Half-saturation Constant For Uptake Research Articles

Nitrate and phosphate uptake kinetics of the harmful diatom Coscinodiscus wailesii, a causative organism in the bleaching of aquacultured Porphyra thalli

The large diatom Coscinodiscus wailesii is one of the problematic species which indirectly cause bleaching damage to “Nori” ( Porphyra thalli) cultivation through competitive utilization of nutrients during its bloom. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of C. wailesii, Harima-Nada strain. Maximum uptake rates ( ρ max), obtained by short-term experiments, were 58.3 and 95.5 pmol cell −1 h −1 for nitrate and 41.9 and 59.1 pmol cell −1 h −1 for phosphate at 9 and 20 °C, respectively. The half saturation constants for uptake ( K s ) were 2.91 and 5.08 μM N and 5.62 and 6.67 μM P at 9 and 20 °C, respectively. The ρ max values of C. wailesii, much higher than those of other marine phytoplankton species, suggest that C. wailesii is able to take up large amounts of nutrients from the water column. On the other hand, V max/ K s ( V max; V max = ρ max/ Q 0, Q 0; minimum cell quota) values of C. wailesii, which is a better measure to evaluate the competitive ability for nutrient uptake, were low in dominant diatom species. This parameter indicates that C. wailesii is disadvantaged compared to other diatom species in competing for nutrients, and the decreasing nutrient concentrations from winter to spring is an important factor limiting C. wailesii blooming in early spring.

Nitrate and phosphate uptake kinetics of the harmful diatom Eucampia zodiacus Ehrenberg, a causative organism in the bleaching of aquacultured Porphyra thalli

The diatom Eucampia zodiacus Ehrenberg is a harmful diatom which indirectly causes bleaching of aquacultured Nori (Porphyra thalli) through competitive utilization of nutrients during bloom events. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of E. zodiacus, Harima-Nada strain. Maximum uptake rates (ρmax), which were obtained by short-term experiments, were 0.777 and 0.916pmolcell−1h−1 for nitrate and 0.244 and 0.550pmolcell−1h−1 for phosphate at 9 and 20°C, respectively. The half-saturation constants for uptake (Ks) were 2.59 and 2.92μM N and 1.83 and 4.85μM P at 9 and 20°C, respectively. Although the maximum specific uptake rate (Vmax; Vmax=ρmax/Q0, Q0; minimum cell quota) and Vmax/Ks for nitrate at 9°C are about 1/2 of those obtained at the optimum temperature (20°C), they are still higher than those obtained for many other phytoplankton at their optimum temperature conditions for uptake. These results suggest that E. zodiacus utilizes nitrogen efficiently at low water temperature, and it is one of the important factors causing the serious damage to Porphyra thalli by bleaching due of this species. For phosphate, the Ks values of E. zodiacus were higher than those reported for other species; the Vmax and Vmax/Ks values were much lower than those of other diatoms such as Skeletonema costatum (Greville) Cleve. These results suggest that E. zodiacus is disadvantaged compared to other diatom species during competitive utilization of phosphate.

