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Increased seawater pCO2 has the potential to alter phytoplankton biochemistry, which in turn may negatively affect the nutritional quality of phytoplankton as food for grazers. Our aim was to identify how Antarctic phytoplankton, Pyramimonas gelidicola, Phaeocystis antarctica, and Gymnodinium sp., respond to increased pCO2. Cultures were maintained in a continuous culture setup to ensure stable CO2 concentrations. Cells were subjected to a range of pCO2 from ambient to 993 µatm. We measured phytoplankton response in terms of cell size, cellular carbohydrate content, and elemental, pigment and fatty acid composition and content. We observed few changes in phytoplankton biochemistry with increasing CO2 concentration which were species-specific and predominantly included differences in the fatty acid composition. The C:N ratio was unaffected by CO2 concentration in the three species, while carbohydrate content decreased in Pyramimonas gelidicola, but increased in Phaeocystis antarctica. We found a significant reduction in the content of nutritionally important polyunsaturated fatty acids in Pyramimonas gelidicola cultures under high CO2 treatment, while cellular levels of the polyunsaturated fatty acid 20:5ω3, EPA, in Gymnodinium sp. increased. These changes in fatty acid profile could affect the nutritional quality of phytoplankton as food for grazers, however, further research is needed to identify the mechanisms for the observed species-specific changes and to improve our ability to extrapolate laboratory-based experiments on individual species to natural communities.

A possible coupling between UV radiation (UVR; 280 to 400 nm) induced stress and the production of dimethylsulfoniopropionate (DMSP), the precursor of the climate-regulating gas dimethylsulfide (DMS), was investigated in the marine prymnesiophyte Emiliania huxleyi. To this end, axenic cultures of E. huxleyi were exposed to a range of UVR doses for 2 consecutive days. During and after these treatments, growth, photosynthetic activity, cell size, DNA damage, sugar accumulation and DMSP concentrations were followed, The vulnerability of E. huxleyi for relatively low UVR doses was demonstrated by the inhibition of growth and the simultaneous occurrence of DNA damage. Also, mean cell size increased and sugars accumulated as a result of the UVR treatments. In contrast, no effect was observed on the optimal quantum yield of Photosystem II (PSII), a measure of the efficiency of photosynthesis. With increasing UVR dose, cellular DMSP content increased, However, the intrdcellular DMSP concentrations remained constant at the level typical for the applied temperature and salinity conditions, due to accompanying increase in cell size. The increased cellular DMSP content did not compensate, therefore, for the decreased growth rates, resulting in an overall decrease in the total amount of DMSP produced in the cultures. The UVR effects as induced in this study are assumed to be severe as compared with natural solar conditions, especially because high in situ UVAR (315 to 400 nm) may ameliorate UVBR damage by activation of photorepair. Yet the presented results imply that when (increased) UV(B)R causes growth rate reduction of E. huxleyi in situ, DMSP fluxes are likely to be reduced too.

Natural marine phytoplankton assemblages from Bahia Bustamante (Chubut, Argentina, 45 degreesS, 66.5 degreesW), mainly consisting of cells in the picoplankton size range (0.2-2 mum), were exposed to various UVBR (280-315 nm) and UVAR (315-400 mn) regimes in order to follow wavelength-dependent patterns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneously, UVR induced photosynthetic inhibition was studied in radiocarbon incorporation experiments. Biological weighting functions (BWFs) for photoinhibition and for CPD induction, the latter measured in bare calf thymus DNA, differed in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In contrast, BWFs for inhibition of photosynthesis and CPD accumulation were fairly similar in the UVBR region, especially above 300 um. Incubation of phytoplankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD.MB-1 at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of U-VBR or UVR did not cause significant CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. Fligh CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had been killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblages from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure. (C) 2001 Elsevier Science BY All rights reserved.

