Susceptibility of Two Southern Ocean Phytoplankton Key Species to Iron Limitation and High Light

Although iron (Fe) availability primarily sets the rate of phytoplankton growth and primary and export production in the Southern Ocean, other environmental factors, most significantly light, also affect productivity. As light availability strongly influences phytoplankton species distribution in low Fe-waters, we investigated the combined effects of increasing light (20, 200, and 500 μmol photons m-2 s-1) in conjunction with different Fe (0.4 and 2 nM) availability on the physiology of two ecologically relevant phytoplankton species in the Southern Ocean, Chaetoceros debilis (Bacillariophyceae) and Phaeocystis antarctica (Haptophyceae). Fe-deficient cells of P. antarctica displayed similar high growth rates at all irradiances. In comparison, Fe-deplete C. debilis cells grew much slower under low and medium irradiance and were unable to grow at the highest irradiance. Interestingly, Fe-deficient C. debilis cells were better protected against short-term excessive irradiances than P. antarctica. This tolerance was apparently counteracted by strongly lowered growth and particulate organic carbon production rates of the diatom relative to the prymnesiophyte. Overall, our results show that P. antarctica was the more tolerant species to changes in the availability of Fe and light, providing it a competitive advantage under a high light regime in Fe-deficient waters as projected for the future

Early Exanthema Upon Vemurafenib Plus Cobimetinib Is Associated With a Favorable Treatment Outcome in Metastatic Melanoma: A Retrospective Multicenter DeCOG Study

Background: The combination of BRAF and MEK inhibitors has become standard of care in the treatment of metastatic BRAF V600-mutated melanoma. Clinical factors for an early prediction of tumor response are rare. The present study investigated the association between the development of an early exanthema induced by vemurafenib or vemurafenib plus cobimetinib and therapy outcome. Methods: This multicenter retrospective study included patients with BRAF V600-mutated irresectable AJCC-v8 stage IIIC/D to IV metastatic melanoma who received treatment with vemurafenib (VEM) or vemurafenib plus cobimetinib (COBIVEM). The development of an early exanthema within six weeks after therapy start and its grading according to CTCAEv4.0 criteria was correlated to therapy outcome in terms of best overall response, progression-free (PFS), and overall survival (OS). Results: A total of 422 patients from 16 centers were included (VEM, n=299; COBIVEM, n=123). 20.4% of VEM and 43.1% of COBIVEM patients developed an early exanthema. In the VEM cohort, objective responders (CR/PR) more frequently presented with an early exanthema than non-responders (SD/PD); 59.0% versus 38.7%; p=0.0027. However, median PFS and OS did not differ between VEM patients with or without an early exanthema (PFS, 6.9 versus 6.0 months, p=0.65; OS, 11.0 versus 12.4 months, p=0.69). In the COBIVEM cohort, 66.0% of objective responders had an early exanthema compared to 54.3% of non-responders (p=0.031). Median survival times were significantly longer for patients who developed an early exanthema compared to patients who did not (PFS, 9.7 versus 5.6 months, p=0.013; OS, not reached versus 11.6 months, p=0.0061). COBIVEM patients with a mild early exanthema (CTCAEv4.0 grade 1-2) had a superior survival outcome as compared to COBIVEM patients with a severe (CTCAEv4.0 grade 3-4) or non early exanthema, respectively (p=0.047). This might be caused by the fact that 23.6% of patients with severe exanthema underwent a dose reduction or discontinuation of COBIVEM compared to only 8.9% of patients with mild exanthema. Conclusions: The development of an early exanthema within 6 weeks after treatment start indicates a favorable therapy outcome upon vemurafenib plus cobimetinib. Patients presenting with an early exanthema should therefore be treated with adequate supportive measures to provide that patients can stay on treatment

Immune Checkpoint Blockade for Metastatic Uveal Melanoma: Re-Induction following Resistance or Toxicity

