Ronald J. W. Visser

The effect of iron limitation on the photophysiology of Phaeocystis antarctica (Prymnesiophyceae) and Fragilariopsis cylindrus (Bacillariophyceae) under dynamic irradiance

The effects of iron limitation on photoacclimation to dynamic irradiance were studied in Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) W. Krieg. in terms of growth rate, photosynthetic parameters, pigment composition, and fluorescence characteristics. Under dynamic light conditions mimicking vertical mixing below the euphotic zone, P. antarctica displayed higher growth rates than F. cylindrus both under iron (Fe)–replete and Fe‐limiting conditions. Both species showed xanthophyll de‐epoxidation that was accompanied by low levels of nonphotochemical quenching (NPQ) during the irradiance maximum of the light cycle. The potential for NPQ at light levels corresponding to full sunlight was substantial in both species and increased under Fe limitation in F. cylindrus. Although the decline in Fv/Fm under Fe limitation was similar in both species, the accompanying decrease in the maximum rate of photosynthesis and growth rate was much stronger in F. cylindrus. Analysis of the electron transport rates through PSII and on to carbon (C) fixation revealed a large potential for photoprotective cyclic electron transport (CET) in F. cylindrus, particularly under Fe limitation. Probably, CET aided the photoprotection in F. cylindrus, but it also reduced photosynthetic efficiency at higher light intensities. P. antarctica, on the other hand, was able to efficiently use electrons flowing through PSII for C fixation at all light levels, particularly under Fe limitation. Thus, Fe limitation enhanced the photophysiological differences between P. antarctica and diatoms, supporting field observations where P. antarctica is found to dominate deeply mixed water columns, whereas diatoms dominate shallower mixed layers.

OXIDATIVE STRESS RESPONSES IN THE MARINE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE) DURING PHOTOACCLIMATION(1).

The enzyme superoxide dismutase (SOD) holds a key position in the microalgal antioxidant network. The present research focused on oxidative stress responses in the Antarctic diatom Chaetoceros brevis F. Schütt during transition to excess (including ultraviolet radiation [UVR]) and limiting irradiance conditions. Over a 4 d period, cellular responses of thiobarbituric acid reactive substances (TBARS, a general oxidative stress indicator), SOD activity, photosynthetic and xanthophyll cycle pigments, PSII efficiency, and growth were determined. In addition, oxidative responses were measured during a daily cycle. Changing irradiance conditions significantly affected growth rates of C. brevis. PSII efficiency decreased significantly during periodic excess irradiance and increased under low irradiance conditions. Transition to excess irradiance increased the ratio of xanthophyll to light‐harvesting pigments, whereas the opposite was observed for cultures transferred to low irradiance. This acclimation process was completed after 2 d in the new irradiance environment. SOD activity increased significantly after the first day regardless of the new irradiance environment but returned to preexposure values on the fourth day. We hypothesize that SOD activity may be temporarily elevated in C. brevis after irradiance shifts, thereby reducing oxidative stress when photoacclimation is in progress.

Excessive irradiance and antioxidant responses of an Antarctic marine diatom exposed to iron limitation and to dynamic irradiance

The synergistic effects of iron limitation and irradiance dynamics on growth, photosynthesis, antioxidant activity and excessive PAR (400-700 nm) and UV (280-400 nm) sensitivity were investigated for the Antarctic marine diatom Chaetoceros brevis. Iron-limited and iron-replete cultures were exposed to identical daily irradiance levels, supplied as dynamic (20-1350 micromol m(-2) s(-1)) and constant (260 micromol m(-2) s(-1)) irradiance. After acclimation, growth, maximal quantum yield of PSII (F(v)/F(m)), pigment composition, and the activities of the antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) were determined. Then, excessive irradiance sensitivity was assessed by monitoring pigment composition, F(v)/F(m) and viability loss during a single excessive PAR and UV treatment. Iron limitation reduced growth rates, F(v)/F(m) dynamics, and cellular pigments pools. Cellular pigment concentrations were higher under dynamic irradiance than under constant irradiance but this difference was less pronounced under iron limitation compared to iron-replete conditions. SOD and APX activities increased during dynamic irradiance under iron limitation, suggesting increased radical formation around PSII. Despite these physiological differences, no effects on growth were observed between constant and dynamic irradiance cultivation in iron-limited and iron-replete cells. The applied culturing conditions did not affect glutathione reductase activity in C. brevis. F(v)/F(m) and xanthophyll de-epoxidation dynamics during excessive irradiance were not different for iron-limited and replete cells and viability loss was not found during excessive irradiance. This study revealed photoacclimation differences between iron-limited and iron-replete C. brevis cultures that did not affect growth rates and excessive irradiance sensitivity after acclimation to constant and dynamic irradiance.