Experiments were conducted during October 1998 in Lake Titicaca, Bolivia (16 degrees S, 68 degrees W, 3810 m a.s.l), to determine the effects of solar ultraviolet radiation (UVR) on phytoplankton photosynthetic rates and DNA damage. Water samples were taken daily and incubated ir? situ or in simulated in situ conditions using sharp cut-off filters to eliminate various portions of the UVR spectrum. The total inhibition of photosynthesis due to WR in surface waters was 85 %; the greatest part of this inhibition (65 %) was due to WAR (315-400 nm), the rest (20 %) being due to UVBR (280-315 nm). The inhibition of photosynthesis decreased with depth so that there were no significant differences among treatments at 1.3 optical depths (K-PAR). The loss of carbon assimilation in the integrated production over the euphotic zone (4.6 optical depths) was 17.4 %, with 14% due to UVAR and an additional 3.4% due to UVBR. Lake Titicaca phytoplankton had a threshold for inhibition of photosynthesis at about 0.3 W m(-2) for UVBR and 5 W m(-2) for UVAR, below which no inhibition was detected. Above this threshold, photosynthetic inhibition increased steadily, with UVAR having the greatest effect. Analysis of biological weighting functions (BWFs) indicated that phytoplankton from Lake Titicaca was less sensitive to UVR than phytoplankton from other regions. DNA damage (evaluated through the formation of cyclobutane pyrimidine dimers, CPDs) was observed for a simulated worst-case situation (i.e, samples incubated in full sunlight) and significant CPD accumulation was found as a result of UVBR exposure, but not with UVAR. However, absolute levels of damage were relatively low when compared with results obtained at other locations, also suggesting the low sensitivity of Lake Titicaca phytoplankton to UVR. It seems that UVBR stress in these organisms acts via at least two cellular targets: the photosynthetic apparatus and nuclear DNA. Our results suggest that an eventual enhancement of UVBR, due to ozone depletion, would have little impact on the phytoplankton of Lake Titicaca.

In situ experiments were conducted at various depths in the water column to determine the impact of solar UV radiation (280 to 400 nm) upon photosynthesis and DNA of natural phytoplankton assemblages from mid-latitudes of Patagonia (Bahia Bustamante, Chubut, Argentina; 45 degreesS, 66.5 degreesW). The effects of UV radiation were significant at the surface; however, the impact decreased rapidly with depth: at 3 m there was no measurable DNA damage accumulation, whereas at 6 m photosynthetic inhibition was almost zero. UV-A radiation (315 to 400 nm) was mostly responsible for photosynthetic inhibition, while UV-B radiation (280 to 315 nm) had a lesser effect on this process. However, UV-B radiation was very effective in damaging the DNA through the formation of cyclobutane pyrimidine dimers (CPDs) in surface waters. The high initial CPD level found in the natural phytoplankton assemblage decreased when samples were incubated at 3 or 6 m, indicating that at these depths repair, dilution or disappearance of damage occurred. Phytoplankton assemblages were dominated by cells less than 2 mum in effective diameter; this cell size category seems to be more resistant to photosynthetic inhibition, but vulnerable to CPD accumulation, as compared with larger eukaryotic phytoplankters (i.e., Phaeodactylum sp.).

Accumulation and removal of UVBR (ultraviolet-B radiation: 280 to 315 nm)-induced DNA damage (cyclobutane pyrimidine dimers; CPDs) were studied in 2 size fractions (0.2 to 0.8 and 0.8 to 10 mum) of natural populations of tropical marine bacterio- and phytoplankton off Curacao, Netherlands Antilles. Plankton was either directly sampled from the surface layer (mixed situation), or incubated in UVR-transparent bags at 2 water depths (simulated non-mixed situation). A DNA dosimeter was used to measure biologically effective UVBR doses as well as attenuation of biologically effective radiation in the surface layer. A significant difference in CPD level between the 2 plankton size fractions could not be measured. In the mixed as well as in the non-mixed situation, DNA damage was accumulated between 09:00 and 14:00 h irrespective of the variability in incident UVBR. Surface layer samplings on consecutive days showed a clear diel pattern of increase and decrease, indicating that, in the field, DNA damage is repaired or diluted during the afternoon or at night. No significant repair of DNA damage could be measured in samples that were incubated during the afternoon at 0.2 or 10 m depth. The UVBR sensitivity of surface-incubated plankton was expressed as the ratio between damage induced in the cells to damage induced in (unshielded and unrepaired) bare DNA. In both plankton size classes this ratio fluctuated around 0.32, In conclusion, the results show that plankton cells present in the clear waters off Curacao undergo UVBR stress despite the fact that they are subjected to vertical mixing. No significant difference in CPD levels between 2 size fractions of plankton could be measured, suggesting that in this plankton population other aspects, e.g. repair capacities or cell morphology, were more important than cell size.

