity in _Prorocentrum_ sp., and it is known that violaxanthin is involved in the xanthophyll cycle under prolonged high light stress [66]. _P. antarctica_ and _Prorocentrum_ sp. responded differently to iron and vitamin B\({}_{12}\) limitation. _Prorocentrum_ sp. exhibited a lower increase in the Chl-a content in the DFB than in the -B12 and CTRL treatments, as well as a lower Chl-a/cell ratio (Table 1).