Short-term antioxidative responses of 15 microalgae exposed to excessive irradiance including ultraviolet radiation

Short-term photosensitivity and oxidative stress responses were compared for three groups of marine microalgae: Antarctic microalgae, temperate diatoms and temperate flagellates. In total, 15 low-light-acclimated species were exposed to simulated surface irradiance including ultraviolet radiation (SSI). Photosensitivity was assessed as the rate of recovery of Fv/Fm in the hours following SSI treatment. Before, during and after the SSI treatment, oxidative stress responses were assessed by following xanthophyll content and cycling, and activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase, and glutathione redox status. When acclimated to low irradiance, antioxidant levels were not group specific. Superoxide dismutase activity was positively correlated with cell size, whereas in general, ascorbate peroxidase activity appeared to be lower and glutathione redox status appeared to be higher in the Antarctic than in the temperate species. After SSI exposure, the strong inhibition of PSII was followed by variable rates of recovery, although four species remained photosynthetically inactive. SSI tolerance appeared unrelated to geographic or taxonomic background, or to cell size. PSII recovery was enhanced in species with decreasing superoxide dismutase activity, glutathione redox status and increased xanthophyll cycle activity. We conclude that antioxidant responses are highly species specific and not related to the geographic or taxonomic background. Furthermore, xanthophyll cycling seems more important than antioxidants. Finally, it can be hypothesized that glutathione could function as a stress sensor and response regulator.

Differences in acclimation potential of photosynthesis in seven isolates of the tropical to warm temperate macrophyte Valonia utricularis (Chlorophyta)

A. Eggert, R.J.W. Visser, P.R. Van Hasselt and A.M. Breeman. 2006. Differences in acclimation potential of photosynthesis in seven isolates of the tropical to warm temperate macrophyte Valonia utricularis (Chlorophyta). Phycologia 45: 546–556. DOI: 10.2216/05-03.1 The potential to acclimate photosynthesis to sub- and supra-optimal temperatures was investigated in seven isolates of Valonia utricularis (Roth) C. Agardh, a green macrophyte with a tropical to warm-temperate distribution. Photosynthesis–light response curves were obtained by measuring chlorophyll a fluorescence characteristics of algae grown at optimal (25°C), sub- and supra-optimal temperatures. Suboptimal temperatures were chosen to support 30% of the maximum relative growth rate in each isolate. Thermal acclimation was investigated by comparing short-term and long-term temperature effects on the initial rate of increase of the relative electron transport rate (rETR) and the maximum rETR under light-saturating conditions. Isolates from the northeast Atlantic and the Mediterranean all showed a strong potential to acclimate maximum rETR to suboptimal growth temperatures, i.e. short-term temperature effects were diminished after acclimation. However, photoinhibition, measured as a decrease of the maximal quantum yield (Fv/Fm), was found when plants were grown at 30°C. The isolates reduced light harvesting at 30°C by decreasing total chlorophyll content and by increasing the chlorophyll a/b ratio. Up-regulation of photoprotective processes by the xanthophyll cycle pigments was not observed. In contrast, isolates from the Indo-west Pacific were unable to acclimate photosynthesis to suboptimal growth temperatures and these temperatures were strongly photoinhibiting, even though adjustments on the pigment level were observed. All Indo-west Pacific isolates reached comparable maximum rETR values at 30° and 25°C. Thus, the Atlantic/Mediterranean isolates had a stronger potential to acclimate photosynthetic rates at suboptimal growth temperatures compared to the Indo-west Pacific isolates, which was accompanied by losses at 30°C. The results are discussed in a biogeographical context.

