We present a methodology, called fast repetition rate (FRR) fluorescence, that measures the functional absorption cross-section (sigmaPS II) of Photosystem II (PS II), energy transfer between PS II units (p), photochemical and nonphotochemical quenching of chlorophyll fluorescence, and the kinetics of electron transfer on the acceptor side of PS II. The FRR fluorescence technique applies a sequence of subsaturating excitation pulses ('flashlets') at microsecond intervals to induce fluorescence transients. This approach is extremely flexible and allows the generation of both single-turnover (ST) and multiple-turnover (MT) flashes. Using a combination of ST and MT flashes, we investigated the effect of excitation protocols on the measured fluorescence parameters. The maximum fluorescence yield induced by an ST flash applied shortly (10 &mgr;s to 5 ms) following an MT flash increased to a level comparable to that of an MT flash, while the functional absorption cross-section decreased by about 40%. We interpret this phenomenon as evidence that an MT flash induces an increase in the fluorescence-rate constant, concomitant with a decrease in the photosynthetic-rate constant in PS II reaction centers. The simultaneous measurements of sigmaPS II, p, and the kinetics of Q-A reoxidation, which can be derived only from a combination of ST and MT flash fluorescence transients, permits robust characterization of the processes of photosynthetic energy-conversion.

The increases in atmospheric pCO 2 over the last century are accompanied by higher concentrations of CO 2 (aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)-fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO 2 (aq). We examined the influence of pCO 2 [preindustrial $ 250 ppmv (low), ambient $ 400, future $ 900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom-forming cyanobacterium contributing substantial inputs of 'new N' to the oligotrophic subtropical and tropical oceans. High pCO 2 enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO 2 cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO 2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO 2 . Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO 2 to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.

The photosynthesis‐irradiance (PE) relationship links indices of phytoplankton biomass (e.g. chl) to rates of primary production. The PE curve can be characterized by two variables: the light‐limited slope (αb) and the light‐saturated rate (Pbmax) of photosynthesis. Variability in PE curves can be separated into two categories: that associated with changes in the light saturation index, Ek (=Pbmax/αb) and that associated with parallel changes in αband Pbmax (i.e. no change in Ek). The former group we refer to as “Ek‐dependent” variability, and it results predominantly from photoacclimation (i.e. physiological adjustments in response to changing light). The latter group we refer to as “Ek‐independent” variability, and its physiological basis is unknown. Here, we provide the first review of the sporadic field and laboratory reports of Ek‐independent variability, and then from a stepwise analysis of potential mechanisms we propose that this important yet largely neglected phenomenon results from growth rate–dependent variability in the metabolic processing of photosynthetically generated reductants (and generally not from changes in the oxygen‐evolving PSII complexes). Specifically, we suggest that as growth rates decrease (e.g. due to nutrient stress), reductants are increasingly used for simple ATP generation through a fast (<1s) respiratory pathway that skips the carbon reduction cycle altogether and is undetected by standard PE methodologies. The proposed mechanism is consistent with the field and laboratory data and involves a simple new “twist” on established metabolic pathways. Our conclusions emphasize that simple reductants, not reduced carbon compounds, are the central currency of photoautotrophs.

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum (IMS101) showed that increasing CO 2 partial pressure (pCO 2 ) enhances N 2 fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO 2 , its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses of Trichodesmium IMS101 grown under a matrix of low and high levels of pCO 2 (150 and 900 matm) and irradiance (50 and 200 mmol photons m 22 s 21 ). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO 2 and light levels in the cultures. The pCO 2 -dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO 2 stimulated rates of N 2 fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO 2 . HCO 3 2 was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO 2 uptake was enhanced under high pCO 2 . A comparison between carbon fluxes in vivo and those derived from 13 C fractionation indicates high internal carbon cycling, especially in the low-pCO 2 treatment under high light. Light-dependent oxygen uptake was only detected under low pCO 2 combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO 2 effect on N 2 fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO 2 . These findings are supported by a complementary study looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.

Marine phytoplankton account for about 50% of all global net primary productivity (NPP). Active fluorometry, mainly Fast Repetition Rate fluorometry (FRRf), has been advocated as means of providing high resolution estimates of NPP. However, not measuring CO2-fixation directly, FRRf instead provides photosynthetic quantum efficiency estimates from which electron transfer rates (ETR) and ultimately CO2-fixation rates can be derived. Consequently, conversions of ETRs to CO2-fixation requires knowledge of the electron requirement for carbon fixation (Φe,C, ETR/CO2 uptake rate) and its dependence on environmental gradients. Such knowledge is critical for large scale implementation of active fluorescence to better characterise CO2-uptake. Here we examine the variability of experimentally determined Φe,C values in relation to key environmental variables with the aim of developing new working algorithms for the calculation of Φe,C from environmental variables. Coincident FRRf and 14C-uptake and environmental data from 14 studies covering 12 marine regions were analysed via a meta-analytical, non-parametric, multivariate approach. Combining all studies, Φe,C varied between 1.15 and 54.2 mol e− (mol C)−1 with a mean of 10.9±6.91 mol e− mol C)−1. Although variability of Φe,C was related to environmental gradients at global scales, region-specific analyses provided far improved predictive capability. However, use of regional Φ e,C algorithms requires objective means of defining regions of interest, which remains challenging. Considering individual studies and specific small-scale regions, temperature, nutrient and light availability were correlated with Φ e,C albeit to varying degrees and depending on the study/region and the composition of the extant phytoplankton community. At the level of large biogeographic regions and distinct water masses, Φ e,C was related to nutrient availability, chlorophyll, as well as temperature and/or salinity in most regions, while light availability was also important in Baltic Sea and shelf waters. The novel Φ e,C algorithms provide a major step forward for widespread fluorometry-based NPP estimates and highlight the need for further studying the natural variability of Φe,C to verify and develop algorithms with improved accuracy.