Re-induction with immune checkpoint blockade (ICB) needs to be considered in many patients with uveal melanoma (UM) due to limited systemic treatment options. Here, we provide hitherto the first analysis of ICB re-induction in UM. A total of 177 patients with metastatic UM treated with ICB were included from German skin cancer centers and the German national skin cancer registry (ADOReg). To investigate the impact of ICB re-induction, two cohorts were compared: patients who received at least one ICB re-induction (cohort A, n = 52) versus those who received only one treatment line of ICB (cohort B, n = 125). In cohort A, a transient benefit of overall survival (OS) was observed at 6 and 12 months after the treatment start of ICB. There was no significant difference in OS between both groups (p = 0.1) with a median OS of 16.2 months (cohort A, 95% CI: 11.1–23.8) versus 9.4 months (cohort B, 95% CI: 6.1–14.9). Patients receiving re-induction of ICB (cohort A) had similar response rates compared to those receiving ICB once. Re-induction of ICB may yield a clinical benefit for a small subgroup of patients even after resistance or development of toxicities

Seawater carbonate chemistry and photo-physiological fitness, growth and carbon production of the Antarctic cryptophyte Geminigera cryophila

Ecophysiological studies on Antarctic cryptophytes to assess whether climatic changes such as ocean acidification and enhanced stratification affect their growth in Antarctic coastal waters in the future are lacking so far. This is the first study that investigates the combined effects of the increasing availability of pCO2 (400 and 1000 μatm) and irradiance (20, 200 and 500 μmol photons m-2 s-1) on growth, elemental composition and photo-physiology of the Antarctic cryptophyte Geminigera cryophila. Under ambient pCO2, this species was characterized by a pronounced sensitivity to increasing irradiance with complete growth inhibition at the highest light intensity. Interestingly, when grown under high pCO2 this negative light effect vanished, and it reached the highest rates of growth and particulate organic carbon production at the highest irradiance compared to the other tested experimental conditions. Our results for G. cryophila reveal beneficial effects of ocean acidification in conjunction with enhanced irradiance on growth and photosynthesis. Hence, cryptophytes such as G. cryophila may be potential winners of climate change, potentially thriving better in more stratified and acidic coastal waters and contributing in higher abundance to future phytoplankton assemblages of coastal Antarctic waters

Seawater carbonate chemistry and particulate organic carbon accumulation of two Antarctic diatom species

Impacts of rising atmospheric CO2 concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii the effects of moderate and high natural solar radiation combined with either ambient or future pCO2 on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO2 concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO2 in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (Fv/Fm) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom‐forming Antarctic coastal diatoms might increase carbon contents under future pCO2 conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO2 sequestration potential of diatom communities in the future coastal Southern Ocean

Impact of light and iron availability on Antarctic phytoplankton ecophysiology

Although iron (Fe) availability sets primarily the rate of phytoplankton growth and primary and export production in the Southern Ocean, other environmental factors, most significantly light, also affect productivity. Due to wind-induced vertical mixing, the total irradiance dose can be reduced, but exposing phytoplankton also to periods of excessive irradiance when residing near the surface. As these dynamic alterations between low and high irradiance in low Fe-waters are important drivers of species distribution, we investigated the effects of light (20, 200 and 500 μmol photons m−2 s−1) in combination with low and high Fe availability (0.4 and 2 nM Fe) on the physiology of the two ecologically relevant species Chaetoceros debilis and Phaeocystis antarctica. Fe-limited cells of P. antarctica displayed similar high growth rates at all irradiances. In comparison, Fe-limited C. debilis cells grew much slower under low and medium irradiance and were unable to grow at the highest irradiance. Compared to C. debilis Fe-limited P. antarctica cells fixed more particulate organic carbon at all irradiances. When exposed to short-term excessive irradiances C. debilis could cope better than P. antarctica under low Fe conditions, but this was at the expense of lowered carbon production. Overall, our results show that P. antarctica was more tolerant to changes in the availability of Fe and light, providing it a competitive advantage under a dynamic light regime in Fe-deficient waters

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high solar radiation.

Impacts of rising atmospheric CO2 concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO2 on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO2 concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO2 in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (Fv/Fm) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom-forming Antarctic coastal diatoms might increase carbon contents under future pCO2 conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO2 sequestration potential of diatom communities in the future coastal Southern Ocean

Photophysiological strategies to iron limitation and high light differ between two Antarctic key phytoplankton species

