Silicon Metabolism in Diatoms: Implications for Growth

Abstract

Diatoms are the world's largest contributors to biosilicification and are one of the predominant contributors to global carbon fixation. Silicon is a major limiting nutrient for diatom growth and hence is a controlling factor in primary productivity. Because our understanding of the cellular metabolism of silicon is limited, we are not fully knowledgeable about intracellular factors that may affect diatom productivity in the oceans. The goal of this review is to present an overview of silicon metabolism in diatoms and to identify areas for future research. Numerous studies have characterized parameters of silicic acid uptake by diatoms, and molecular characterization of transport has begun with the isolation of genes encoding the transporter proteins. Multiple types of silicic acid transporter gene have been identified in a single diatom species, and multiple types appear to be present in all diatom species. The controlled expression and perhaps localization of the transporters in the cell may be factors in the overall regulation of silicic acid uptake. Transport can also be regulated by the rate of silica incorporation into the cell wall, suggesting that an intracellular sensing and control mechanism couples transport with incorporation. Sizable intracellular pools of soluble silicon have been identified in diatoms, at levels well above saturation for silica solubility, yet the mechanism for maintenance of supersaturated levels has not been determined. The mechanism of intracellular transport of silicon is also unknown, but this must be an important part of the silicification process because of the close coupling between silica incorporation and uptake. Although detailed ultrastructural analyses of silica deposition have been reported, we know little about the molecular details of this process. However, proteins occluded within silica that promote silicification in vitro have recently been characterized, and the application of molecular techniques holds the promise of great advances in this area. Cellular energy for silicification and transport comes from aerobic respiration without any direct involvement of photosynthetic energy. As such, diatom silicon metabolism differs from that of other major limiting nutrients such as nitrogen and phosphorous, which are closely linked to photosynthetic metabolism. Cell wall silicification and silicic acid transport are tightly coupled to the cell cycle, which results in a dependency in the extent of silicification on growth rate. Silica dissolution is an important part of diatom cellular silicon metabolism, because dissolution must be prevented in the living cell, and because much of the raw material for mineralization in natural assemblages is supplied by dissolution of dead cells. Perhaps part of the reason for the ecological success of diatoms is due to their use of a silicified cell wall, which has been calculated to impart a substantial energy savings to organisms that have them. However, the growth of diatoms and other siliceous organisms has depleted the oceans of s...

Structure and mechanical properties of selected protective systems in marine organisms

“…Diatoms, commonly found throughout the ocean, produce rigid external cell walls (or exoskeletons) formed from hydrated amorphous silica and range in diameters from 10-150 μm [117,118]. Despite their often microscopic size, the sheer biomass of diatoms causes them to account for 40% of the primary production of carbon [119] and the majority of biogenic silica (silica transformed from dissolved silicate into skeletal material) [120] in the oceans.…”

“…14c) [128]. The structure of the diatom exoskeleton is controlled by the availability of the necessary nutrient: silicic acid (the naturally available silica precursor in the oceans), causing both the strength and structure to diminish when depleted [117].…”

The relationship between light intensity and nutrient uptake kinetics in six freshwater diatoms

“…Si uptake kinetics of the present experiment belongs to one of the three classic models of Si uptake, surge uptake (Martin-Jézéquel et al, 2000). Surge uptake occurred when intracellular Si level was very low and the maximum uptake rate could be obtained in this process (Conway and Harrison, 1977;Conway et al, 1976).…”

“…The affinity differences in Si uptake can be influenced by various types of silicic acid transporter (SIT) genes and the synthesis of higher affinity transporters (Martin-Jézéquel et al, 2000). Thamatrakoln et al (2006) pointed that there were structural differences of SITs between centrics and pennates.…”

“…Leynaert et al (2009) observed that small size phytoplankton tended to dominate in nutrient-poor water columns, whereas large size phytoplankton dominated in nutrient-rich regions. Martin-Jézéquel et al (2000) found that silicification of diatoms can be influenced by light which improves Si uptake by promoting phytoplankton growth (Xu et al, 2009). In Monterey Bay, Si uptake of algae in upwelled waters increased with the growing light intensity (Kudela, 2008).…”