phytoplankton productivity

Primary productivity by phytoplankton accounts for approximately half of the carbon fixed by photosynthesis on a planetary scale. Clearly, measurement of phytoplankton on wide spatial and temporal scales has an enormous potential for developing our understanding of ocean productivity and verifying climate change models.

While satellite remote sensing operates on the widest possible spatial scales for evaluation, historically, validation of satellite data has relied on photosynthetron-based measurement of Carbon 14 (¹⁴C) fixation, but this is slow and labour-intensive.

SPOTLIGHT ON PHYTOPLANKTON
➤ What are phytoplankton?
➤ Phytoplankton dynamics
➤ Phytoplankton and climate change
➤ Studying phytoplankton
Photosynthetron-based measurement of ¹⁴C fixation cannot be applied on meaningful spatial and temporal scales, which means that there is inevitably an under-sampling of the oceanic environment for phytoplankton. The key scientific direction for Chelsea Technologies is to develop widely accessible instrumentation that science can use on significantly wider spatial and temporal scales than ¹⁴C fixation, at comparable levels of accuracy and precision.

¹⁴C fixation and phytoplankton productivity

The ¹⁴C tracer method is the historical standard for photosynthetic measurement of phytoplankton. It provides the only direct measurement of carbon capture and has a long and distinguished historical sequence of datasets against which to compare new data. However, the ¹⁴C tracer method is inherently time consuming, requires radioisotope consumables, and doesn’t provide the data over wide temporal and spatial scales that the new generation of climate and phytoplankton science requires.

“the ¹⁴C tracer method is inherently time consuming, and therefore doesn’t provide the data over wide temporal and spatial scales that the new generation of climate and phytoplankton science requires”

The typical process with ¹⁴C starts with a sample retrieved on a research vessel at a single spot. This sample is then filtered, and a carefully measured and controlled amount of ¹⁴C is added with a pipette in a laboratory in a precise concentration and volume. The phytoplankton sample then uses this carbon to photosynthesise (“incubation”) over a duration of 2-24 hours under carefully controlled light conditions (a “photosynthetron”), and ¹⁴C is then measured at the end to calculate the carbon capture.

Photosynthetron-based ¹⁴C tracer methodology used for phytoplankton productivity

To give some more detail, photosynthesis-irradiance (PE) curves eventually derived from the ¹⁴C tracer method are an important tool used to characterize the physiology of phytoplankton. The PE curve describes the response relationship of the photosynthetic rate as a function of light. Since light drives photosynthesis, the PE curve provides the data to analyse this important process.

Phytoplankton photosynthetic physiology can be investigated through single turnover active fluorometry (STAF) approaches, which carries the unique potential to autonomously collect data at high spatial and temporal resolutions. Chelsea Technologies’ LabSTAF is a new generation fluorometer that uses single turnover active fluorometry to measure very low levels of photosynthesis, ideal for oligotrophic zonesThe approach to generating PE curves is to incubate samples over a range of light levels (Lewis and Smith 1983). Several variations on the photosynthetron theme have been used, but all use a single light source that is differentially attenuated to each position of the photosynthetron to create a light gradient. Temperature is regulated by a controlled water jacket. Each position of the photosynthetron corresponds to a different light-level resulting in parallel incubations.

Individual incubations, all of the same period, are assayed for ¹⁴C incorporation, thus generating a PE curve. Because of the sensitivity of the ¹⁴C method for measuring photosynthesis, the photosynthetron-based approach is the conventional technique for measuring PE curves in most aquatic environments. Such conventional photosynthetron scans yield high quality data, and their utility has been demonstrated in numerous laboratory and field studies (Henley 1993; Marra et al. 2000; Sathyendranath et al. 1999). However, there are shortcomings that limit their scalability.

