*2.1. Sample Collection and Pretreatment*

Surface lake water and fresh algal scum of the phytoplankton *Microcystis* were collected at the aggregate stage of a cyanobacterial bloom along the shore of Lake Taihu (trestle area), China, in 2021 [42] (Figure S1). Samples (i.e., fresh algal scum) were stored at 4 ◦C and transported to the laboratory, where they were immediately filtered through a sterile 20-μmpore nylon net (47-mm diameter, Merck Millipore, Ireland) to exclude the interference of the largest algal aggregates, protozoa, and non-living particulates in the microbial degradation system. This filtration for lake water through a 20-μm-pore-size filter (47-mm diameter, Merck Millipore, Cork, Ireland) was performed to separate particle-associated bacteria or large cells from small and free-living bacteria [43]. Then, the chlorophyll a (Chl a) concentration was measured (18.61 μg L−1) and considered the low biomass in summer. Chlorophyll a concentration was similar to the annual average in the lake center, and therefore, this was considered as a control set [44]. Meanwhile, harvested algal scum were concentrated through 20-μm bolting silk to partly dewater and remove obvious impurities; then, these samples with 90% moisture were defined as algal organic matter.

We considered the algal organic matter as a whole, which could be divided into two fractions by the freeze-thaw method. First, 10 mL algal organic matter-sterile water solutions with multiple batches (0.03 g mL−<sup>1</sup> fresh weight) were shaken on a shaker, and we then obtained a destructed cyanobacterial complex mixture; the solids of these mixture were concentrated using a 0.2-μm membrane filter (47-mm diameter, PC, Merck Millipore, Cork, Ireland), and the filtrates were obtained. Destruction of cells in the above mixture was performed using the freeze-thaw method with three successive recycles, as described in a previous study [45]. The efficiency of cell destruction was confirmed using an optical microscope [46].

#### *2.2. Batch Experiment*

#### 2.2.1. Long-Term Microbial Degradation of Algal Organic Matter

At room temperature (20–25 ◦C) and under dark conditions, the cyanobacterial scum sampled from Lake Taihu shore and then pretreated as the 0.5 L algal organic mattersterile water solution (0.03 g mL−<sup>1</sup> fresh weight) was added to 9.5 L of filtered lake water containing bacterial communities and mixed in an acid-cleaned glass container to conduct a long-term (80-d) degradation experiment as the algal-derived DOM group. The purpose of setting this algal biomass concentration, i.e., ~1.5 g L−<sup>1</sup> (fresh weight), was to simulate the real accumulation situation along the lakeshore as best as possible. The purpose of using dark incubation conditions was to prevent the possible growth of cyanobacteria that may have been present in filtered lake water. Therefore, the possible growth of cyanobacteria will interfere with the experimental results. Simultaneously, the long-term degradation of

the filtered lake water without added cyanobacterial scum was set as the control group (i.e., natural lake water group). Both groups had three replicates, and all containers were covered tightly to avoid direct contact with the atmosphere. By collecting water samples at different intervals during the 80-d degradation process, the dynamic changes in planktonic bacterial abundance in lake water, and the concentrations and compositions of dissolved organic carbon (DOC) and nutrients were analyzed.
