Abstract
The element stoichiometry of bacteria has received considerable attention because of their significant role in marine ecosystems. However, relatively little is known about the composition of major structural elements of the unicellular heterotrophic protists—thraustochytrids, despite their widely recognized contribution to marine nutrient cycling. Here, we analyze the cell volume and elemental C, N, H, and S cell content of seven cultured thraustochytrids, isolated from different marine habitats, in the exponential and stationary growth phases. We further derive the relationships between the cell volume and elemental C and N content of the cultured thraustochytrids. The cell volumes varied significantly (p < 0.001) among the isolates, with median values of 96.9 and 212.5 μm3 in the exponential and stationary phases, respectively. Our results showed a significantly higher percentage of C (64.0 to 67.5) and H (9.9 to 13.2) but a lower percentage of N (1.86 to 2.16) and S (0.34 to 0.91) in the stationary phase, along with marked variations of C and N fractions among isolates in the exponential phase. The cell C (5.7 to 203.7 pg) and N (0.65 to 6.1 pg) content exhibited a significant (p < 0.001) linear relationship with the cell volume (27.7 to 510 μm3). On further analysis of the relationship across the two growth phases, we found the equation (cell C (pg) = 0.356 × cell volume (μm3) + 20.922) for stationary phase cells more appropriate for C estimation of natural thraustochytrids. This study provides the first experimental evidence of higher cell C density than the current estimate and relatively larger C contribution of thraustochytrids than bacteria to the marine organic pool.
Highlights
Introduction published maps and institutional affilLabyrinthulomycetes are widely distributed, saprotrophic, or only weakly parasitic fungus-like microorganisms, ubiquitous in estuarine and marine environments [1].The morphologically described labyrinthulomycetes consist of a small group of almost exclusively marine genera—the thraustochytrids—with typically saprotrophic or bacteriotrophic, and occasionally holozoic nutrition [1,2]
The cell mass values of seven different thraustochytrid isolates ranged from 11.4 ± 3 (PKU#SW8) to 82.1 ± 5.5 pg (PKU#Mn4) in the exponential phase, and from 79.1 ± 1.7 (PKU#SW8) to 306.4 ± 20 pg (PKU#Mn4) in the stationary phase (Figure 1a)
Compared to the exponential phase cells, the stationary phase cells of all the isolates showed a marked increase in their dry weight mass
Summary
The cell mass values of seven different thraustochytrid isolates ranged from 11.4 ± 3. Compared to the exponential phase cells, the stationary phase cells of all the isolates showed a marked increase in their dry weight mass. The cell diameter of the exponential phase cells ranged from 3.75 ± 0.18 to 7.15 ± 0.22 μm while that of the stationary phase cells ranged from 5.72 ± 0.13 to 9.91 ± 0.18 μm (Figure 1b). The cell volume of the isolates ranged from 27.7 ± 4.0 (PKU#SW8) to 191.7 ± 17.5 μm (PKU#Mn4) in the exponential phase, and from 97.9 ± 6.5 (PKU#Mn16) to 510 ± 27.9 μm (PKU#Mn4) in the stationary phase (Figure 1c). The cell volumes varied significantly (p < 0.001) among the isolates, with the median values of 96.9 ± 22.8 and 212.5 ± 54 μm in the exponential and stationary phases, respectively.
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