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Primary Productivity Measurements in Freshwater: Converting Photosynthetic Electron Transfer Rates to Carbon Fixation Rates

Final Report Summary - PROMON (Primary Productivity Measurements in Freshwater: Converting Photosynthetic Electron Transfer Rates to Carbon Fixation Rates)

Local environmental and global (climate) change directly affect freshwater ecosystems via trophic dynamics and biogeochemical cycles, of which phytoplankton are an integral component. Through photosynthesis, phytoplankton take up CO2 and sequester O2, and thus, they are critical for ecosystem health and productivity. However, to this day, the relationships between environmental conditions, phytoplankton community composition and primary productivity (i.e. photosynthetic CO2 fixation) are still poorly characterized. Furthermore, even though CO2 fixation is considered one of the best indicators for the growth potential of phytoplankton, primary productivity estimates are rarely included in ecosystem assessments.
With the advancement of rapid, in situ high-resolution fast repetition rate (FRR) fluorescence techniques, such routine measurements are now possible and enable users to link primary productivity directly to environmental controls at the time of sampling. The major drawback of this technique, however, is that it measures the rate at which electrons flow from water through photosystem II (PSII) – the so called linear electron transfer rate of PSII (ETRPSII). Many applications (climate models, trophic and fisheries assessments, ecosystem management) work in a photosynthetic currency of fixed CO2. Thus, the ETRPSII need to be converted to CO2 fixation rates via the electron requirement for carbon fixation (Φe,C), which is the ratio of the ETRPSII:CO2 fixation (in mol e- (mol C)-1). From previous work we know that changes in environmental factors (e.g. light and nutrient availability) and phytoplankton community composition may affect both, ETRPSI¬I and CO2 fixation, albeit to varying degrees.
Thus, the overall scientific objective of this project was to assess the individual and combined effects of environmental conditions and phytoplankton community composition on the variability of Φe,C. To address this objective, we carried out two different types of studies using phytoplankton cultures (year 1) and a field study with natural phytoplankton communities from the Římov drinking water reservoir (year 2).
For the culture experiments, three different species were initially grown as semi-continuous batch cultures in fully nutrient replete medium, which represents optimal growth conditions. After an initial sampling to determine phytoplankton biomass, bio-optical and biochemical properties as well as carbon fixation and electron transfer rates to derive Φe,C, the amount of nutrients in the medium was reduced to 1/10th of the optimum concentration and changes in biomass, cell properties and photosynthesis were followed for up to 8 consecutive days to determine how cells acclimate to changes in nutrient availability and how the acclimation responses affect Φe,C. We tested one nutrient form at a time and repeated the experiment three times simulating NO3-, PO43- and NH4+ limitation, respectively.
From these experiments, we found that there exists large variability in the different photosynthesis proxies (O2 evolution, carbon fixation and electron transfer), which is determined not only by nutrient availability (i.e. nutrient concentration) and nutrient form (NO3-, PO43-, NH4+) but also by species composition. Thus, we can now elucidate the relative importance of these three factors (nutrient concentration, type of nutrient and species) in determining Φe,C.
We also discovered that the incubation time of the carbon fixation measurements and the physiological state (steady state vs. non-steady state cultures) have a huge impact on ΦeC. The duration of the carbon fixation measurement determines whether carbon fixation resembles net primary productivity (NPP), gross primary productivity (GPP) or something between the two. All of them vary with growth rate and species. In the past, researchers used short term (20 minute) incubations, however, this may not hold for the future. We are currently investigating the effect of using NPP and GPP in determining the variability in ΦeC. This may have profound impacts on how researchers derive ΦeC in the future and thus convert those easy-to-measure electron transfer rates to ecological meaningful carbon fixation rates.
In addition to the above mentioned measurements, we also collected a large dataset on the concentration of reaction centres 2 ([RCII]), which is directly influenced by nutrient limitation too and will help us interpreting the observed changes in ΦeC. Furthermore, [RCII] is one of the big unknowns in the derivation of ETRPSII, but is rarely measured. With those additional measurements, we will be able to determine how novel and recently published ETRPSII algorithms perform under different scenarios of nutrient limitation, which will be a high-impact contribution for the entire community of Fast Repetition Rate (FRR) fluorometry users.
During the bioassay study in the Římov drinking water reservoir, we used natural phytoplankton communities, i.e. we filled large 10 L carboys with water from the reservoir and placed them inside a corral, which was moored to a monitoring side near the dam. Half of the carboys were amended with the nutrient in question (NO3 or PO43-) and the other half remained without nutrient addition (control samples). All carboys were sampled daily for up to 5 days to determine changes in phytoplankton biomass (by measuring chlorophyll concentrations), phytoplankton community composition (by counting cells under a microscope) as well as carbon fixation and electron transfer rates (by 14C uptake and FRR fluorometry, respectively) to then calculate ΦeC. To study how phytoplankton community composition affects these photosynthesis proxies, we measured them on whole water samples and after dividing them into small (<20 µm) and large (20-200 µm) phytoplankton by filtering samples through filters with different pore sizes. Because NO3- and PO43- limitation are two of the most common forms of nutrient limitation in freshwater systems and because NH4+ concentrations were relatively high during the sampling period, we only studied the effect of NO3- and PO43- and omitted NH4+.
The results of these two bioassays showed that small (mainly flagellates) and large (mostly diatoms) phytoplankton responded differently to changes in nutrient availability, and thus, have varying effects on bulk primary productivity and subsequently on Φe,C. They also confirmed observations from the culture study that the type of nutrient limitation (i.e. NO3- or PO43- stress) is an important determinant of primary production and Φe,C.
Overall, the results of this project have significant socio-economic impact. First, the resultant data will enable researchers and environmental agencies to implement fast, cost effective and high-resolution measurements of primary productivity in a wide variety of aquatic ecosystems, and hence assess the growth potential of phytoplankton. Second, knowledge of the influence of the availability of different nutrients on phytoplankton community composition, photosynthesis and growth will be important for water managers and policy makers because it will aid them in assessing the effects of changes in land use patterns and climate change on biochemical cycles and subsequently on alterations in phytoplankton community composition and bloom dynamics. The latter should be monitored regularly, particularly in habitats used for recreational purposes or drinking water because there are a large variety of toxic species, which pose considerable threats to wildlife and human beings. Understanding how nutrient limitation affects different types of phytoplankton and their growth potential through measurements of primary productivity is a critical step in these kinds of assessments.

Contact details:
Prof. Ondřej Prášil, Institute of Microbiology, Czech Academy of Sciences, 37981 Třeboň, Czech Republic, phone: +420 384-340-430, Email: