Showing papers by "Aude Leynaert published in 2004"

Effect of iron deficiency on diatom cell size and silicic acid uptake kinetics

Abstract: We studied the silicic acid uptake kinetics of the pennate diatom Cylindrotheca fusiformis grown under a wide range of iron concentrations (from Fe-limiting to Fe-sufficient conditions) to assess the effect of iron availability on diatom cell size, silicon content, and silicic acid uptake kinetic parameters. As the iron stress increased, the growth rate slowed, cell size decreased, and silicification increased. A series of Si kinetic uptake experiments (from Si-limiting to Si-sufficient conditions) performed at different iron concentrations demonstrated the extent of the colimitation domain, where the specific Si uptake rate (VSi) varied as a function of both silicic acid and Fe availability. A decrease in maximal specific uptake rate of silicic acid (VSi-max) under iron limitation was observed along with a decrease in half-saturation constant for silicic acid uptake ( KSi). Because VSi-max and KSi vary in the same direction, the specific affinity for silicic acid does not change under iron stress. The variation in cell size is an acclimation to low nutrient concentrations. Our study shows that Si uptake kinetics parameters, especially VSi-max, are strongly related to cell size, which is itself constrained by the degree of iron limitation. Thus, the surface to volume ratio should be related to silicic acid flows in size-based biogeochemical models of planktonic ecosystems. Another way to describe the observed variations of Si uptake over the full range of Si and Fe concentrations tested in the present study would be to implicitly take into account the variations in cell size through a scaling of the maximal specific uptake rate (Vmax) by the iron-limitation term in a multiplicative manner. Diatoms account for about 40% of the primary production in highly productive coastal and nutrient-replete oceanic waters (Nelson et al. 1995) and play a major role in the export of organic carbon (Buesseler 1998). Hence, they are important in aquatic ecosystems and in the global carbon cycle. Studying the factors controlling the contribution of diatoms to total production is therefore essential for our understanding of the mechanisms controlling the efficiency of the biological pump of carbon in various regions of the oceans. In particular, their role in high-nutrient low-chlorophyll (HNLC) regions is unclear. These areas have elevated nitrate and phosphate concentrations throughout the year in surface waters, but chlorophyll levels remain ‘‘lower than expected’’ (Minas et al. 1986). Several hypotheses have been proposed to explain the paradox of these areas: high grazing pressure (Landry et al. 1997), unfavorable light-mixing regime (Nelson and Smith 1991), and limitation by nutrients other than nitrate or phosphate, in particular by the micronutrient iron