“The key scientific direction for Chelsea Technologies is to develop widely accessible instrumentation that science can use for climate studies on significantly wider spatial and temporal scales than ¹⁴C fixation, at comparable levels of accuracy and precision”

In particular, the size, power consumption, duration and difficulty makes conventional photosynthetrons an impractical tool when numerous PE curves are required. Conventional photosynthetrons are bulky and require large coldwater recirculators to maintain temperature. The source of light emits heat, and a refrigerated water recirculator is required to maintain in situ temperatures. Another concern is that the combined water chiller/photosynthetron setup requires a lot of space and electrical power, both of which can be limited in field or laboratory settings.

But, as the number of curves increases, the processing of the numerous parallel incubations also increases, thus making the generation of multiple PE curves labour intensive. Finally, conventional photosynthetrons use different combinations of neutral density screening to modify the levels of light in each position. Although technically sound, in practice it is time-consuming and difficult to precisely set the light level for each of the positions, often leading to inappropriate light gradients and ill-defined PE curves. Scientists using the ¹⁴C fixation method are well aware of these issues. The process is still slow, manual, with a spotlight on one particular sample, as opposed to wide geospatial scales that science requires.

How STAF revolutionises phytoplankton research

The FLC-generated rPE curve for phytoplankton productivity analysis comprises an automated set of STAF measurements made at several photon irradiance levels between darkness and saturating light. The FLC-generated dataset is comparable to the ¹⁴C tracer-based photosynthesis – photon irradiance (PE) curve

STAF technologies enable exactly this, to make measurements over much wider spatial and temporal scales than are possible with ¹⁴C fixation with continuing reference to parallel ¹⁴C fixation data for comparison and data integrity checks. Chelsea’s development of our dual incubation method was the vital stepping stone as it paves the way for a standard methodology to be developed for parallel measurements that eliminates the requirement for spectral correction between the two.

Chelsea’s experiments made in Crete, the Jerico project which generated electron per carbon values of between 8 and 18, provided the necessary data that users who have run long-term ¹⁴C based measurements should consider integrating STAF technologies within their existing toolset. In other words, scientists who want to improve their rate of experimentation can legitimately use both methods side by side; ¹⁴C for a once a day detailed granular spot result, together with STAF technologies giving results 10s of 1000s of times per day that can be calibrated against the ¹⁴C result.

Satellite remote sensing and phytoplankton productivity

Satellite remote sensing operates on the widest possible spatial and temporal scales. The advent of ocean colour sensors introduced a new understanding of the dynamics inherent to phytoplankton biomass and productivity. Satellite remote sensing allows insights into such spatiotemporal dynamics as bloom start, peak and end timing, duration, maximum chlorophyll-a concentrations, spatial extent, rates of increase and decrease, and bloom chlorophyll-a concentration. Software is typically used to extract and map these phenology metrics. However, the relatively large uncertainties, and limited number of spectral bands all point to the need of further improvement in data availability and accuracy with future satellite sensors. Few satellite products maintain reasonable accuracy, and satellite sensors typically have both a high coefficient of determination (variability) and a high median absolute percentage error (Kahru et al, 2014). The low accuracy at medium and high chlorophyll-a is caused by the poor retrieval of remote sensing reflectance.

“… users who have run long-term ¹⁴C based measurements should consider integrating STAF technologies within their existing toolset”

Satellites, therefore, which have been scanning the oceans with their sensors for the past three decades, are an imperfect solution at best: although they can be used to gauge the amount of the plant pigment chlorophyll-a in the water – as an indicator of how high the general concentration of phytoplankton is, distinguishing between different types of phytoplankton remains extremely challenging. Moreover, nor does satellite data show phytoplankton at anything other than the surface, and verification using photosynthetron-based measurement of ¹⁴C fixation is necessary.

Underway systems

STAF technologies can also be used in so-called underway systems to study phytoplankton. An underway system is one in which a LabSTAF system is plumbed into the water that naturally gets taken onboard and discharged, so the LabSTAF takes continuous readings whilst the vessel is underway. This has the benefit of giving data over greater geospatial scales than a stationary research vessel, or spot samples. The success of this method has been demonstrated in the Mediterranean (please contact us for case studies).