Cyanobacteria, also known as blue-green (micro)algae, are able to sustain many types of chemical stress because of metabolic adaptations that allow them to survive and successfully compete in a variety of ecosystems, including polluted ones. As photoautotrophic bacteria, these microorganisms synthesize aromatic amino acids, which are precursors for a large variety of substances that contain aromatic ring(s) and that are naturally formed in the cells of these organisms. Hence, the transformation of aromatic secondary metabolites by cyanobacteria is the result of the possession of a suitable “enzymatic apparatus” to carry out the biosynthesis of these compounds according to cellular requirements. Another crucial aspect that should be evaluated using varied criteria is the response of cyanobacteria to the presence of extracellular aromatic compounds. Some aspects of the relationship between aromatic compounds and cyanobacteria such as the biosynthesis of aromatic compounds, the influence of aromatic compounds on these organisms and the fate of aromatic substances inside microalgal cells are presented in this paper. The search for this information has suggested that there is a lack of knowledge about the regulation of the biosynthesis of aromatic substances and about the transport of these compounds into cyanobacterial cells. These aspects are of pivotal importance with regard to the biotransformation of aromatic compounds and understanding them may be the goals of future research.

The dynamic increase in the commercial application of antimicrobial derivatives of boronic acids, and potential impact of their presence in aquatic systems, supports the necessity to study the toxicity of these substances towards microorganisms of crucial meaning in the environment. One example of the mentioned derivatives is tavaborole (5-fluoro-substituted benzoxaborole), a pharmaceutical agent with antifungal activity. Cyanobacteria were used as model organisms, which are photoautotrophic prokaryotes, as representative aquatic bacteria and photoautotrophs associated with the plant kingdom. To the best of our knowledge, we investigated this issue for the first time. In order to recognize the under-stress response of those microorganisms, the concentration of photopigments—a key factor in the activity of photosynthetic apparatus—was measured spectrophotometrically. We found that the 3-piperazine bis(benzoxaborole) significantly suppressed the growth of halophilic and freshwater cyanobacteria, at a concentration 3.0 mM and 0.3 mM, respectively. Our results also showed that the tested substances at micromolar concentrations stimulated the growth of cyanobacteria, particularly in the freshwater strain Chroococcidiopsis thermalis. The tested substances acted with various strengths, depending on their structure and concentration; nevertheless, they had a greater influence on the synthesis of phycobiliproteins (e.g., lowered their concentration) than on the formation of chlorophyll and carotenoids.

Phycobiliproteins - colorful proteins produced by microalgae and cyanobacteria, owe their unique properties to prosthetic groups, which are open-chain tetrapyrroles. As they are produced in the biosynthesis process, phycobiliproteins possess special biotechnological potential also due to their useful physicochemical properties. A classic example is the use of these proteins - especially phycocyanin with an intense blue color - as natural dyes in the food, cosmetic and pharmaceutical industries. In the products to which they are added, they are responsible not only for color, but also for various biological activities, providing these products with additional values. In this category, the most frequently mentioned are the antioxidant properties of phycobiliproteins, which may be used in the treatment of neurodegenerative diseases, and the fluorescent properties, especially of phycoerythrin, which also predispose these proteins to use in biomedical research. The results of contemporary work on the acquisition and use of phycobiliproteins indicate that the key to the wider use of these substances is to understand the process of biosynthesis of individual groups of these colored proteins, as well as developing ways to effectively control their production. Due to the different physicochemical properties and biological activity of individual, structurally diverse phycobiliproteins, the specificity of the actions taken is extremely important in this case. Since there are real reasons to increase the scale of production of these substances by modifying the conditions of microalgae cultivation, we believe that obtaining larger amounts of specific phycobiliproteins in pure form will intensify the current use and open up new possibilities for the use of these valuable proteins.