Impact of nitrogen availability upon the electron requirement for carbon fixation in Australian coastal phytoplankton communities

Abstract

Nitrogen (N) availability affects phytoplankton photosynthetic performance and regulates marine primary production (MPP) across the global coast and oceans. Bio‐optical tools including Fast Repetition Rate fluorometry (FRRf) are particularly well suited to examine MPP variability in coastal regions subjected to dynamic spatio‐temporal fluctuations in nutrient availability. FRRf determines photosynthesis as an electron transport rate through Photosystem II (ETRPSII), requiring knowledge of an additional parameter, the electron requirement for carbon fixation (KC), to retrieve rates of CO2‐fixation. KC strongly depends upon environmental conditions regulating photosynthesis, yet the importance of N‐availability to this parameter has not been examined. Here, we use nutrient bioassays to isolate how N (relative to other macronutrients P, Si) regulates KC of phytoplankton communities from the Australian coast during summer, when N‐availability is often highly variable. KC consistently responded to N‐amendment, exhibiting up to a threefold reduction and hence an apparent increase in the efficiency with which electrons were used to drive C‐fixation. However, the process driving this consistent reduction was dependent upon initial conditions. When diatoms dominated assemblages and N was undetectable (e.g., post bloom), KC decreased predominantly via a physiological adjustment of the existing community to N‐amendment. Conversely, for mixed assemblages, N‐addition achieved a similar reduction in KC through a change in community structure toward diatom domination. We generate new understanding and parameterization of KC that is particularly critical to advance how FRRf can be applied to examine C‐uptake throughout the global ocean where nitrogen availability is highly variable and thus frequently limits primary productivity.

Diurnal regulation of photosynthetic light absorption, electron transport and carbon fixation in two contrasting oceanic environments

“…High excitation pressure also triggers the upregulation of heat dissipation mechanisms in the pigment antenna (here estimated as NPQNSV), and several studies have reported a correlation between Φe,C and NPQNSV (Hughes et al, 2018a;Schuback et al, 2015Schuback et al, , 2016bSchuback et al, , 2017aZhu et al, 2017). We observed such a correlation at both of our sampling sites (Fig.…”

Effects of Temperature and Nutrient Supply on Resource Allocation, Photosynthetic Strategy, and Metabolic Rates of Synechococcus sp.

“…Where nutrients are limiting at higher temperatures there may also be an increased uncoupling between the light and dark reactions of photosynthesis as energy and reductant is used in nutrient uptake and cellular maintenance rather than carbon fixation (Hughes et al. ).…”

Uncertainties in variable fluorescence and 14C methods to estimate primary production: a case study in the coastal waters off the Korean peninsula

“…However, temperature did not show any significant correlation with other variables except Φ e,CC and K PAR . The effect of temperature on Φ e,C may be indirect, as it reflects the state of stratification, which in turn modulates nutrient supply and light availability in the upper layer (Mousing et al 2014, Hughes et al 2018. The negative correlation of temperature with nutrients, K PAR , and chl a supports this interpretation, although only the correlation with K PAR was significant.…”