Kevin Wyman

THE ELEMENTAL COMPOSITION OF SOME MARINE PHYTOPLANKTON1

We analyzed the cellular content of C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Cd, and Mo in 15 marine eukaryotic phytoplankton species in culture representing the major marine phyla. All the organisms were grown under identical culture conditions, in a medium designed to allow rapid growth while minimizing precipitation of iron hydroxide. The cellular concentrations of all metals, phosphorus, and sulfur were determined by high‐resolution inductively coupled plasma mass spectrometry (HR‐ICPMS) and those of carbon and nitrogen by a carbon hydrogen nitrogen analyzer. Accuracy of the HR‐ICPMS method was validated by comparison with data obtained with 55Fe radioactive tracer and by a planktonic reference material. The cellular quotas (normalized to P) of trace metals and major cations in the biomass varied by a factor of about 20 among species (except for Cd, which varied over two orders of magnitude) compared with factors of 5 to 10 for major nutrients. Green algae had generally higher C, N, Fe, Zn, and Cu quotas and lower S, K, Ca, Sr, Mn, Co, and Cd quotas than coccolithophores and diatoms. Co and Cd quotas were also lower in diatoms than in coccolithophores. Although trace element quotas are influenced by a variety of growth conditions, a comparison of our results with published data suggests that the measured compositions reflect chiefly the intrinsic (i.e. genetically encoded) trace element physiology of the individual species. Published field data on the composition of the planktonic biomass fall within the range of laboratory values and are generally close to the approximate extended Redfield formula given by the average stoichiometry of our model species (excluding the hard parts): While clearly this elemental stoichiometry varies between species and, potentially, in response to changes in the chemistry of seawater, it provides a basis for examining how phytoplankton influence the relative distributions of the ensemble of major and trace elements in the ocean.

PHOTOADAPTATION AND THE “PACKAGE” EFFECT IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)

In the marine unicellular chlorophyte, Dunaliella tertiolecta Butcher, the spectrally averaged m vivo absorption cross section, normalized to chlorophyll a (so‐called a* values), vary two‐fold in response to changes in growth irradiance. We used a kinetic approach to examine the specific factors which account for these changes in optical properties as cells photoadapt. Using Triton X‐100 to solubilize membranes, we were able to differentiate between “package” effects and pigmentation effects. Our analyses suggest that 43–49% of the variability in a* is due to changes in pigmentation, whereas 51–57% is due to the “package” effect. Further analyses revealed that changes in cell sue did not significantly affect packaging, while thylakoid stacking and the transparency of thylakoid membranes were important factors. Our results suggest that thylakoid membrane protein/lipid ratios change during photoadaptation, and these changes influence the effective rate of light harvesting per unit chlorophyll a.

Relationship of steady-state photosynthesis to fluorescence in eucaryotic algae

Abstract The change in fluorescence yield (Δφ) was measured in five species of eucaryotic algae using a ‘pump and probe’ flash technique. The half-time for the oxidation of Q, which was measured by varying the delay time between actinic (pump) and measuring (probe) flashes, averaged 400 μs and was unaffected by background irradiance between 10 12 and 10 16 quanta · cm −1 · s −1 . The absorption cross-section of PS II traps was measured by varying the intensity of the actinic flash. These cross-sections did not change (± 10%) with background irradiance. The cross-section data can be fitted to a cumulative one-hit Poisson distribution. In the steady state, the relationship between δφ and photosynthetic oxygen evolution was highly nonlinear and cannot be explained by energy transfer between PS II units. Using the criteria of Δφ, about 15% of the PS II traps remain open at light saturation of O 2 evolution. Conversely, at low irradiance levels, capable of stimulating much less than 1% of the maximum steady-state photosynthetic rate, the fluorescence yield decreases by as much as 25% from the dark-adapted value. Furthermore, the data suggest that a long-lived quencher of fluorescence is formed at moderate to continuous irradiance levels, at least 10 16 quanta · cm −2 · s −1 . Our results suggest that cyclic electron flow around PS II occurs under normal physiological conditions and is especially pronounced in chlorophytes.

Kinetics of light-intensity adaptation in a marine planktonic diatom

The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.

Effects of Iron Limitation on Photosystem II Composition and Light Utilization in Dunaliella tertiolecta

The effects of iron limitation on photosystem II (PSII) composition and photochemical energy conversion efficiency were studied in the unicellular chlorophyte alga Dunaliella tertiolecta. The quantum yield of photochemistry in PSII, inferred from changes in variable fluorescence normalized to the maximum fluorescence yield, was markedly lower in iron-limited cells and increased 3-fold within 20 h following the addition of iron. The decrease in the quantum yield of photochemistry was correlated with increased fluorescence emission from the antenna. In iron-limited cells, flash intensity saturation profiles of variable fluorescence closely followed a cumulative one-hit Poisson model, suggesting that PSII reaction centers are energetically isolated, whereas in iron-replete cells, the slope of the profile was steeper and the calculated probability of energy transfer between reaction centers increased to >0.6. Immunoassays revealed that in iron-limited cells the reaction center proteins, D1, CP43, and CP47, were markedly reduced relative to the peripheral light-harvesting Chl-protein complex of PSII, whereas the [alpha] subunit of cytochrome b559 was about 10-fold higher. Spectroscopic analysis established that the cytochrome b559 peptide did not contain an associated functional heme. We conclude that the photochemical conversion of absorbed excitation energy in iron-limited cells is limited by the number of photochemical traps per unit antenna.

Inhibition of PS II photochemistry by PAR and UV radiation in natural phytoplankton communities.

The effects of PAR and UV radiation on PS II photochemistry were examined in natural phytoplankton communities from coastal waters off Rhode Island (USA) and the subtropical Pacific. The photochemical energy conversion efficiency, the functional absorption cross section and the kinetics of electron transfer on the acceptor side of PS II were derived from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phytoplankton communities displayed marked decreases in PS II photochemical energy conversion efficiency that were correlated with increased PAR. In the coastal waters, the changes in photochemical energy conversion efficiency were not statistically different for samples treated with supplementary UV-B radiation or screened to exclude ambient UV-B. Moreover, no significant light-dependent changes in the functional absorption cross section of PS II were observed. The rate of electron transfer between QA- and QB was, however, slightly reduced in photodamaged cells, indicative of damage on the acceptor side. In the subtropical Pacific, the decrease in photochemical energy conversion efficiency was significantly greater for samples exposed to natural levels of UV-A and/or UV-B compared with those exposed to PAR alone. The cells displayed large diurnal changes in the functional absorption cross section of PS II, indicative of non-photochemical quenching in the antenna. The changes in the functional absorption cross section were highly correlated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited reaction centers rapidly recover (within an hour or two) to their preillumination values. Thus, while we found definitive evidence for photoinhibition of PS II photochemistry in both coastal and open ocean phytoplankton communities, we did not find any effect of UV-B on the former, but a clear effect on the latter. The results of this study indicate that the effects of UV-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body and the mixing rate, as on the wavelength and energy distribution of the radiation and the absorption cross sections of the biophysical targets.

The divinyl-chlorophyll a/b-protein complexes of two strains of the oxyphototrophic marine prokaryote Prochlorococcus – characterization and response to changes in growth irradiance

Apoproteins of the antenna complexes of Prochlorococcus marinus clone SS120 (= CCMP 1375) and Prochlorococcus sp. clone MED4 (= CCMP 1378) cross-reacted with an antibody against the 30 kDa CP 5 complex of Prochlorothrix hollandica antenna. For the MED4 strain, which has a high divinyl-chlorophyll a to divinyl-chlorophyll b (DV-Chl a/b) ratio ranging from 11.4 to 15.0 (w/w), the major antenna proteins had an apparent molecular mass of 32.5 kDa. In contrast for the SS120 strain, which has a low DV-Chl a/b ratio ranging from 1.1 to 2.2, antenna apoproteins were observed in the range 34–38 kDa. For both strains, these apoproteins decreased at high growth irradiance but more markedly in the latter. Partially purified antenna fractions had a DV-Chl a/b ratio ca. 7-fold lower for SS120 than for MED4 at 30 μmol photons m-2 s-1. For both strains, the 77 K fluorescence emission spectra of whole thylakoids displayed a major peak at 685 nm and a broad but very low shoulder above 700 nm. Energetic coupling of the antenna to both PS II and PSI reaction centers was demonstrated for SS120 by the strong contribution of DV-Chl b in both the 77 K excitation fluorescence spectra and the oxidized minus reduced absorption difference spectra of P700. The PS I to PS II ratio of Prochlorococcus SS120 was determined as being 0.7 ± 0.1 at low light.

Light absorption and utilization among hermatypic corals: a study in Jamaica, West Indies

AbstractThe chlorophyll specific absorption coefficient (

Quantum requirements for growth and fatty acid biosynthesis in the marine diatom Phaeodactylum tricornutum (Bacillariophyceae) in nitrogen replete and limited conditions.

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