A multi-spectral fluorescence induction and relaxation (FIRe) technique for physiological and taxonomic analysis of phytoplankton communities

Jung

et al. 2020

JGR Oceans

Phytoplankton in the Arctic Ocean are subject to nitrogen limitation in the summer, however, how severely the nitrogen stress affects phytoplankton physiology remains largely unknown. In the summers of 2015-2018, we examined the distribution of phytoplankton photophysiological properties across two contrasting regions of the Arctic Ocean with distinctly different levels of nitrogen availability in the upper water column. We quantified the extent of nitrogen stress using a highly sensitive fluorescence induction and relaxation system to obtain continuous underway measurements and via discrete sample analyses of phytoplankton physiology, as well as nutrient enrichment incubations. The results revealed vast regions in the Chukchi Sea where phytoplankton photosynthesis was severely nitrogen-stressed. Thereby, the maximum quantum yield of photochemistry in photosystem II showed only a small decrease (12 ± 9%) relative to its nutrient-replete values, while the maximum photosynthetic electron transport rates under saturating irradiance were impaired to a greater extent (40 ± 17%). This phytoplankton photosynthesis response is indicative of a severe nitrogen limitation, which results in dramatic reduction in growth and net primary production rates. Nutrient enrichment incubations also revealed a marked increase in large-size phytoplankton growth (>10 μm) after the nitrogen stress was alleviated, suggesting that the larger cells were more susceptible to nitrogen stress. These results are important for understanding how regional nitrogen fluxes control variability in the primary production and phytoplankton community structure and how these processes might change with rapid climate changes in the Arctic Ocean. Plain Language Summary Nutrient availability is the main bottom-up controls of phytoplankton physiology and growth in the upper ocean. The distribution of nutrient limitation in the global ocean varies greatly in space and time, so phytoplankton responses to this factor are essential for understanding the marine ecosystem. Although nitrogen limitation was previously shown in the Arctic Ocean in the summer, how nitrogen stress affects phytoplankton physiology remains largely unknown. This study investigates, with high spatial resolution, the distribution of phytoplankton physiological status and quantifies the effects of nitrogen stress in the western Arctic Ocean. Our results revealed severe nitrogen limitation in the summer that results in dramatic reduction in growth and net primary production in this region of the ocean. Therefore, alterations in nitrogen fluxes along with climate change in the Arctic Ocean would be important for controlling phytoplankton growth and primary production in this region.

Section: Methodsmentioning

confidence: 99%

Effects of Nitrogen Limitation on Phytoplankton Physiology in the Western Arctic Ocean in Summer

Jung

et al. 2020

JGR Oceans

“…1). These instruments are conceptually similar to previous FRR (Kolber et al 1998) and FIRe systems (Gorbunov and Falkowski 2005), but exhibit approximately 20-fold higher sensitivity and signal-to-noise ratio, compared to their predecessors. This extreme sensitivity is crucial for sampling in oligotrophic regions, which constitute 30% of the world ocean (Lin et al 2016).…”

Section: Methodsmentioning

confidence: 91%

“…The peak excitation intensity in the measuring volume of the FIRe instrument is optimally adjusted to ensure closure of PSII reaction centers and saturation of fluorescence within approximately 100 μs (i.e., a single electron turnover in PSII) (Gorbunov et al 2020), which is crucially important for accurate retrievals of functional cross sections and quantum yields (Gorbunov et al 1999). The fluorescence signal is collected from a central, 8 mm diameter, portion of the sample volume, isolated by a red band pass interference filter (680 nm, with 20 nm bandwidth), and detected by an avalanche photodiode (SD630-70-72-500, Advanced Photonics).…”

Section: Methodsmentioning

confidence: 99%

“…σ PSII is calculated from the rate of fluorescence rise during the single turnover flash (Kolber et al 1998). This rate is proportional to the product of σ PSII and the photon flux density of excitation light (Gorbunov and Falkowski 2005). Retrievals of σ PSII in absolute units (Å 2 ) require accurate calibration of the average photon flux density in the measuring volume, that is, the volume from which the signal is recorded (Gorbunov and Falkowski 2005).…”

Section: Functional Absorption Cross-sectionsmentioning

confidence: 99%

“…This rate is proportional to the product of σ PSII and the photon flux density of excitation light (Gorbunov and Falkowski 2005). Retrievals of σ PSII in absolute units (Å 2 ) require accurate calibration of the average photon flux density in the measuring volume, that is, the volume from which the signal is recorded (Gorbunov and Falkowski 2005). Because amplitudebased measurements of absolute ETR depend on σ PSII , accurate calibration of excitation intensities is crucially important.…”

Section: Functional Absorption Cross-sectionsmentioning

confidence: 99%

See 2 more Smart Citations

Using chlorophyll fluorescence kinetics to determine photosynthesis in aquatic ecosystems

Limnology & Oceanography

Falkowski

2020

Variable fluorescence techniques are increasingly used to assess phytoplankton photosynthesis. All fluorescence techniques and models for photosynthetic electron transport rates (ETRs) are amplitude-based and are subject to errors, especially when phytoplankton growth is nutrient-limited. Here we develop a new, kinetic-based approach to measure, directly and in absolute units, ETRs and to estimate growth rates in phytoplankton. We applied this approach to investigate the effects of nitrogen limitation on phytoplankton photophysiology and growth rates. Nutrient stress leads to a decrease in the quantum yield of photochemistry in Photosystem II (F v /F m); however, the relationship between F v /F m and growth rates is highly nonlinear, which makes it impossible to quantify the reduction in phytoplankton growth rates from F v /F m alone. In contrast, the decline in growth rates under nitrogen stress was proportional to the decrease in kinetic-based photosynthetic rates. Our analysis suggests the kinetic fluorescence measurements markedly improve the accuracy of ETR measurements, as compared to classical amplitude-based measurements. Fluorescence-based methods for primary production rely on measurements of ETRs and then conversion to carbon fixation rates by using the electron yields of carbon fixation. The electron yields exhibit 10-fold variability in natural phytoplankton communities and are strongly affected by nutrient limitation. Our results reveal that a decrease in the growth rates and the electron yields of carbon fixation is driven by, and can be quantified from, a decrease in photosynthetic turnover rates. We propose an algorithm to deduce the electron yields of carbon fixation, which greatly improve fluorescence-based measurements of primary production and growth rates.

FIRe glider: Mapping in situ chlorophyll variable fluorescence with autonomous underwater gliders

Carvalho

Limnology & Ocean Methods

Oliver

et al. 2020

Nutrient and light availability regulate phytoplankton physiology and photosynthesis in the ocean. These physiological processes are difficult to sample in time and space over physiologically and ecologically relevant scales using traditional shipboard techniques. Gliders are changing the nature of data collection, by allowing a sustained presence at sea over regional scales, collecting data at resolution not possible using traditional techniques. The integration of a fluorescence induction and relaxation (FIRe) sensor in a Slocum glider allows autonomous high‐resolution and vertically‐resolved measurements of photosynthetic physiological variables together with oceanographic data. In situ measurements of variable fluorescence under ambient light allows a better understanding of the physical controls of primary production (PP). We demonstrate this capability in a laboratory setting and with several glider deployments in the Southern Ocean. Development of these approaches will allow for the in situ evaluation of phytoplankton light stress and photoacclimation mechanisms, as well as the role of vertical mixing in phytoplankton dynamics and the underlying physiology, especially in remote locations and for prolonged duration.