Emiliania huxleyi (Lohm.) Hay et Miller is an important component of the phytoplankton in open ocean waters. The sensitivity of this cosmopolitan alga to natural levels of UVB radiation has never been tested. Since DNA is believed to be a major target of natural UVB radiation (UVBR: 280-315 nm) in living cells, experiments with E. huxleyi were performed using growth rate reduction and DNA damage as indicators of UVBR stress. Specific growth rate, cell volume, pigment content, and CPD (cyclobutane pyrimidine dimer) formation (a measure for DNA damage) were followed during and after prolonged exposure of a series of cultures to a range of UVBR levels. E. huxleyi was found to be very sensitive to UVBR: at a daily weighted UVBR dose of only 400 J.m(-2).d(-1) (BEDDNA300nm), growth was halted. At this UVBR level, both cell volume and contents of the major photosynthetic and photoprotective pig pigments had increased. The UVBR vulnerability of huxleyi cannot be explained by a high potential for cyclobutane thymine dimer formation (the most abundant CPD type) due to a high T content of nuclear DNA: the CG content of this E huxleyi strain is high (68%) compared with other species. The high UVBR sensitivity may be related to the stage of the cell cycle during UVBR exposure, in combination with low repair capacity, It is concluded that E. huxleyi may experience UVBR stress through the formation of cyclobutane pyrimidine dimers, with subsequent low repair capacity and thereby arrest of the cell cycle.

UVBR (ultraviolet-B radiation: 280 to 315 nm)-induced DNA damage, measured as cyclobutane pyrimidine dimers (CPDs), was determined in size fractions of natural populations of bacterio- and phytoplankton collected in marine tropical waters. Mean biologically effective UVBR doses in the wind-mixed layer were calculated from DNA dosimeter data. Phytoplankton species composition in these waters was monitored using flow cytometry and pigment analyses. In terms of (divinyl-)chlorophyll a concentrations, prochlorophytes and cyanobacteria comprised the largest fraction of the phytoplankton, except in a eutrophic bay at Curacao an island located in the southern Caribbean. In terms of cell numbers and amount of DNA, small prochlorophytes and marine bacteria dominated. Small but detectable levels of UVBR-induced DNA damage were found at all locations. In general, more DNA damage was found in the small size fraction (0.2 to 1 mu m) than in the larger size fraction (1 to 10 mu m). The greatest amount of damage was found in the small size fraction collected in the central Atlantic Ocean (20 CPDs/10(6) nucleotides), despite the fact that UVBR doses were much higher at other locations. The calculated mean biologically effective UVBR doses in the wind-mixed layer were 2 to 17 times lower as compared with incident UVBR doses. CPD levels determined in cultures of the cyanobacterium Synechococcus sp, subjected to UVBR doses similar to those in the wind-mixed layer corresponded with CPD levels measured in the 1 to 10 mu m fraction in the field. Our results indicate that UVBR vulnerability is size dependent. Furthermore, the low CPD levels observed in these field communities may be explained by the low mean biologically effective doses received by the cells as a result of wind-induced mixing.

