
In line with our predictions, camelina and barley straw impacted the structure of phytoplankton communities in the studied microcosms differently (Fig. 8). That influence was evident in the number of phytoplankton taxa, total abundance, and the abundance of the particular taxonomic groups and dominating species.
Summary of the main effects of decomposing camelina straw comparison to barley straw on phytoplankton communities’ structure.
Our research revealed that barley straw was the most efficient in reducing the total number of taxa (Supplementary Table S1) and the number of exclusive species (Supplementary Table S2). Meanwhile, aquaria containing camelina straw had more representatives of chlorophytes and euglenoids (which prefer high organic contents) than aquaria with barley straw or no straw (Fig. 2). That indicates that camelina straw positively impacts the diversity of these algae groups. By contrast, conditions lacking straw appeared more conducive to the number of cyanobacterial and diatom species. Moreover, chlorophytes, diatoms, and cyanobacteria had the highest number of representatives/taxa in all three types of aquaria (Fig. 2) compared to the remaining taxonomic groups of algae, which is characteristic of eutrophic water bodies19. The predominance of representatives of these groups resulted from the fact that the water for the study was collected from a lake with high nutrient levels, but also from the fact that during the investigation, a regular supply of nutrients in constant concentrations was provided in all aquaria to replicate the trophic conditions found in eutrophic lakes and ponds affected by ongoing human activities. The selection of appropriate phytoplankton communities (with species composition characteristic of eutrophic water bodies) for this type of experimental research is crucial because it is precisely the water bodies with a high trophic state and with species preferring such conditions that require the most reclamation treatments due to excessive phytoplankton development (especially cyanobacteria). Regrettably, the available research lacks sufficient data on the impact of decomposing straw on phytoplankton species composition. Most studies (e.g10,14). , prioritize analyzing quantitative structure while neglecting the biodiversity aspect. However, investigating the influence of straw on changes in species composition in eutrophic reservoirs is vital because these changes will, in turn, result in potential substantial changes in the food webs of aquatic ecosystems in the future.
In the case of the total phytoplankton abundance in investigated microcosms, temporal changes in each type of aquaria were observed, but also between three types of aquaria (Fig. 3), which was mainly related to the direct effect of camelina and barley straw, but also to some physicochemical parameters of water (especially oxygenation). The decrease in the overall average number of phytoplankton abundance in aquaria with control resulted from the high oxygenation and pH (Fig. 3, Supplementary Table S6), which reached the highest average values in these aquaria (Table 1). High and increasing concentrations of dissolved oxygen inhibited phytoplankton growth. Similar observations were made in other studies20,21 in artificial, microalgal culture systems with air saturation. According to Peng et al.22, the reduction of cell growth or even culture collapse at high concentrations of dissolved oxygen is caused by inhibiting photosynthesis because of photochemical damage to photosynthetic apparatus and other cellular components. In our experiments, the values of oxygen concentrations have increased at least twice as compared to the values noted in the lake, from which water for the aquaria was taken (Supplementary Table S6). An additional factor causing the decrease of the total phytoplankton abundance over time was the increase in pH value, especially in control (Supplementary Table S6), related to the increase in oxygenation. According to Kawecka & Eloranta23, the optimal pH range for most freshwater planktonic algal growth is 6.5–8.5. These values were exceeded in aquaria with straw, but only on the last two dates of the study, and they reached a value of approximately pH = 9 in the last week of the experiment. In the water reservoir from which water was collected for the aquaria, the pH was lower than that in the aquaria during the experiment in the laboratory and amounted to 7.2, which would suggest that this acidity level was optimal for the development of the algae species that occurred in the water we used. However, our statistical analysis showed that the inhibitory effect of changes in the pH of aquarium water on the abundance of phytoplankton and most taxonomic groups was not as significant as the effect of oxygenation (Fig. 6, Supplementary Table S5). The lower pH in the aquaria with straws (compared to the control) could be caused by the release of phenolic acids from the decomposing straw.
