This study demonstrated the effects of bamboo extract on the growth of three algal species based on batch culture experiments. Both mono- and co-culture experiments suggest that the extract can suppress the growth of the cyanobacterium M. aeruginosa, while it can promote the growth of the competitive diatom N. palea. Multiple pathways of the effect of bamboo on cyanobacteria are possible, including direct and indirect effects of the extract. Few studies have shown such opposing effects of extracts between cyanobacteria and other algal species; thus, these observations might be specific to bamboo and related plants. The remarkable effect of bamboo extract on colony-forming cyanobacteria suggests that bamboo may perform advantageously in field ponds and lakes where colony-forming cyanobacteria occur, as allelopathic effects of other plants are usually tested for singular cells without protecting systems, which rarely occur in the stages of proliferation.
We revealed that the growth of M. aeruginosa was largely inhibited by bamboo extract at a concentration of 5 g L−1. Although the effect on growth was less clear in 1.0 and 2.5 g L−1 treatments, physiological changes induced by these treatments were suggested by the culture pH and stress indicators. An increase in pH to >10 in cultures is normal for M. aeruginosa, which is adapted to higher pH and the availability of CO2 in water [59]. Lower pH in the extract treatments than in the control suggests a decline in algal photosynthetic activity by the extract. The stress indicators MDA content and activities of antioxidant enzymes (SOD, POD, and CAT) behaved in a concentration-dependent manner, except for the 5.0 g L−1 treatment. The minimum levels of these indicators in the 5.0 g L−1 treatment could suggest a malfunction of cells rather than a reduced stress in the treatment.
In this study, extracts of 1 g L−1 and lower concentrations had little or no effect on the growth of M. aeruginosa. Although previous studies using other plants often showed effects of extracts with concentrations of 1–10 g L−1 or higher [e.g., 58,60], effects with much lower concentrations have also been reported (e.g., 0.2 g L−1 of leaves of Eucalyptus trees, [61]; 0.05 g L−1 of decomposed barley straw, [62]). Thus, the algistatic level of bamboo extract in this study was not specifically high among reported effective plants.
No apparent decrease in the growth of S. obliquus and N. palea, even in the 5 g L−1 treatment, suggests that the bamboo extract can selectively inhibit the growth of cyanobacteria. Selective inhibitory effects of plant extracts on cyanobacteria have been demonstrated in other studies [63, 64], which could be attributed to the lack of a defense system in cyanobacteria against allelochemicals such as those developed in green algae and diatoms. Our results rather showed a promotive effect of bamboo extract on the growth of N. palea in the 5 g L−1 treatment. Few studies have shown such opposing effects between cyanobacteria and other algal species simultaneously. Eladel et al. [65] reported that extract of rice straw inhibited the growth of cyanobacteria (Anabaena), while stimulating the growth of green algae (Chlorella). According to their review, extracts from barley and rice straws often inhibit the growth of cyanobacteria, including Microcystis, while stimulating the growth of green algae and diatoms as a result of high nutrient (NO3 and PO4) release [65]. It is interesting that both straw plants and bamboo belong to the family Poaceae and have many common properties, including high silica content [46].
It appears that, in the co-culture systems without bamboo extract, M. aeruginosa and S. obliquus affected each other with less competitive advantage, whereas M. aeruginosa was competitively superior to N. palea. Growth parameters of each species were lower in the co-culture system than in the mono-culture system, except for the r of S. obliquus. It is known that S. obliquus can outcompete M. aeruginosa in cultures without herbicides or with low to moderate pH and temperature conditions [e.g., 49,66]. The diatom N. palea has been shown to be a poor competitor for nutrients among various algal species [67].
The percent change in growth parameters from the mono-culture to co-culture suggests that the relationship between M. aeruginosa and its competitor was modified by the bamboo extract. A greater percentage decrease in the µ of M. aeruginosa in the 5.0 g L−1 treatment than in the control implies that negative impacts from competitors increased in the presence of the extract. Impaired growth of M. aeruginosa in co-culture was also observed as a non-sigmoidal increase (e.g., decrease, slow exponential increase) of cells in the 2.5 and 5.0 g L−1 treatments. In contrast, a lower percentage decrease in µ of S. obliquus and N. palea for the 5.0 g L−1 treatment compared to the control implies that the negative impact by M. aeruginosa was reduced by the extract. The numerical superiority between M. aeruginosa and N. palea in the control was completely reversed in the 5.0 g L−1 treatment. The greater effects of N. palea on M. aeruginosa in the 5.0 g L−1 treatment compared to those of S. obliquus may be attributed to its periphytic nature, as discussed in more detail in the following paragraph.