This study focuses on the effects of ultraviolet radiation (UVR) on bacterioplankton. The effect of different parts of the sunlight spectrum on the leucine and thymidine incorporation and on the induction of DNA damage in natural bacterial populations in the coastal Caribbean Sea off Curacao were investigated. DNA photodamage in microorganisms and biodosimeters was quantified by the number of cyclobutane dimers (thymine dimers). Increasing DNA damage during the day was found when incubated in full surface solar radiation. When UVBR was excluded no DNA damage was observed, indicating that thymine dimers were only formed by WE radiation. The amount of thymine dimers in the >0.8 mu m fraction was only one-third of the amount of induced thymine dimers in the

Effects of iron stress on chromatic adaptation were studied in natural phytoplankton communities collected in the Pacific region of the Southern Ocean. Iron enrichment experiments (48 to 72 h) were performed, incubating plankton communities under white, green and blue light respectively, with and without addition of 2 nM Fe. Pigment ratios were affected by iron addition only to a minor extent. The pigment composition as dictated by the light conditions was similar for both the iron-enriched and the unamended bottles. Upon iron addition, phytoplankton auto-fluorescence, as estimated by flow cytometry, decreased markedly, indicating iron stress of the endemic phytoplankton community. It was concluded that iron did not control chromatic adaptation via the pigment composition, but exerted a clear effect on the efficiency of electron transfer.

Sensitivity to photoinhibition from near-surface irradiance was determined in coastal phytoplankton from Potter Cove (King George Island, Antarctica) in relation to hydrographic conditions shaped by summertime meltwater influx. Slow (indicative for PSII damage) and fast (indicative for xanthophyll cycle activity) relaxing non-photochemical chlorophyll fluorescence quenching (NPQ) was measured during recovery from excess photosynthetically active radiation (PAR) and PAR + ultraviolet radiation (UVR) for 3 stations differentially affected by sediment influx. Additionally, assemblages were incubated outside to study xanthophyll cycle activity during PAR and PAR+UVR exposure. For comparison, NPQ and xanthophyll cycle activity were determined for high (125 mu mol photons m(-2) s(-1)) and low (20 mu mol photons m(-2) s(-1)) light acclimated Chaetoceros brevis (Bacillariophyceae) and Phaeocystis antarctica (Haptophyceae) cultures. Pigment analysis (CHEMTAX) revealed diatom dominance (average 74%) at all locations, with haptophytes being of minor importance (average 13%). All stations were stratified by a vertical density gradient, whereas sediment influx was significantly higher at the innermost station. Taxonomic composition, NPQ characteristics, and photoprotective pigment ratios were not different among stations. At the chlorophyll maximum, phytoplankton showed high light acclimation at all locations, and fast relaxing NPQ dominated after excess PAR and PAR+UVR. Samples from below the chlorophyll maximum were enriched in haptophytes, showed lower photoprotective pigment ratios, reduced capacity for NPQ, and increased contribution of slow relaxing NPQ. The photoprotective capacity of high and low light acclimated C. brevis and P. antarctica differed significantly. P. antarctica induced less NPQ and there was a stronger slow relaxing NPQ component than in the diatom, particularly after low light acclimation. Although total NPQ decreased and slow relaxing NPQ increased after UVR in field samples, NPQ and the xanthophyll cycle de-epoxidation state of cultures were not affected by UVR. A higher photoprotective capacity of diatoms over haptophytes may explain their dominance in stratified meltwater-affected coastal Antarctic waters.

Natural marine phytoplankton assemblages from Bahia Bustamante (Chubut, Argentina, 45 degreesS, 66.5 degreesW), mainly consisting of cells in the picoplankton size range (0.2-2 mum), were exposed to various UVBR (280-315 nm) and UVAR (315-400 mn) regimes in order to follow wavelength-dependent patterns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneously, UVR induced photosynthetic inhibition was studied in radiocarbon incorporation experiments. Biological weighting functions (BWFs) for photoinhibition and for CPD induction, the latter measured in bare calf thymus DNA, differed in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In contrast, BWFs for inhibition of photosynthesis and CPD accumulation were fairly similar in the UVBR region, especially above 300 um. Incubation of phytoplankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD.MB-1 at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of U-VBR or UVR did not cause significant CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. Fligh CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had been killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblages from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure. (C) 2001 Elsevier Science BY All rights reserved.

