Si–C interactions during degradation of the diatom Skeletonema marinoi

Abstract

International audienceWhile a relationship between ballast and carbon in sedimenting particles has been well-documented, the mechanistic basis of this interaction is still under debate. One hypothesis is that mineral ballast protects sinking organic matter from degradation. To test this idea, we undertook a laboratory experiment using the diatom Skeletonema marinoi to study in parallel the dissolution of one of the most common mineral ballasts, biogenic silica (bSiO2), and the associated degradation of organic matter. Three different models were applied to our results to help elucidate the mechanisms driving bSiO2 dissolution and organic compound degradation. Results of this modeling exercise suggest that the diatom frustule is made up of two bSiO2 phases that dissolve simultaneously, but at different rates. In our experiments, the first phase was more soluble (View the MathML source) and made up 31% of the total bSiO2. In this phase, bSiO2 was mainly associated with membrane lipids and the amino acids glutamic acid, tyrosine, and leucine. The second phase was more refractory (View the MathML source), and contained more neutral lipid alcohols and glycine. Until it dissolved, the first bSiO2 phase effectively protected much of the organic matter from degradation: particulate organic carbon (POC) degradation rate constants increased from 0.025 to 0.082 d−1 after the total dissolution of this phase, and particulate organic nitrogen (PON) degradation rate constants increased from 0.030 to 0.094 d−1. Similar to POC and PON, the total hydrolyzable amino acids (THAA) degradation rate constant increased from 0.054 to 0.139 d−1 after dissolution of the first bSiO2 phase. The higher THAA degradation rate constant is attributed to a pool of amino acids that was produced during silicification and enclosed between the two silica phases. This pool of amino acids might come from the incorporation of silica deposition vesicles into the diatom wall and might not be directly associated with bSiO2. In contrast, most lipid degradation was not prevented by association with the more soluble bSiO2 phase, as the average lipid degradation rate constant decreased from 0.048 to 0.010 d−1 after 17 d of degradation. This suggests that most lipids were associated with rather than protected by silica, except pigments that appeared resistant to degradation, independently from silica dissolution. When the only organic compounds remaining were associated with the second bSiO2 phase, degradation rate constants decreased greatly; concentrations changed only slightly after day 25

Microbial mediation of benthic biogenic silica dissolution

“…It has been hypothesized by Lee et al (2000) and Hedges et al (2001) that mineral ballast (BSi, CaCO 3 , and terrigenous compounds) inhibit carbon mineralization before reaching the sediment surface. Then, the correlation of benthic Si release and O 2 uptake in the dataset could be a result of BSi protecting the cytoplasmic OM from decay, as proposed by Armstrong et al (2002) and Moriceau et al (2009) for sinking particles in the water column. Applied to the sediment, this would explain correlating BSi dissolution and OM mineralization without the need of bacterial mediation of BSi dissolution.…”

PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies

“…Laboratory experiments show that the diatom frustule is made of two biogenic silica phases which dissolve simultaneously, but at different rates (e.g., Kamatani et al, 1980;Van Capellen et al, 2002;Moriceau et al, 2009;Loucaides et al, 2012). The first phase dissolves significantly faster than the second phase.…”

“…In fact, observations are rather contradictory on this ballast effect ). In particular, the greater efficiency of the vertical sedimentation of organic matter when associated with calcite and biogenic silica may be due rather to the protection of an organic matter fraction by the inorganic matrix (Moriceau et al, 2009;Engel et al, 2009). …”

“…The first phase dissolves significantly faster than the second phase. It is associated with membrane lipids and amino acids and represents about one-third of the frustule (Moriceau et al, 2009). However, the existence of these two phases is still a matter of debate as it has been hypothesized to be a result of the experimental design of the dissolution experiments (Loucaides et al, 2012).…”

Biological silicon removal from coal seam gas water

“…Macronutrients: Diatoms are primarily formed using carbon, nitrogen, silicon, sulphur, potassium and phosphorous (Macronutrients) [56], [57], [33], [58], [60]. For diatoms to form, these elements must be biologically available from the water, or introduced as carbon which is sometimes is introduced from the atmosphere.…”