As iron (Fe) and light availability strongly influence phytoplankton species distribution in low Fe-waters, we investigated the combined effects of increasing light (20, 200 and 500 μmol photons m−2 s−1) in conjunction with different Fe (0.4 and 2 nM Fe) availability on the physiology of two ecologically relevant phytoplankton species in the Southern Ocean, Chaetoceros debilis (Bacillariophyceae) and Phaeocystis antarctica (Haptophyceae). Combining Fast Repetition Rate fluorometry, elemental composition and pigment analyses with cell-based modelling, new insights on the photophysiological strategies of the two tested species were gained. Fe-deficient cells of P. antarctica displayed similar high growth rates at all irradiances. In comparison, Fe-deplete C. debilis cells grew much slower under low and medium irradiance and were unable to grow at the highest irradiance. Interestingly, Fe-deficient C. debilis cells were better protected against short-term excessive irradiances than P. antarctica. Next to similar electron transfer rates, Fe-deplete C. debilis cells displayed faster re-oxidation of the primary electron acceptor Qa, indicating operation of a putative plastid plastoquinol terminal oxidase, known to create a proton gradient in the thylakoid lumen. In line with this, high xanthophyll activity was shown by its high cellular diadinoxanthin content along with very high NPQ activities. Such strategy was especially efficient after short-term exposure to high irradiance, as seen by the high potential of Fv/Fm recovery (~70-80%), pointing towards a high tolerance of C. debilis to short-term high light stress. This tolerance was, however, counteracted by strongly lowered growth and particulate organic carbon production rates of the diatom relative to the prymnesiophyte. The prymnesiophyte also possessed high photoprotective capabilities, with strong alternative electron cycling activities (electron cycling around photosystem I, Mehler and/or photorespiration) being more important than xanthophyll cycling. Overall, our results suggest that P. antarctica could outcompete C. debilis in Fe-deficient waters at all light regimes

Ocean acidification and high irradiance stimulate the photo-physiological fitness, growth and carbon production of the Antarctic cryptophyte Geminigera cryophila

Ecophysiological studies on Antarctic cryptophytes to assess whether climatic changes such as ocean acidification and enhanced stratification affect their growth in Antarctic coastal waters in the future are lacking so far. This is the first study that investigates the combined effects of the increasing availability of pCO2 (400 and 1000 μatm) and irradiance (20, 200 and 500 μmol photons m-2 s-1) on growth, elemental composition and photophysiology of the Antarctic cryptophyte Geminigera cryophila. Under ambient pCO2, this species was characterized by a pronounced sensitivity to increasing irradiance with complete growth inhibition at the highest light intensity. Interestingly, when grown under high pCO2 this negative light effect vanished, and it reached the highest rates of growth and particulate organic carbon production at the highest irradiance compared to the other tested experimental conditions. Our results for G. cryophila reveal beneficial effects of ocean acidification in conjunction with enhanced irradiance on growth and photosynthesis. Hence, cryptophytes such as G. cryophila may be potential winners of climate change, potentially thriving better in more stratified and acidic coastal waters and contributing in higher abundance to future phytoplankton assemblages of coastal Antarctic waters

Ocean acidification and changing light availability triggered stress responses in Antarctic diatoms in laboratory and field incubation experiments.

The effects of climate change, including ocean acidification (OA), on future Southern Ocean phytoplankton community’s species composition and consequent impacts on primary production and carbon export are largely unknown. Further, changes in light availability induced by decreased vertical mixing of surface waters can lead to higher light availability enhancing potentially phytoplankton productivity in Antarctic coastal and shelf areas. Therefore, a suite of laboratory and field experiments with the two ecologically relevant Antarctic diatom species as well as a natural phytoplankton community from the Western Antarctic Peninsula were conducted under different pCO2 levels and irradiance regimes. In the two diatom species Fragilariopsis curta and Odontella weisflogii, OA did not stimulate, but inhibited growth and carbon fixation under low and medium light whereas this effect was amended under high growth irradiances. Under different dynamic light regimes, however, OA stimulated carbon fixation in the two diatoms. Yet, reduced photosynthetic efficiencies in both species indicated that this was likely a stress response to OA. In CO2-light incubation experiments with a natural community of the West Antarctic Peninsula, OA led to a decline of overall diatom abundances, including Fragilariopsis and Odontella, triggering thereby the dominance of the prymnesiophyte Phaeocystis antarctica, an inefficient vector for carbon export. In summary, these findings highlight that, under different irradiance regimes, OA is likely to induce a species shift, away from diatoms, within phytoplankton communities and to alter primary productivity of the coastal Southern Ocean with important implications for biogeochemical cycles in the future