Responses of elemental and biochemical composition of Chaetoceros muelleri to growth under varying light and nitrate : phosphate supply ratios and their influence on critical N: P

Abstract

The critical nitrogen-to-phosphorous ratio (N : P) defines the transition between N and P limitation of growth rate and is not a biological constant. To test the effect of environmental conditions on the critical N : P, we cultured the diatom Chaetoceros muelleri in chemostats with inflow nitrate : phosphate ratios ranging from 5 to 90 mol N (mol P)Ϫ1 at two photon flux densities (PFDs; 50 and 700 mol photons m Ϫ2 s Ϫ1). The nitrate : phosphate ratio marking the transition between N and P limitation increased from between 15-30 mol N : mol P at the high PFD to 45 mol N : mol P at the low PFD. The particulate ratio marking this transition increased from 16-23 mol N : mol P at low PFD to 35 mol N : mol P at high PFD. Cell phosphorus and RNA contents decreased with increasing N : P ratio up to the critical N : P ratio for each PFD, above which they remained stable. In contrast, cell dry weight, chlorophyll a, C, N, and protein were not influenced by nitrate : phosphate in the inflow medium, although they were influenced by PFD. Total protein per RNA increased with increasing N : P ratio at the low light conditions, suggesting increased ratio of protein synthesis per RNA. Our results showed the effect of PFD, growth rate, or both on the critical N : P ratio. Agreement was found in the assessment of the transition between N and P limitation on the basis of nutrient enrichment bioassays, cellular elemental (N and P) quotas, and cell RNA content. Our results are consistent with theoretical predictions of higher N requirements under low light conditions due to the coupling of the photosynthetic mechanism with N uptake. In contrast, P-rich cellular components, such as RNA, were dependent on P availability rather than light.

Performance of the Redfield Ratio and a Family of Nutrient Limitation Indicators as Thresholds for Phytoplankton N vs. P Limitation

“…The ranges spanned by these two indicators fit reasonably well with observed ranges of critical N:P supply ratios in phytoplankton cultures (for example, Terry and others 1985;Leonardos and Geider 2004). Although threshold N:P supply ratios are known to differ among species and also to be strongly influenced by growth conditions (Leonardos and Geider 2004), it seems reasonable to interpret both the DIN:DIP and BAN:BAP concentration ratios as proxies for the actual N:P supply ratios experienced by the initial plankton communities in our experiments.…”

Interpretation of fast repetition rate (FRR) fluorescence: signatures of phytoplankton community structure versus physiological state

“…Briefly, PSII photosynthetic unit sizes were determined from O 2 -flash yield measurements using a custom-built LED flash system (Suggett et al 2003). Pigment concentrations were measured by high-performance liquid chromatography (HPLC) via identification and co-elution with pigment standards (see Leonardos & Geider 2004). Finally, PSII fluorescence excitation spectra (400 to 715 nm) were measured with the monochromator on the detector set to 730 nm emission (F 730 ) upon samples pre-treated with DCMU (Lutz et al 2001, Suggett et al 2004.…”

“…1). Here, high light grown cells were allowed to grow at a higher dilution (growth) rate than were low light grown cells (Leonardos & Geider 2004). Lower values of F v /F m were not observed at the lower growth rates, but for cells grown under higher growth PFDs (at the higher growth rate).…”

“…In batch mode (s) nutrients were not replenished post-inoculation and values of F v /F m and σ PSII were recorded for cells growing at a single light intensity (50 µmol photons m -2 s -1 ) through exponential (E) and stationary (S) phases. Each data point represents measurements made on consecutive days; E and S phases were identified by simultaneous daily quantification of cell number (Leonardos & Geider 2004 (Kolber et al 1988, Geider et al 1993a, Kolber & Falkowski 1993), a common maximum value for F v /F m cannot be assigned to all algae (see discussion by Suggett et al 2004). …”

Nutrient and chlorophyll a anomaly in red-tide periods of 2003–2008 in Sishili Bay, China

“…As a result, high nutrient and COD concentrations are still found near its estuary, even though the river has been dammed. In most marine environments, nitrogen appears to be the limiting factor of primary production; accordingly, the concentration of TIN, PO 4 -P and molar N/P ratio can control the pattern and function of the ecosystem of a bay, as well as alter the structure, production, and biomass of phytoplankton (Balode et al, 1998;Lenton and Watso, 2000;Leonardos and Geider, 2004;McQuatters-Gollop et al, 2007). However, more N than P entered Sishili Bay from terrestrial sources during rainfall events, as indicated by the increasing trend of TIN concentrations during precipitation.…”