Xanthophyll cycle activity and photosynthesis of Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) during fluctuating solar radiation

van de Poll W.H., Buma A.G.J., Visser R.J.W., Janknegt P.J., Villafañ V.E. and Helbling E.W. 2010. Xanthophyll cycle activity and photosynthesis of Dunaliella tertiolecta (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) during fluctuating solar radiation. Phycologia 49: 249–259. DOI: 10.2216/08-83.1 Short-term ultraviolet (UV) radiation (280–400 nm) effects on xanthophyll cycle activity and photosynthesis were assessed during fluctuating irradiance (60- and 10-min cycles – saturating irradiance to near-zero irradiance) for the marine algae Thalassiosira weissflogii (Bacillariophyceae) and Dunaliella tertiolecta (Chlorophyceae). Laboratory cultures were cycled, as above, up and down the water column for 8 h under solar radiation, during which photosystem II (PSII) quantum yield in the light [(Fm′ − Ft)/Fm′] was monitored over 1-min intervals. In addition, pigment composition, xanthophyll de-epoxidation state and carbon assimilation were assessed during the fluctuating irradiance cycles. Although PSII quantum yield in the light of both species mirrored irradiance, the PSII response to irradiance fluctuations changed over time as PSII quantum yield was downregulated at midday. This coincided with maximal xanthophyll de-epoxidation that developed during the course of the day for both species. In contrast to the de-epoxidation levels, nonphotochemical quenching (NPQ) and PSII quantum yield in the light fluctuated with the irradiance dynamics at noon in both species. Maximal xanthophyll de-epoxidation and NPQ at noon was lower under photosynthetically active radiation (PAR) + UV than under PAR exposure for T. weissflogii during the 10-min cycle, whereas this was not found for the 60-min cycle and in D. tertiolecta. Synthesis of xanthophyll cycle pigments occurred in both species, and was faster for D. tertiolecta during PAR + UV than during PAR exposure. Carbon incorporation and on most occasions PSII quantum yield in the light were lower during UV exposure for both species, regardless of xanthophyll de-epoxidation state. UV effects on carbon assimilation were higher during 10-min than during 60-min irradiance fluctuation cycles. However, the 10-min irradiance fluctuation cycle appeared to enhance overall carbon assimilation in D. tertiolecta but depressed productivity of T. weissflogii, as compared with the 60-min cycles.

Wavelength-dependent xanthophyll cycle activity in marine microalgae exposed to natural ultraviolet radiation

The wavelength dependency of xanthophyll cycling in two marine microalgae (Thalassiosira weissflogii and Dunaliella tertiolecta) was studied by establishing biological weighting functions (BWFs) during exposure to natural ultraviolet radiation. High-(HL) and low-(LL) light-acclimated cultures of both species were exposed outdoors for up to 60 min under a series of UVR (280–400 nm) cut-off filters, after which the de-epoxidation state of xanthophyll cycle pigments, radiocarbon assimilation and photochemical quantum yield were measured. Exposures were repeated 4–8 times during the daily cycle to create exposure–response curves for each wavelength condition. UVR affected the three target processes significantly in both species and biological weights increased with decreasing wavelength, particularly in the UVBR region (280–315 nm). Minor wavelength dependency was observed between 315 and 360 nm. After BWF normalization to 300 nm, the LL cultures showed highly similar responses when comparing the three target processes, while the BWFs for the HL cultures differed significantly. The observed enhanced xanthophyll cycling activity in the UVR region implied that xanthophylls had an active role in diminishing UVR stress. However, this enhancement seems to be an indirect effect of damage within the dark reactions of photosynthesis. Hence, another vital target process further downstream in the photosynthetic process, possibly involved in the dark reactions, seems to be responsible for the high similarity in BWFs.

The pigment composition of Phaeocystis antarctica (Haptophyceae) under various conditions of light, temperature, salinity, and iron

