Seasonality of phytoplankton growth limitation by iron and manganese in subantarctic waters

Abstract

Phytoplankton indirectly influence climate through their role in the ocean biological carbon pump. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Using three shipboard bioassay experiments on three separate voyages, we evaluated the seasonality of iron (Fe) and manganese (Mn) co-limitation of subantarctic phytoplankton growth south of Tasmania, Australia. We observed a strong seasonal variation in a phytoplankton Fe limitation signal, with a summer experiment showing the greatest response to Fe additions. An autumn experiment suggested that other factors co-limited phytoplankton growth, likely low silicic acid concentrations. The phytoplankton responses to Mn additions were subtle and readily masked by the responses to Fe. Using flow cytometry, we observed that Mn may influence the growth of some small phytoplankton taxa in late summer/autumn, when they represent an important part of the phytoplankton community. In addition, Mn induced changes in the bulk photophysiology signal of the spring community. These results suggest that the importance of Mn may vary seasonally, and that its control on phytoplankton growth may be associated with specific taxa.

Response of Southern Ocean Resource Stress in a Changing Climate

“…Fe limitation covers 99.5% of the Southern Ocean (7.73 × 10 7 km 2 ) and is most widespread for diatoms and least extensive for picophytoplankton (98.7% and 87.6% of the total area of limitation, Figures 1b and 1d, respectively) because of the higher modeled Q Fe but similar magnitude Q Fe,req . In the reference period, picophytoplankton have the strongest Mn limitation (Figure 1h), consistent with results of shipboard experiments conducted in the sub‐Antarctic region of the Pacific sector (Latour et al., 2023). In our model, picophytoplankton have different pattern of Fe and Mn limitation (Figures 1d–1h), because of their small size that lessens Fe limitation and opens up the scope for Mn regulation of their increased growth rates.…”

Characterization of a Southern Ocean deep chlorophyll maximum: Response of phytoplankton to light, iron, and manganese enrichment

“…2, 3). This suggests that as growth rates increase (7-to 10-fold compared to the LL control; Table 2) and community composition changes, cellular Mn demands exceed supply for some phytoplankton species within the community, as observed in subantarctic waters (Latour et al 2023b). In addition to PSII, Mn is required in metalloenzymes, including superoxide dismutase, the expression of which is thought to be influenced by light and Fe limitation but is poorly characterized in SO phytoplankton (McCain et al 2021;McCain and Bertrand 2022).…”

“…Photophysiology, macronutrients, and Chl a concentrations were measured onboard. The photochemical efficiency (F v /F m ) and functional absorption cross-section of PSII (σ PSII ) were measured using a Fast Repetition Rate Fluorometry (445 nm excitation; Soliense) on LL-acclimated (≤ 2 μmol photons m À2 s À1 ) communities following Latour et al (2023b). Macronutrients were measured by segmented flow analysis (Rees et al 2018).…”