Evelyn Lawrenz

Spectral fluorometric characterization of phytoplankton community composition using the Algae Online Analyser

The utility of a multiple-fixed-wavelength spectral fluorometer, the Algae Online Analyser (AOA), as a means of quantifying phytoplankton biomass and community composition was tested using natural communities from two southeastern United States estuaries, North Inlet, South Carolina, and the Neuse River Estuary, North Carolina. Estimates of biomass (as chlorophyll a) were correlated with HPLC values and variations (usually over-estimates) were consistent with effects of light intensity and nutrient availability on fluorescence quenching. AOA estimates of taxonomic structure were consistent with those from HPLC-derived marker pigments by ChemTax, with both methods indicating domination by chromophytes and green algae in North Inlet and chromophytes and cyanobacteria in the Neuse. We recommend frequent calibration by discrete sample collection, and calibration with species representative of the region of interest. Overall, the AOA appears to be a useful tool for monitoring of phytoplankton community composition, especially as an early warning system for the detection of harmful algal blooms.

Taxonomic Discrimination of Phytoplankton by Spectral Fluorescence

Chlorophyll fluorescence techniques are used widely in both laboratory and field studies to assess the abundance and physiological responses of cyanobacteria, microalgae, macroalgae and vascular plants, as described in other chapters in this volume. Most of the instruments used in these studies excite fluorescence in the blue region of the spectrum and measure chlorophyll fluorescence (peak ca. 685 nm) at ambient temperature. Fluorescence is generally detected using a photomultiplier tube (PMT), which is very sensitive to intensity but insensitive to spectral quality. Cross-talk between the light source used to excite fluorescence and the detector is prevented by the use of cut-off filters on both the emitter and the PMT, or by the use of emitters with narrow wavebands, such as light-emitting diodes (LEDs) or lasers, and a long-pass filter on the detector. With the advent of LEDs, which have a very high efficiency (intensity of light output per unit power input) compared to the xenon flash-lamps used in many older instruments, commercially-available fluorometers can have very low power demands and be both small and sensitive (detection limits are typically <1 mg m−3 of Chla). This makes them ideal for unattended monitoring such as on platforms, moorings or gliders.

Construction, figures of merit, and testing of a single-cell fluorescence excitation spectroscopy system

Characterization of phytoplankton community composition is critical to understanding the ecology and biogeochemistry of the oceans. One approach to taxonomic characterization takes advantage of differing pigmentation between algal taxa and thus differences in fluorescence excitation spectra. Analyses of bulk water samples, however, may be confounded by interference from chromophoric dissolved organic matter or suspended particulate matter. Here, we describe an instrument that uses a laser trap based on a Nikon TE2000-U microscope to position individual phytoplankton cells for confocal fluorescence excitation spectroscopy, thus avoiding interference from the surrounding medium. Quantitative measurements of optical power give data in the form of photons emitted per photon of exposure for an individual phytoplankton cell. Residence times for individual phytoplankton in the instrument can be as long as several minutes with no substantial change in their fluorescence excitation spectra. The laser trap was found to generate two-photon fluorescence from the organisms so a modification was made to release the trap momentarily during data acquisition. Typical signal levels for an individual cell are in the range of 10(6) photons/s of fluorescence using a monochromated 75 W Xe arc lamp excitation source with a 2% transmission neutral density filter.

Differential effects of changes in spectral irradiance on photoacclimation, primary productivity and growth in Rhodomonas salina (Cryptophyceae) and Skeletonema costatum (Bacillariophyceae) in simulated blackwater environments

The underwater light field in blackwater environments is strongly skewed toward the red end of the electromagnetic spectrum due to blue light absorption by colored dissolved organic matter (CDOM). Exposure of phytoplankton to full spectrum irradiance occurs only when cells are mixed up to the surface. We studied the potential effects of mixing‐induced changes in spectral irradiance on photoacclimation, primary productivity and growth in cultures of the cryptophyte Rhodomonas salina and the diatom Skeletonema costatum. We found that these taxa have very different photoacclimation strategies. While S. costatum showed classical complementary chromatic adaption, R. salina showed inverse chromatic adaptation, a strategy previously unknown in the cryptophytes. Transfer of R. salina to periodic full spectrum light (PFSL) significantly enhanced growth rate (μ) by 1.8 times and primary productivity from 0.88 to 1.35 mg C · (mg Chl−1) · h−1. Overall, R. salina was less dependent on PFSL than was S. costatum, showing higher μ and net primary productivity rates. In the high‐CDOM simulation, carbon metabolism of the diatom was impaired, leading to suppression of growth rate, short‐term 14C uptake and net primary production. Upon transfer to PFSL, μ of the diatom increased by up to 3‐fold and carbon fixation from 2.4 to 6.0 mg C · (mg Chl−1) · h−1. Thus, a lack of PFSL differentially impairs primarily CO2‐fixation and/or carbon metabolism, which, in turn, may determine which phytoplankton dominate the community in blackwater habitats and may therefore influence the structure and function of these ecosystems.

Fast reactivation of photosynthesis in arctic phytoplankton during the polar night1

Arctic microalgae experience long periods of continuous darkness during the polar night, when they are unable to photosynthesize. Despite numerous studies on overwintering strategies, such as utilization of stored energy products, formation of resting stages, reduction of metabolic rates and heterotrophic lifestyles, there have been few attempts to assess the in situ physiological state and restoration of the photosynthetic apparatus upon re‐illumination. In this study, we found diverse and active marine phytoplankton communities during the polar night at 78°N. Furthermore, we observed rapid changes (≤20 min) in the efficiency of photosynthetic electron transport upon re‐illumination. High photosynthetic capacity and net primary production were established after 24 h of re‐illumination. Our results suggest that some Arctic autotrophs maintain fully functional photosystem II and downstream electron acceptors during the polar night even though the low in situ net primary production levels measured in January prove that light was not sufficient to support any measurable primary production. Due to low temperatures resulting in low respiratory rates as well as the absence of photodamage during the polar night, maintenance of basic photosynthetic machinery may actually pose relatively low metabolic costs for algal cells. This could allow Arctic microalgae to endure the polar night without the formation of dormant stages, enabling them to recover and take advantage of light immediately upon the suns return during the winter–spring transition.