The authors inform about the composition and distribution of phytoplanktonic community between the first 75 m of depth in Bransfield Strait y around the Elephant island, during the ANTAR X expedition in the 1999 Austral Summer (22nd–29th January 1999). The higher cellular concentration (500 cel/mL) was given by the autotrophic nanoplankton, with a high density mainly on the bay stations and down the first 25 m of depth. Among the most representative species we have Leucocryptos marina, Phaeocystis antarctica, the Monadas and the pennate diatoms., Se da a conocer la composición y distribución de la comunidad fitoplanctónica en los primeros 75 m de profundidad en el Estrecho de Bransfield y alrededores de la Isla Elefante, durante la Expedición ANTAR X (22-29 enero de 1999) verano austral de 1999. Las mayores concentraciones celulares (>500 cel/mL) estuvieron dadas por el nanoplancton, con núcleos de altas densidades principalmente en las estaciones marginales y en los primeros 25 m de profundidad. Las especies más representativas fueron Leucocryptos marina, Phaeocystis antarctica, Monadas y las diatomeas Pennatas.

Experiments were conducted during October 1998 in Lake Titicaca, Bolivia (16 degrees S, 68 degrees W, 3810 m a.s.l), to determine the effects of solar ultraviolet radiation (UVR) on phytoplankton photosynthetic rates and DNA damage. Water samples were taken daily and incubated ir? situ or in simulated in situ conditions using sharp cut-off filters to eliminate various portions of the UVR spectrum. The total inhibition of photosynthesis due to WR in surface waters was 85 %; the greatest part of this inhibition (65 %) was due to WAR (315-400 nm), the rest (20 %) being due to UVBR (280-315 nm). The inhibition of photosynthesis decreased with depth so that there were no significant differences among treatments at 1.3 optical depths (K-PAR). The loss of carbon assimilation in the integrated production over the euphotic zone (4.6 optical depths) was 17.4 %, with 14% due to UVAR and an additional 3.4% due to UVBR. Lake Titicaca phytoplankton had a threshold for inhibition of photosynthesis at about 0.3 W m(-2) for UVBR and 5 W m(-2) for UVAR, below which no inhibition was detected. Above this threshold, photosynthetic inhibition increased steadily, with UVAR having the greatest effect. Analysis of biological weighting functions (BWFs) indicated that phytoplankton from Lake Titicaca was less sensitive to UVR than phytoplankton from other regions. DNA damage (evaluated through the formation of cyclobutane pyrimidine dimers, CPDs) was observed for a simulated worst-case situation (i.e, samples incubated in full sunlight) and significant CPD accumulation was found as a result of UVBR exposure, but not with UVAR. However, absolute levels of damage were relatively low when compared with results obtained at other locations, also suggesting the low sensitivity of Lake Titicaca phytoplankton to UVR. It seems that UVBR stress in these organisms acts via at least two cellular targets: the photosynthetic apparatus and nuclear DNA. Our results suggest that an eventual enhancement of UVBR, due to ozone depletion, would have little impact on the phytoplankton of Lake Titicaca.

We investigate biological mechanisms that generate oscillations and chaos in multispecies competition models. For this purpose, we use a competition model concerned with competition for abiotic essential resources. Because phytoplankton and plants consume quite a number of abiotic essential resources, the model is particularly relevant for phytoplankton communities and terrestrial vegetation. We show that the predicted dynamics depend crucially on the relationship between the resource requirements and the resource consumption characteristics of the species. More specifically, the model predicts that competition generates (1) stable coexistence if species consume most of the resources for which they have high requirements, (2) oscillations and chaos if species consume most of the resources for which they have intermediate requirements, and (3) competitive exclusion with a winner that depends on the initial conditions if species consume most of the resources for which they have low requirements. The theoretical predictions are compared with available data on resource utilization patterns of phytoplankton species.

