2.3.3. Bacterial Abundance (BA)

Samples for analyses of BA were preserved with a final concentration of 0.5% glutaraldehyde and stored at −80 ◦C. BA was measured using an LSRFortessa flow cytometer (BD Biosciences, San Jose, CA, USA) by staining with SYBR Green I. Bacteria were enumerated according to a previously described method [53].

#### *2.4. Statistical Analysis*

The linear regression model was used by OriginPro 8.1 software (OriginLab, Northampton, MA, USA) to characterize the following relationships: (1) among the free-living bacterial abundance and CDOM absorption coefficient at 355 nm for the entire process in the bulk algal-derived DOM group; (2) among free-living bacterial abundance and the main components of FDOM identified by PARAFAC analysis for the entire process in bulk algal-derived DOM, algal-derived filtrate, and algal residual exudative organic matter groups; and (3) among free-living bacterial abundance and the fluorescence intensity of humic-like components for the day 40–80, day 20–80, and day 31–80 phases in the bulk algal-derived DOM, algal-derived filtrate, and algal residual exudative organic matter groups, respectively. To examine the significance of the temporal changes in the main components of FDOM identified by PARAFAC analysis during the experiment, we mainly aimed at comparing the bulk algal-derived DOM and control groups, and the algal-derived filtrate, and algal residual exudative organic matter groups. One-way analysis of variance (ANOVA) was performed using the data analysis function of OriginPro 8.1 software.

#### **3. Results and Discussion**

#### *3.1. Algal Organic Matter Analysis*

The initial properties of the selected algal-derived DOM and its different fractions are summarized in Table 1. In brief, the initial bulk algal-derived DOM was composed of 67.15% carbon, 26.38% nitrogen, and 6.47% phosphorus, whereas the initial algal-derived filtrate and residual exudative organic matter were composed of 77.86% and 58.33% carbon, 19.55% and 39.06% nitrogen, and 2.59% and 2.60% phosphorus, respectively. These values showed that the carbon component of the algal-derived filtrate was higher than that of the bulk algal-derived DOM; the nitrogen component of algal residual exudative organic matter was higher than that of the bulk algal-derived DOM. In contrast, the phosphorus components of the algal-derived filtrate and algal residual exudative organic matter were lower than that of the bulk algal-derived DOM. After freeze-thaw treatment, the CDOM absorption coefficients at 355 nm for initial algal-derived filtrate and algal residual exudative organic matter were approximately 0.79 and 0.63 times lower than that of initial bulk algal-derived DOM, respectively. Obviously, the FDOM composition of the initial bulk algal-derived DOM contained only two components (i.e., protein-like C1 component and humic-like C2 component), and C1 was the main fraction in the bulk algal-derived DOM. Surprisingly, through the comparison among the FDOM composition for initial bulk algal-derived DOM, filtrate, and algal residual exudative organic matter, it appeared that the humic-like C2 component was enriched in the algal-derived filtrate, which was approximately > 6 times higher than that of the other two groups.


**Table 1.** Physicochemical properties of bulk algal-derived dissolved organic matter (DOM), algalderived filtrate, and algal residual exudative organic matter. ND—not determined. DOC—dissolved organic carbon; DON—dissolved organic nitrogen; DOP—dissolved organic phosphorus.