The pigment composition of Phaeocystis antarctica was monitored under various conditions of light, temperature, salinity, and iron. 19′‐Hexanoyloxyfucoxanthin (Hex‐fuco) always constituted the major light‐harvesting pigment, with remarkably stable ratios of Hex‐fuco‐to‐chl a under the various environmental conditions. Increased pigment‐to‐chl a ratios at low irradiance confirmed the light‐harvesting function of Fucoxanthin (Fuco), 19′‐Hexanoyloxy‐4‐ketofucoxanthin (Hex‐kfuco), 19′‐butanoyloxyfucoxanthin (But‐fuco), and chl c2 and c3. Increased pigment‐to‐chl a ratios at high irradiance, low iron concentrations, and to a lesser extent at high salinity confirmed the photoprotective function of diadinoxanthin, diatoxanthin, and ß,ß‐carotene. Pigment ratios were not always according to expectations. The consistent increase in But‐fuco/chl at high temperature, high salinity, and low iron suggests a role in photoprotection rather than in light harvesting. Low Hex‐kfuco/chl ratios at high salinity were consistent with a role as light harvester, but the high ratios at high temperature were not, leaving the function of Hex‐kfuco enigmatic. Dedicated experiments were performed to test whether or not the light‐harvesting pigment Fuco could be converted into its structural relative Hex‐fuco, and vice versa, in response to exposure to light shifts. Rapid conversions could not be confirmed, but long‐term conversions cannot be excluded. New pigment ratios are proposed for chemotaxonomic applications. The ratios will improve pigment‐based diagnosis of algal species in waters dominated by P. antarctica.

Antioxidative responses of two marine microalgae during acclimation to static and fluctuating natural UV radiation

Photoacclimation properties were investigated in two marine microalgae exposed to four ambient irradiance conditions: static photosynthetically active radiation (PAR: 400–700 nm), static PAR + UVR (280–700 nm), dynamic PAR and dynamic PAR + UVR. High light acclimated cultures of Thalassiosira weissflogii and Dunaliella tertiolecta were exposed outdoors for a maximum of 7 days. Dynamic irradiance was established by computer controlled vertical movement of 2 L bottles in a water filled basin. Immediate (<24 h), short‐term (1–3 days) and long‐term (4–7 days) photoacclimation was followed for antioxidants (superoxide dismutase, ascorbate peroxidase and glutathione cycling), growth and pigment pools. Changes in UVR sensitivity during photoacclimation were monitored by measuring UVR‐induced inhibition of carbon assimilation under standardized UV conditions using an indoor solar simulator. Both species showed immediate antioxidant responses due to their transfer to the outdoor conditions. Furthermore, upon outdoor exposure, carbon assimilation and growth rates were reduced in both species compared with initial conditions; however, these effects were most pronounced in D. tertiolecta. Outdoor UV exposure did not alter antioxidant levels when compared with PAR‐only controls in both species. In contrast, growth was significantly affected in the static UVR cultures, concurrent with significantly enhanced UVR resistance. We conclude that antioxidants play a minor role in the reinforcement of natural UVR resistance in T. weissflogii and D. tertiolecta.

Atlantic Advection Driven Changes in Glacial Meltwater: Effects on Phytoplankton Chlorophyll-a and Taxonomic Composition in Kongsfjorden, Spitsbergen

Phytoplankton biomass and composition was investigated in a high Arctic fjord (Kongsfjorden, 79˚N, 11˚40’E) using year round weekly pigment samples collected from October 2013 to December 2014. In addition, phytoplankton dynamics supplemented with physical and chemical characteristics of the 2014 spring bloom (April –June 2014) were assessed in two locations in Kongsfjorden. The goal was to elucidate effects of Atlantic advection on spatial phytoplankton chlorophyll-a (chl-a) and taxonomic composition. Chl-a declined during the polar night to a minimum of 0.01 mg m-3, followed by a 1000-fold increase until May 28. Atlantic advection prevented sea ice formation and increased springtime melting of marine terminating glaciers. This coincided with spatial and temporal differences in abundances of flagellates (prasinophytes, haptophytes, cryptophytes, and chrysophytes) and diatoms in early spring. More flagellated phytoplankton were observed in the non-stratified central Kongsfjorden, whereas diatoms were more abundant in the stratified inner fjord. Contrasting conditions between locations were reduced when glacial melt water stratification expanded towards the mouth of the fjord, mediating a diatom dominated surface bloom at both locations. We suggest that glacial melt water governs spring bloom spatial timing and composition in the absence of sea ice driven stratification. The spring bloom exhausted surface nutrient concentrations by the end of May. The nutrient limited post bloom period (June-October) was characterized by reduced biomass and pigments of flagellated phytoplankton, consisting of prasinophytes, haptophytes, chrysophytes and to a lesser extent cryptophytes and peridinin-containing dinoflagellates.