Beschreibung: The kelp Alaria esculenta represents a key species in high Arctic marine fjord ecosystems. However, the European Arctic is currently experiencing extensive environmental change. Glacial fjord systems, such as Kongsfjorden (Spitsbergen, Svalbard), are subjected to rising temperature, increased freshwater inflow from glaciers and melting snow and a changing ultraviolet (UV) radiation regime related to stratospheric ozone depletion. Thus, in addition to natural seasonality, sessile organisms require acclimation in order to adapt to an environment in transition. We examined the physiological and ultrastructural responses of A. esculenta to the combined exposure to hyposalinity and UV radiation. Photosynthetic quantum yield slightly decreased during a low-salinity treatment of 7 d. Exposure to UV radiation also lowered quantum yield, but specimens previously treated with hyposalinity were significantly less susceptible to UV than nontreated individuals. Concomitant with a loss of chlorophyll during the hyposaline treatment, phlorotannin and antioxidant contents were maintained, and samples treated with low salinities exhibited higher UV-screening characteristics as demonstrated by significantly higher absorption ratios at 300/680 nm. Ultrastructural analyses revealed a treatment-dependent swelling of cell walls and accumulations of phlorotannin-containing vesicles. Our findings point to a strategy by which kelps apply a fast and cost-efficient redistribution of phlorotannins rather than increased synthesis as a general stress response to different environmental drivers in contrast to stress-specific responses. The notion that acclimation to one stressor (low salinity) reflects increased tolerance towards a second stressor (UV radiation) supports the concept of 'cross-acclimation' as established for higher plants but not yet for seaweeds.

Beschreibung: Kelps, perennial brown seaweeds of the order Laminariales, are foundational species in Arctic coastal ecosystems. Presently, their ability to persist under polar night conditions might be significantly affected by increasing winter temperatures. We assessed physiological parameters (photosynthesis, pigment content, respiration, carbohydrate storage) in two species of Arctic kelp, the boreal-temperate Saccharina latissima and the Arctic-endemic Laminaria solidungula, during the polar night 2016/17. Algae were sampled from Kongsfjorden, Svalbard, 78° 55' N, shortly before the onset of the dark period in October, and at the end of the polar night in early February. Analyses were conducted for different tissue sections along the phylloid (Meristem, Centre Region, Distal Region). Data suggest that kelp maintain their photosynthetic competence throughout the entire winter period, as indicated by PE-curve-parameters, and photosynthetic pigment contents. Overall laminarin content was reduced by 96 % in S. latissima, and by 90 % in L. solidungula during winter indicating that this storage glucan fuelled metabolic function during the polar night. Marked differences in laminarin content between the phylloid regions and across species indicated specific adaptive mechanisms between boreal-temperate and Arctic-endemic kelp. We suggest that laminarin turnover represents a sensitive parameter to assess kelp physiology under a changing temperature regime.

Beschreibung: 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.