The ability of bamboo extract to control M. aeruginosa by promoting the growth of competitors was enhanced in the colony-forming M. aeruginosa. In co-culture with colony-forming M. aeruginosa, M. aeruginosa increased and N. palea decreased in the control, whereas in the 5 g L−1 treatment, M. aeruginosa decreased by less than one-seventh and N. palea increased more than 105-fold. It appears that the colony triggered a rapid death of M. aeruginosa, rapid growth of N. palea, or both in the bamboo extract. A unique feature of N. palea is its adhesion to, and gliding movement on, the substrate, which enables them to invade and consume the colony of M. aeruginosa [51, 52]. We observed that the colonies of M. aeruginosa, which were initially intact and floating, became partially fragmented, etiolated, and sank (settled to the bottom) in the extract treatment. The invasion of M. aeruginosa by N. palea was possibly accelerated by the settlement of the colony. However, the process that induced the loss of floating ability of the colonies, whether they were degraded by the extract or by the invasion of N. palea, is unknown.
Our supplemental observations show the importance of bamboo as a substrate for the growth of N. palea (Fig. 6). The colony-forming M. aeruginosa with N. palea taken from Lake Taihu was inoculated equally into six vials (8 mL) with BG-11 medium, and a toothpick made of bamboo was placed in two of the six vials. Incubation conditions were the same as those in the main experiments. There was no visible change in the color of the cultures for several weeks, but the color in vials with toothpicks changed to yellowish after 7–8 weeks (Fig. 6a). Microscopic observations at day 60 revealed that in the vials with toothpicks all M. aeruginosa had sunk and N. palea had increased substantially (Fig. 6b). In addition, a large number of N. palea formed flocks and adhered to the surface of the toothpicks (Fig. 6c, 6d, 6e). The yellowish color was primarily N. palea that adhered to the vial walls. The lack of coloring of the culture together with a lack of decrease in total number (float + sink) of M. aeruginosa suggest that the leachate from the toothpicks was limited. This implies that the toothpick contributed as a growing substrate for N. palea and subsequently as a source of invaders for the colony of M. aeruginosa.
We suggest that bamboo can suppress M. aeruginosa through inhibiting its growth by dissolved chemicals and through stimulating the growth of competitors, especially diatoms, either as a result of dissolved chemicals or of providing plant material as a substrate for growth (Fig. 7). The effective extract concentration in this study (i.e., 1–5 g L−1) is not easy to obtain in the field other than in small ponds, because 1–5 kg of bamboo is needed for 1 m3 of water. Planting bamboo stands along the shoreline would increase DOM associated with bamboo and also increase the available substrates for diatoms and green algae (Fig. 7). In addition, bamboo poles are known to be effective substrates for developing periphyton biofilms dominated by green algae and diatoms [43–45]. Periphyton has also garnered the attention of researchers for controlling the growth of cyanobacteria [55, 56]. Bamboo enables such synergetic effects of DOM and substrates in controlling cyanobacteria, which would be more feasible for application in the field.
The colony of M. aeruginosa, which plays a key role in dominance and bloom formation, may also be a clue to degeneration. The colony (sheath and mucilage), which mainly consists of extracellular polysaccharides, protects cells from predators, and mitigates stress under less suitable environmental conditions [54, 59]. The colony also provides nutrients for their cells under low nutrient conditions and it harbors a symbiotic bacterial community [68, 69]. However, the results of the present and previous studies [52] suggest that the colony can turn into a substrate for attachment and food for certain algal species. It is assumed that the barrier system of the colony is weakened by a certain stress from the external environment (e.g., in the presence of bamboo extract), which subsequently allows the invasion of competitors.
The control of cyanobacteria using bamboo might be categorized as a biological control in a conventional classification. However, using bamboo differs from common case, where a plant or animal that directly suppresses cyanobacteria, is introduced and maintained in an aquatic system. Using bamboo also differs from cases that focus on allelochemicals of plants, in which selectivity, algistatic level, dosage, and persistence, are major concerns, more similar to chemical control. Because we expect that modulating the water- and habitat-environment to enhance competitors of cyanobacteria is the major way that bamboo effects are mediated, interventions using bamboo could be termed habitat-environmental control. This study revealed the potential roles of bamboo extract in controlling cyanobacteria using widespread species; however, several important questions, including relevant chemicals, variations among plant organs and among life stages, the processes involved in the increase and decrease of algal species, and the actual effects in the field need to be addressed through further studies.