Nitrogen dynamics during the Arabian Sea Northeast Monsoon

Abstract This investigation focused on the weaker and less well understood of the two Arabian Sea monsoonal wind phases, the NE Monsoon, which persists for 3–4 months in the October to February period. Historically, this period has been characterized as a time of very low nutrient availability and low biological production. As part of the US JGOFS Arabian Sea Process Study, 17 stations were sampled on a cruise in January 1995 (late NE Monsoon) and, 15 stations were sampled on a cruise in November 1995 (early NE Monsoon). Only the southern most stations (10° and 12°N) and one shallow coastal station were as nutrient-depleted as had been expected from the few relevant prior studies in this region. Experiments were conducted to ascertain the relative importance of different nitrogenous nutrients and the sufficiency of local regeneration processes in supplying nitrogenous nutrients utilized in primary production. Except for the southern oligotrophic stations, the euphotic zone concentrations of NO3− were typically 5–10-fold greater than those of NO2− and NH4+. There was considerable variation (20–40-fold) in nutrient concentration both within and between the two sections on each cruise. All nitrogenous nutrients were more abundant (2–4-fold) later in the NE Monsoon. Strong vertical gradients in euphotic zone NH4+ concentration, with higher concentrations at depth, were common. This was in contrast to the nearly uniform euphotic zone concentrations for both NO3− and NO2−. Half-saturation constants for uptake were higher for NO3− (1.7 μmol kg−1 (s.d.=0.88, n=8)) than for NH4+ (0.47 μmol kg−1 (s.d.=0.33, n=5)). Evidence for the suppressing effect of NH4+ on NO3− uptake was widespread, although not as severe as has been noted for some other regions. Both the degree of sensitivity of NO3− uptake to NH4+ concentration and the half-saturation constant for NO3− uptake were correlated with ambient NO3− concentration. The combined effect of high affinity for low concentrations of NH4+ and the effect of NH4+ concentration on NO3− uptake resulted in similarly low f-ratios, 0.15 (s.d.=0.07, n=15) and 0.13 (s.d.=0.08, n=17), for early and late observations in the NE Monsoon, respectively. Stations with high f-ratios had the lowest euphotic zone NH4+ concentrations, and these stations were either very near shore or far from shore in the most oligotrophic waters. At several stations, particularly early in the NE Monsoon, the utilization rates for NO2− were equal to or greater than 50% the utilization rates for NO3−. When converted with a Redfield C : N value of 6.7, the total N uptake rates measured in this study were commensurate with measurements of C productivity. While nutrient concentrations at some stations approached levels low enough to limit phytoplankton growth, light was shown to be very important in regulating N uptake at all stations in this study. Diel periodicity was observed for uptake of all nitrogenous nutrients at all stations. The amplitude of this periodicity was positively correlated with nutrient concentration. The strongest of these relationships occurred with NO3−. Ammonium concentration strongly influenced the vertical profiles for NO3− uptake as well as for NH4+ uptake. Both NO2− and NH4+ were regenerated within the euphotic zone at rates comparable to rates of uptake of these nutrients, and thus maintenance of mixed layer concentrations did not require diffusive or advective fluxes from other sources. Observed turnover times for NH4+ were typically less than one day. Rapid turnover and the strong light regulation of NH4+ uptake allowed the development and maintenance of vertical structure in NH4+ concentration within the euphotic zone. In spite of the strong positive effect of light on NO2− uptake and its strong negative effect on NO2− production, the combined effects of much longer turnover times for this nutrient and mixed layer dynamics resulted in nearly uniform NO2− concentrations within the euphotic zone. Responses of the NE Monsoon planktonic community to light and nutrients, in conjunction with mixed layer dynamics, allowed for efficient recycling of N within the mixed layer. As the NE Monsoon evolved and the mixed layer deepened convectively, NO2− and NO3− concentrations increased correspondingly with the entrainment of deeper water. Planktonic N productivity increased 2-fold, but without a significant change the new vs. recycled N proportionality. Consequently, NO3− turnover time increased from about 1 month to greater than 3 months. This reflected the overriding importance of recycling processes in supplying nitrogenous nutrients for primary production throughout the duration of the NE Monsoon. As a result, NO3− supplied to the euphotic zone during the NE Monsoon is, for the most part, conserved for utilization during the subsequent intermonsoon period.

EVALUATION OF COMPETITIVE ABILITY OF STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) UNDER P LIMITATION1

ABSTRACTThe desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Qp) and the surplus P:C ratio (Qsp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Qp) was 1.85 μg P·mg C−1in S. luetkemuellerii and 6.09 μg P·mg C−1in M. aeruginosa, whereas the respective Qp of P saturnted organisms (Qs) were 43 and 63 μg P·mg C−1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Qs/Q0). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa.The initial uptake of Pi pulses in the organisms was not inhibited by rapid (<1 h) internal feedback mechanisms and the short term uptake rote could be expressed solely as a function of ambient Pi. The maximum cellular C‐based uptake rate (Vm) in P starved M. aeruginosa was up to 50 times higher than that of S. luetkemuellerii. It decreased with increasing growth rate (P status) in the former species and remained fairly constant in the latter. The corresponding cellular P‐based value (Um= Vm/Qp) decreased with growth rate in both species and was about 10 times higher in P started M. aeruginosa than in S. luetkemuellerii. The average half saturation constant for uptake (Km) was equal for both species (22 μg P·L−1) and varied with the P status. S. luetkemuellerii exhibited shifts in the uptake rate of Pi that were characterized by increased affinity (Um/Km) at low Pi, concentrations (<4 μg P·L−1) compared to that at higher concentrations. The species thus was well adapted to uptake at low ambient Pi, but M. aeruginosa was superior in Pi uptake under steady state and transient conditions when the growth rate was lower than 0.75 d−1. Moreover, M. aeruginosa was favored by pulsed addition of Pi. M. aeruginosa relpased Pi at a higher rate than S. luetkemuellerii. Leakage of Pi from the cells caused C‐shaped μ vs. Pi curves. Therefore, no unique Ks for growth could be estimated. The maximum growth rate (μm) (23° C) was 0.94 d−1for S. luetkemuellerii and 0.81 d−1for M. aeruginosa.The steady state concentration of Pi (P*) was lower in M. aeruginosa than in S. luetkemuellerii at medium growth rates. The concentration of Pi at which the uptake and release of Pi was equal (Pc was, however, lower in S. luetkemuellerii.

