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Abstract
In some parts of the Southern Ocean (SO), even though low surface concentrations of iron (Fe) and manganese (Mn) indicate FeMn co-limitation, we still lack an understanding on how Mn and Fe availability influences SO phytoplankton ecophysiology. Therefore, this study investigated the effects of Fe and Mn limitation alone as well as their combination on growth, photophysiology and particulate organic carbon production of the bloom-forming Antarctic diatom Chaetoceros debilis. Our results clearly show that growth, photochemical efficiency and carbon production of C. debilis were co-limited by Fe and Mn as highest values were only reached when both nutrients were provided. Even though Mn-deficient cells had higher photochemical efficiencies than Fe-limited ones, they, however, displayed similar low growth and POC production rates, indicating that Mn limitation alone drastically impeded the cell’s performance. These results demonstrate that similar to low Fe concentrations, low Mn availability inhibits growth and carbon production of C. debilis. As a result from different species-specific trace metal requirements, SO phytoplankton species distribution and productivity may therefore not solely depend on the input of Fe alone, but also critically on Mn acting together as important drivers of SO phytoplankton ecology and biogeochemistry.
Highlights
Large parts of the Southern Ocean (SO) are classified as high-nutrient, low-chlorophyll regions due to the observed low phytoplankton productivity despite high concentrations of macronutrients
Fe-Mn manipulation experiments were performed with the Antarctic diatom Chaetoceros debilis (Polarstern expedition ‘European iron fertilization experiment (EIFEX)’ ANT-XXI/3, In-Patch, 2004, 49 ̊ 36 S, 02 ̊ 05 E, isolated by Philipp Assmy) which was grown in stock cultures with Fe- and Mn-enriched natural Antarctic seawater (F/2R medium [31])
In order to prevent potential contamination, the experiments were conducted under trace metal clean conditions and all sampling and handling of the incubations was conducted under a laminar flow hood (Class 100, Opta, Bensheim, Germany)
Summary
Large parts of the Southern Ocean (SO) are classified as high-nutrient, low-chlorophyll regions due to the observed low phytoplankton productivity despite high concentrations of macronutrients. The low biomass in these areas results from the very low concentrations of the trace metal iron (Fe) [1], which is required for the optimal growth and cellular function of phytoplankton [2,3,4,5,6]. Fe is integrated in photosystem I and II (PSI and II) and is used in redox reactions in many pathways of the cell, including the electron transport chains of photosynthesis and respiration [4,6,7].
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