The decrease in the overall average number of phytoplankton individuals in aquaria with straw (especially with barley straw) compared to the number in aquaria with control, observed in the first three weeks of the experiment (Fig. 3), suggests an inhibitory effect of straw on planktonic algae at that time, which was confirmed by statistical analyses. The fourth week of research found an opposite tendency: the average abundance values in aquaria with straws were higher than in control. That was only due to a significant increase in the number and share of filamentous green algae (Fig. 4). Their short trichomes accidentally fell into phytoplankton communities, becoming part of them as tychoplankton organisms (originally periphytic), which broke away from compact large structures. They significantly enriched phytoplankton communities until the sixth week of the study, increasing their total abundance. Subsequently, in aquaria with straw, it was recorded that filamentous green algae were replaced mainly by cryptophytes and planktonic chlorophytes (in aquaria with camelina straw) and by planktonic chlorophytes and chrysophytes (in aquaria with barley straw). Thus, our observations have shown that a large share of filamentous green algae does not have to persist long (Fig. 4). Fervier et al.10 stated that macrophytes even replaced filamentous chlorophytes during the experiments using barley straw after time. A similar scenario was entirely possible in the continuation of our investigations because, at the end of the experimental period, an occurrence of growing aquatic higher plants was noted. High concentration and nitrogen inflow favored the development of filamentous green algae (Fig. 6) but should also favor the growth of macrophytes, which will compete with filamentous green algae for nutrients.
When comparing the effects of camelina straw and barley straw on the growth of different algal groups and species, it was found that camelina straw had a more significant inhibiting effect on phytoplankton abundance (Figs. 5, 6 and 7). The decomposition of camelina straw substantially impacted the chrysophytes, cyanobacteria, and dinoflagellates, reducing their abundance (Fig. 6). Remarkably, during our investigations, this variety of straw released significantly greater levels of phenolic acids (primarily gallic and caffeic) than barley straw (Table 1, Supplementary Table S7). Over time, the concentration of these polyphenols increased, selectively acting on systematic groups of algae (Fig. 6). The growth of the three taxonomic algae groups was further exacerbated by additional chemical compounds released by camelina straw, particularly protocatechuic acid and two flavonoids (catechin and quercetin), whose concentrations also progressively increased (Supplementary Table S7). Curiously, barley straw did not emit 2, 5-hydroxybenzoic acid and kaempferol flavonoid.
Eladel et al.13 demonstrated that decomposed rice straw was also found to release gallic and caffeic acids, both of which have inhibiting effects on various cyanobacteria taxa (such as Anabaena sp., Microcystis aeruginosa, Aphanizomenon flos-aquae, Oscillatoria tenuis), chrysophytes (from the genera Dinobryon and Synura), and freshwater algae in general. The results were consistent with ours regarding the inhibitory impact on the same two phytoplankton groups (Fig. 5), even though the straw originates from a different plant. However, certain studies have indicated that rice straw can promote the growth of chlorophytes, such as Chlorella sp.13. For comparison, we discovered that barley straw has a comparable but less potent impact on planktonic chlorophytes (Fig. 5).
Comparing the results of our investigations on camelina straw with other studies regarding the influence of decomposing barley straw on algal growth, the inhibiting effect on cyanobacteria abundance was also found10, but it was a short-term effect. Similarly, some study results also stated the negative impact of barley straw on specific cyanobacteria taxa (Dolichospermum flos-aquae, Microcystis aeruginosa, and Microcystis sp.)24. However, according to Fervier et al.10, barley straw promoted diatom growth, which was not observed in our experiments (Fig. 7). In our experiment, a strong inhibitory effect on algae development was observed for camelina straw, and the impact of barley straw was comparably low (Figs. 6 and 7). Unfortunately, due to the lack of literature on the effect of camelina, we compare the results with data relating to the release of similar polyphenols by straw from other plant species and its impact on algal development.
