Production of Fluorescent Dissolved Organic Matter by Microalgae Strains from the Ob and Yenisei Gulfs (Siberia)

Abstract

Dissolved organic matter (DOM) is an important component of aquatic environments; it plays a key role in the biogeochemical cycles of many chemical elements. Using excitation-emission matrix fluorescence spectroscopy, we examined the fluorescent fraction of DOM (FDOM) produced at the stationary phase of growth of five strains of microalgae sampled and isolated from the Ob and Yenisei gulfs. Based on the morphological and molecular descriptions, the strains were identified as diatoms (Asterionella formosa, Fragilaria cf. crotonensis, and Stephanodiscus hantzschii), green microalgae (Desmodesmus armatus), and yellow-green microalgae (Tribonema cf. minus). Three fluorescent components were validated in parallel factor analysis (PARAFAC): one of them was characterized by protein-like fluorescence (similar to peak T), two others, by humic-like fluorescence (peaks A and C). The portion of fluorescence intensity of humic compounds (peak A) to the total fluorescence intensity was the lowest (27 ± 5%) and showed little variation between species. Protein-like fluorescence was most intense (45 ± 16%), but along with humic-like fluorescence with emission maximum at 470 nm (28 ± 14%), varied considerably for different algae strains. The direct optical investigation of FDOM produced during the cultivation of the studied algae strains confirms the possibility of autochthonous production of humic-like FDOM in the Arctic shelf regions.

Keywords: Arctic; PARAFAC; algae; biogeochemical cycles; dissolved organic matter; fluorescence; molecular biology; morphology.

Figure 1

Asterionella formosa Hassal. Strain ARC01. Slide no. 08438. LM, DIC (a–d). SEM, external view (e,g,i,k), internal view (f,h,j). (a–d) size diminution series; (e,f) general view; (g,h) headpole; (i,j) central part; (k) footpole. Scale bars: 10 μm (a–f), 1 μm (g–k).

Figure 2

Fragilaria cf. crotonensis Kitton. Strain ARC03. Slide no. 08440. LM, DIC (a–h). SEM: (i) external view, (j–l) internal view. (a–h) size diminution series, (i–k) general view, (g,h) valve end. Scale bars: 10 μm (a–h), 5 μm (i–k), 1 μm (l).

Figure 3

Stephanodiscus. hantzschii Grunow in Cleve & Grunow. Strain ARC05. Slide no. 08442. LM, DIC: valve face (a–g), valve mantle (h). SEM: (i,j) external view, (k) internal view. (a–i) size diminution series, (i) fragment of valve, (j,k) general view. Scale bars: 10 μm (a–h), 2 μm (j), 1 μm (k).

Figure 4

Desmodesmus. armatus (Chodat) Hegewald. Strain ARC06. Coenobia with different number of cells (a,b). Scale bars: 10 μm.

Figure 5

Tribonema. cf. minus (Wille) Hazen. Strain ARC10. Filaments—(a,b). Scale bars: 10 μm.

Figure 6

Phylogenetic position of strains Asterionella formosa ARC01 and Fragilaria cf. crotonensis ARC03 (indicated in bold) based on Bayesian inference for the partial 18S rRNA and rbcL genes. Total length of the alignment is 1446 characters. Posterior probabilities exceeding 0.9 of BI (constructed by Beast) are presented in order on the nodes. Strain numbers (if available) and GenBank numbers are indicated for all sequences.

Figure 7

Phylogenetic position of the strain Stepanodiscus hantzschii ARC05 (indicated in bold) based on Bayesian inference for the partial 18S rRNA and rbcL genes. Total length of the alignment is 1408 characters. Posterior probabilities exceeding 0.9 of BI (constructed by Beast) are presented in order on the nodes. Strain numbers (if available) and GenBank numbers are indicated for all sequences.

Figure 8

Phylogenetic position of the strain Desmodesmus armatus ARC06 (indicated in bold) based on Bayesian inference for the partial 18S rRNA gene and ITS1–5.8S rDNA–ITS2 region. Total length of the alignment is 2132 characters. Posterior probabilities exceeding 0.9 of BI (constructed by Beast) are presented in order on the nodes. Strain numbers (if available) and GenBank numbers are indicated for all sequences.

Figure 9

Phylogenetic position of the strain Tribonema cf. minus ARC10 (indicated in bold) based on Bayesian inference for the partial 18S rRNA gene. Total length of the alignment is 1041 characters. Posterior probabilities exceeding 0.9 of BI (constructed by Beast) are presented in order on the nodes. Strain numbers (if available) and GenBank numbers are indicated for all sequences.

Figure 10

EEMs (left panel), excitation and emission spectra (right panel) for each identified PARAFAC component.

Figure 11

Contribution of C1–C3 PARAFAC components to the total FDOM fluorescence intensity for the different strains of microalgae at the stationary phase of growth.

Figure 12

(a) IBCAO map of the Arctic Ocean; the red contour indicates the area of the field survey performed in the Kara Sea. (b) Bathymetry of the southern part of the Kara Sea; red circles indicate the locations of vertical thermohaline profiling; red circles with black contours indicate locations of water sampling in the Gulf of Ob (station no. 3935) and the Yenisei Gulf (station no. 3949).

Figure 13

The vertical temperature (a) and salinity (b) structure along the transect in the Gulf of Ob (15–16 August 2021). Water samples analyzed in this study were collected at station no. 3935 (indicated by red).

Figure 14

The vertical temperature (a) and salinity (b) structure along the transect in the Yenisei Gulf (17–19 August 2021). Water samples analyzed in this study were collected at station no. 3949 (indicated by red).