Phosphorus uptake and photosynthesis by phosphate-limited cultures of the cyanobacterium Microcystis aeruginosa

Microcystis aeruginosa AK1 was grown in phosphorus-limited continuous cultures with an 8:16 light-dark cycle. The photosynthesis and phosphorus uptake kinetics were studied and compared with those of other cyanobacteria. Compared to that of light-limited cultures the light-harvesting capacity of P-limited cultures was low. The maximal photosynthetic rate per unit chlorophyll remained constant whereas the light-harvesting efficiency increased with a decrease in the severity of the limitation. The maximal phosphate uptake rate (V max) and the half saturation constant for uptake (K m) remained constant. Inhibition of V max by accumulated phosphate was observed and the inhibition constant was determined. Due to the high V max and the low P-content, Microcystis outcompeted Oscillatoria in conditions where the supply of phosphorus limited the growth rate. The results are interpreted in an ecological context.

Transport and hydrolysis of disaccharides by Trichosporon cutaneum.

Trichosporon cutaneum is shown to utilize six disaccharides, cellobiose, maltose, lactose, sucrose, melibiose, and trehalose. T. cutaneum can thus be counted with the rather restricted group of yeasts (11 to 12% of all investigated) which can utilize lactose and melibiose. The half-saturation constants for uptake were 10 +/- 3 mM sucrose or lactose and 5 +/- 1 mM maltose, which is of the same order of magnitude as those reported for Saccharomyces cerevisiae. Our results indicate that maltose shares a common transport system with sucrose and that there may be some interaction between the uptake systems for lactose, cellobiose, and glucose. Lactose, cellobiose, and melibiose are hydrolyzed by cell wall-bound glycosidase(s), suggesting hydrolysis before or in connection with uptake. In contrast, maltose, sucrose, and trehalose seem to be taken up as such. The uptake of sucrose and lactose is dependent on a proton gradient across the cell membrane. In contrast, there were no indications of the involvement of gradients of H+, K+, or Na+ in the uptake of maltose. The uptake of lactose is to a large extent inducible, as is the corresponding glycosidase. Also the glycosidases for cellobiose, trehalose, and melibiose are inducible. In contrast, the uptake of sucrose and maltose and the corresponding glycosidases is constitutive.

Cadmium induced inhibition of nitrate Uptake in Anacystis nidulans: Interaction with other divalent cations.

The effect of Cd2+ (4ppm) and a combination of Cd2+ with the non-inhibitory levels of metals like Ca2+ (150ppm), Zn2+ (2ppm), and Hg2+ (0.04ppm) on the uptake of nitrate by Anacystis nidulans IU 625 (ATCC 27144) was investigated. The NO3- uptake kinetics was studied by monitoring short-term uptake rates at different NO3- levels (1-20mM). The values for Vmax (0.8μmol/μg protein/hr) and half saturation constant for uptake (Km 3.03mM) were obtained employing the double reciprocal plot. A 4ppm level of Cd2+ inhibited 50% of the NO3- uptake rate (at saturating NO3- level) compared to Cd2+-less control. Cd2+ inhibition of NO3- uptake (Km 5.88mM) was alleviated as a result of competitive interaction by either Ca2+ (Km 4.34mM) or Zn2+ (Km 4.08mM). Hg2+ ions, however, interacted synergistically with Cd2+ (Km 12.5mM). There was no change in the uptake capacity (Vmax 0.8μmol/μg protein/hr) for NO3- under various treatments.