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The effects of nitrate, phosphate, and iron starvation and resupply on photosynthetic pigments, selected photosynthetic proteins, and photosystem II (PSII) photochemistry were examined in the diatom Phaeodactylum tricornutum Bohlin (CCMP 1327). Although cell chlorophyll a (chl a) content decreased in nutrient‐starved cells, the ratios of light‐harvesting accessory pigments (chl c and fucoxanthin) to chl a were unaffected by nutrient starvation. The chl a‐specific light absorpition coefficient (a*) and the functional absorption cross‐section of PSII (σ) increased during nutrient starvation, consistent with reduction of intracellular self‐shading (i.e. a reduction of the “package effect”) as cells became chlorotic. The light‐harvesting complex proteins remained a constant proportion of total cell protein during nutrient starvation, indicating that chlorosis mirrored a general reduction in cell protein content. The ratio of the xanthophylls cycle pigments diatoxanthin and diadinoxanthin to chl a increased during nutrient starvation. These pigments are thought to play a photo‐protective role by increasing dissipation of excitation energy in the pigment bed upstream from the reaction centers. Despite the increase in diatoxanthin and diadinoxanthin, the efficiency of PSII photochemistry, as measured by the ration of variable to maximum fluorescence (Fv/Fm) of dark‐adapted cells, declined markedly under nitrate and iron starvation and moderately under phosphate starvation. Parallel to changes in Fv/Fm were decreases in abundance of the reaction center protein D1 consistent with damage of PSII reaction centers in nutrient‐starved cells. The relative abundance of the carboxylating enzyme, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), decreased in response to nitrate and iron starvation but not phosphate starvation. Most marked was the decline in the abundance of the small subunit of RUBISCO in nitrate‐starved cells. The changes in pigment content and fluorescence characteristics were typically reversed within 24 h of resupply of the limiting nutrient.

The response of the marine diatom Phaeodactylum

The role of iron in regulating light harvesting and photochemical energy conversion processes was examined in the marine unicellular chlorophyte Dunaliella tertiolecta and the marine diatom Phaeodactylum tricornutum. In both species, iron limitation led to a reduction in cellular chlorophyll concentrations, but an increase in the in vivo, chlorophyll-specific, optical absorption cross-sections. Moreover, the absorption cross-section of photosystem 11, a measure of the photon target area of the traps, was higher in ironlimited cells and decreased rapidly following iron addition. Ironlimited cells exhibited reduced variable/maximum fluorescence ratios and a reduced fluorescence per unit absorption at all wavelengths between 400 and 575 nm. Following iron addition, variable/ maximum fluorescence ratios increased rapidly, reaching 90% of the maximum within 18 to 25 h. Thus, although more light was absorbed per unit of chlorophyll, iron limitation reduced the transfer efficiency of excitation energy in photosystem 11. The half-time for the oxidation of primary electron acceptor of photosystem 11, calculated from the kinetics of decay of variable maximum fluorescence, increased 2-fold under iron limitation. Quantitative analysis of western blots revealed that cytochrome f and subunit IV (the plastoquinone-docking protein) of the cytochrome b6/f complex were also significantly reduced by lack of iron; recovery from iron limitation was completely inhibited by either cycloheximide or chloramphenicol. The recovery of maximum photosynthetic energy conversion efficiency occurs in three stages: (a) a rapid (3-5 h) increase in electron transfer rates on the acceptor side of photosystem 11 correlated with de novo synthesis of the cytochrome b6/f complex; (b) an increase (10-15 h) in the quantum efficiency correlated with an increase in Dl accumulation; and (c) a slow (>18 h) increase in chlorophyll levels accompanied by an increase in the efficiency of energy transfer from the light-harvesting chlorophyll proteins to the reaction centers.Phytoplankton photosynthesis in the world's oceans accounts for approximately 40% of global carbon fixation, yet identification of the factors limiting primary productivity and phytoplankton biomass has been elusive. Low phytoplankton biomass in the surface waters of the central ocean basins (Fig.

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A new suite of multiple regression models was developed that describes relationships between the area of bottom water hypoxia along the northern Gulf of Mexico and Mississippi-Atchafalaya River nitrate concentration, total phosphorus (TP) concentration, and discharge. Model input variables were derived from two load estimation methods, the adjusted maximum likelihood estimation (AMLE) and the composite (COMP) method, developed by the U.S. Geological Survey. Variability in midsummer hypoxic area was described by models that incorporated May discharge, May nitrate, and February TP concentrations or their spring (discharge and nitrate) and winter (TP) averages. The regression models predicted the observed hypoxic area within +/-30%, yet model residuals showed an increasing trend with time. An additional model variable, Epoch, which allowed post-1993 observations to have a different intercept than earlier observations, suggested that hypoxic area has been 6450 km2 greater per unit discharge and nutrients since 1993. Model forecasts predicted that a dual 45% reduction in nitrate and TP concentration would likely reduce hypoxic area to approximately 5000 km2, the coastal goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. However, the COMP load estimation method, which is more accurate than the AMLE method, resulted in a smaller predicted hypoxia response to any given nutrient reduction than models based on the AMLE method. Monte Carlo simulations predicted that five years after an instantaneous 50% nitrate reduction or dual 45% nitrate and TP reduction it would be possible to resolve a significant reduction in hypoxic area. However, if nutrient reduction targets were achieved gradually (e.g., over 10 years), much more than a decade would be required before a significant downward trend in both nutrient concentrations and hypoxic area could be resolved against the large background of interannual variability. The multiple regression models and statistical approaches applied provide improved capabilities for evaluating dual nutrient management strategies to address Gulf hypoxia and a clearer perspective on the strengths and limitations of approaching the problem using regression models.

The present study describes the acute toxicity of eight commercial oil dispersants, South Louisiana sweet crude oil (LSC), and chemically dispersed LSC. The approach used consistent test methodologies within a single laboratory in assessing the relative acute toxicity of the eight dispersants, including Corexit 9500A, the predominant dispersant applied during the DeepWater Horizon spill in the Gulf of Mexico. Static acute toxicity tests were performed using two Gulf of Mexico estuarine test species, the mysid shrimp (Americamysis bahia) and the inland silversides (Menidia beryllina). Dispersant-only test solutions were prepared with high-energy mixing, whereas water-accommodated fractions of LSC and chemically dispersed LSC were prepared with moderate energy followed by settling and testing of the aqueous phase. The median lethal concentration (LC50) values for the dispersant-only tests were calculated using nominal concentrations, whereas tests conducted with LSC alone and dispersed LSC were based on measured total petroleum hydrocarbon (TPH) concentrations. For all eight dispersants in both test species, the dispersants alone were less toxic (LC50s: 2.9 to >5,600 µl/L) than the dispersant-LSC mixtures (0.4-13 mg TPH/L). Louisiana sweet crude oil alone had generally similar toxicity to A. bahia (LC50: 2.7 mg TPH/L) and M. beryllina (LC50: 3.5 mg TPH/L) as the dispersant-LSC mixtures. The results of the present study indicate that Corexit 9500A had generally similar toxicity to other available dispersants when tested alone but was generally less toxic as a mixture with LSC.