We also found that camelina straw (mainly by releasing gallic and caffeic acids) strongly stimulated the growth of filamentous chlorophytes (representatives of the genera Oedogonium, Uronema, and Mougeotia), together with the nitrogen inflow and high temperature (Figs. 6, 3 and 7). Those green algae formed compact, macroscopic structures, and smaller threads accidentally found their way into phytoplankton communities (as a tychoplanktonic species). According to other research10, barley straw did not affect filamentous algae growth. On the contrary, Islami & Filizadeh24 stated that barley straw extract negatively affected the growth of some filamentous green algae taxa (Cladophora glomerata and Spirogyra sp.). In our study, the most considerable appearance of filamentous green algae in aquaria with camelina straw, compared to aquaria with barley straw and control, resulted in a decrease in nitrogen in aquaria with camelina straw, probably because filamentous green algae (together with planktonic green algae) intensively absorbed it. Filamentous chlorophytes compete with microalgae for nutrients and light, thus leading to a secondary effect on phytoplankton. That suggests these microalgae will suffer mainly from nutrient competition with green macroalgae in aquatic ecosystems when the share of filamentous chlorophytes increases.
We also found a positive effect of camelina straw (especially phenolic acid – gallic) not only on filamentous green algae (especially Oedogonium species) but also on small, single-celled chlorophyte Carteria sp. (Fig. 7). Most of the other dominating and non-filamentous, planktonic chlorophytes (e.g., Desmodesmus opoliensis, Scenedesmus ecornis, Tetraedron caudatum, Tetradesmus lagerheimii) were also, but weaker associated with gallic and protocatechuic acids and flavonoid catechin. However, many studies proved that barley straw inhibited small planktonic chlorophytes of the genera Ankistrodesmus, Chlorella, and Scenedesmus 11,14. The species of diatoms dominating in our experiment (especially Staurosira construens and Ulnaria acus) seemed indifferent to the effect of polyphenols and flavonoids. Our results align with those showing that diatoms seemed resistant to straw treatment25. In line with our findings relating to the lack of impact of camelina straw on some phytoplankton taxa, barley straw, in some cases, also does not affect selected phytoplankton species according to data from other researchers26.
The impact of polyphenols and flavonoids on other phytoplankton-dominating taxa was negative (Fig. 7). Notably, most of the dominating cyanobacteria (Planktothrix agardhii, Cuspidothrix issatschenkoi, Chroococcus sp., Hyella sp.), cryptophytes (Cryptomonas marssonii, C. erosa, Cyanomonas acuta) and some chrysophytes taxa (e.g., Chromulinasp.), where inhibited mainly by gallic, protocatechuic and caffeic acids. However, Molversmyr27 stated that the growth of bloom-forming cyanobacteria Planktothrix aghardii was stimulated by barley straw.
Some cyanobacteria taxa occurred in lake water (Aphanizomenon flos-aquae, Dolichospermum spiroides, Microcystis aeruginosa) or even dominated in aquaria (Planktothrix agardhii and Cuspidothrix issatschenkoi) in our study are known to be common species, often causing water blooms in eutrophic ecosystems (Supplementary Tables S1 and S2). In addition, P. agardhii, C. issatschenkoi, D. flos-aquae, Limnothrix redekei, M. aeruginosa, Raphidiopsis raciborskii and representatives of the genera Dolichospermum and Oscillatoria can produce toxins (neuro-, hepato- and dermatotoxins) and pose a severe risk to human and animal health28,29, which further increases the importance of using camelina straw in the fight against cyanobacteria. Dominating P. agardhii is one of the most famous bloom-forming species in many shallow eutrophic lakes of temperate climate zone, especially in summertime when the temperatures are high30, typical for turbid mixed environments19. In contrast to algae from other taxonomic groups, cosmopolitan and ubiquitous cyanobacteria are particularly dangerous in large quantities. In addition, they are not eagerly eaten by filter feeders (e.g., zooplankton), so they have no natural enemies that could regulate their numbers31. Therefore, expanding knowledge about natural methods of limiting their development is essential. According to our results, camelina straw seems more effective in preventing their mass occurrence than the commonly used barley straw.
Surprisingly, in microcosms with camelina and barley straw, a lower total molds and bacteria value was noted than in aquaria without straw (Table 1). The low contribution of these organisms in water caused reduced competition with algae for nutrients, which excludes this biotic factor as potentially beneficial in the “fight” against excessive phytoplankton growth. Therefore, decomposing straw did not indirectly inhibit algal growth.