Physiological responses of the Antarctic diatom species Chaetoceros debilis to different concentrations of iron and manganese.

In many regions of the Southern Ocean, surface concentrations of the trace metals iron and manganese are very low. These elements are required for numerous metabolic pathways in phytoplankton cells and an insufficient supply can lead to reduced growth and alterations in photophysiology. However, little is known about the effects of manganese limitation alone on Antarctic phytoplankton species and its interaction with iron limitation. Therefore, this study focused on the effects of manganese and iron limitation alone as well as their combination on growth, elemental composition and photophysiology of the bloom-forming Antarctic diatom Chaetoceros debilis. Our experimental treatments consisted of four combinations of the two trace metals with two iron-limited treatments and two iron-rich treatments with and without manganese addition, respectively. Limitation by iron alone lowered carbon fixation and photochemical efficiency whereas the limitation with both metals resulted in the highest concentrations of the light-harvesting pigment fucoxanthin. Highest values for growth and carbon fixation were only observed after addition of both trace metals. These findings suggest that C. debilis is co-limited under low iron and manganese concentrations. Gaining more inside on the interplay of various trace metals and their potential co-limitation are valuable to better understand the spatial distribution of phytoplankton key species in the present and the future Southern Ocean.