Spectral water transparency in the Northern Weddell Sea was studied during Austral spring. The depth of the 1-% surface irradiance level (''euphotic depth'') varied between 35 and 109 m and was strongly influenced by phytoplankton biomass. Secchi depths were non-linearly related to euphotic depth. In phytoplankton-poor water, the most penetrating spectral region was restricted to a relatively narrow waveband in the blue (approximately 488 nm), but the range was broader, between 488 and 525 nm when phytoplankton were abundant. Water transparency in the red spectral range was always low and only to a small extent affected by phytoplankton. Two independent procedures were used to quantify the impact of phytoplankton on spectral water transparency: (1) Regression analysis of spectral in situ vertical light attenuation coefficients in the sea, against coincident chlorophyll concentrations. This method gave chlorophyll-specific light attenuation coefficients; the y-intercept could be interpreted as a measure of light attenuation by pure water plus non-algal material. (2) Spectra of in vivo light absorption derived by spectroscopy, using phytoplankton enriched to varying degrees onto filters. Thus chlorophyll-specific absorption cross-sections were determined. Estimates obtained by both procedures were in close agreement, By integrating over the spectrum of underwater irradiance, in situ chlorophyll-specific absorption cross sections of phytoplankton suspensions, related to all photosynthetically active radiation, were calculated. Light absorption by phytoplankton for photosynthesis is accomplished mainly in the blue spectral range. Also dissolved and particulate organic matter contributed to the attenuation of blue light. Because in water poor in phytoplankton, underwater irradiance was progressively restricted to blue light, chlorophyll-specific absorption cross-sections of phytoplankton, averaged over the spectrum of photosynthetically active irradiance, increased with water depth. In water with elevated phytoplankton biomass, overall light attenuation was generally enhanced. However, because the spectral composition of underwater light changed relatively little with depth, except immediately below the water surface, light absorption cross-sections of phytoplankton changed little below 10 m depth. Vertical differences in the proportions of underwater light absorbed by the phytoplankton community here were mainly dependent on biomass variations. Because of the comparatively small attenuation of blue light by non-algal matter, the efficiency of light harvesting by phytoplankton at any given concentration of chlorophyll in Antractic waters is greater than in other marine regions. At the highest phytoplankton biomass observed by us, as much as 70% of underwater light was available for phytoplankton photosynthesis. When phytoplankton were scarce

Emiliania huxleyi (Lohm.) Hay et Miller is an important component of the phytoplankton in open ocean waters. The sensitivity of this cosmopolitan alga to natural levels of UVB radiation has never been tested. Since DNA is believed to be a major target of natural UVB radiation (UVBR: 280-315 nm) in living cells, experiments with E. huxleyi were performed using growth rate reduction and DNA damage as indicators of UVBR stress. Specific growth rate, cell volume, pigment content, and CPD (cyclobutane pyrimidine dimer) formation (a measure for DNA damage) were followed during and after prolonged exposure of a series of cultures to a range of UVBR levels. E. huxleyi was found to be very sensitive to UVBR: at a daily weighted UVBR dose of only 400 J.m(-2).d(-1) (BEDDNA300nm), growth was halted. At this UVBR level, both cell volume and contents of the major photosynthetic and photoprotective pig pigments had increased. The UVBR vulnerability of huxleyi cannot be explained by a high potential for cyclobutane thymine dimer formation (the most abundant CPD type) due to a high T content of nuclear DNA: the CG content of this E huxleyi strain is high (68%) compared with other species. The high UVBR sensitivity may be related to the stage of the cell cycle during UVBR exposure, in combination with low repair capacity, It is concluded that E. huxleyi may experience UVBR stress through the formation of cyclobutane pyrimidine dimers, with subsequent low repair capacity and thereby arrest of the cell cycle.