## 1 Introduction
### Biodeterioration Caused by Microalgae  
Algae are a polyphyletic group of organisms, i.e., not all of them come from the same evolutionary ancestor. Within this group we can find enormous diversity in all aspects. For decades, cyanobacteria were called blue-green algae and were grouped with other algae, mainly because of their ability to photosynthesise. Today, however, cyanobacteria are no longer considered to be algae, but rather are considered to be prokaryotic organisms belonging to the domain of bacteria, whereas all algae are eukaryotic organisms. Nevertheless, in this work we have included cyanobacteria in the term \"microalgae\", since both cyanobacteria and eukaryotic microalgae share important similitudes from an ecological point of view--they both carry out oxygenic photosynthesis, are pioneers in stone colonisation and cause biodeterioration through similar processes. The microalgae that affect the state of conservation of fountains and other stone monuments can be epilithic (growing on the surface of the stone) or endolithic (growing inside it) [9], with the latter being the most damaging and difficult to remove. In any case, the microalgae grow and produce biofilms, which are composed of a set of organisms and the substances they excrete. The microalgae are often embedded in the matrix formed by extracellular polymeric substances (EPS) secreted by the cells themselves. These biofilms affect the stone in different ways. Firstly, they produce an aesthetic alteration, since they form a patina with different natural colourings (light or dark green, orange, brown, etc.) on the stone. Furthermore, biofilms can retain atmospheric dust, pollutants from the air and substances dissolved in water that give them a darker colour. This phenomenon is known as \"soiling\" [10]. These biofilms also cause physical deterioration. Many species of algae and cyanobacteria have sheaths that produce nucilage, which allows them to adhere to the substrate. The nucilage is also used for gliding [11] and to retain water, so that they can survive drought conditions. On absorbing water, the sheaths' volumes vary considerably and their continual expansion and contraction causes the gradual destruction of stony material, which progressively loses granules that go on to form part of the biofilm [12]. The water retention is a fundamental aspect--it fosters mechanical damage due to frost weathering with sharp temperature changes and also allows for the growth of other algae with greater water requirements, so that the communities become more complex. It may also help to support other, more damaging organisms such as fungi and bryophytes [10]. If there is an excessive proliferation of algae, they can affect how the fountains work, for example by blocking spouts, which can speed up other kinds of deterioration. Lastly, microalgae also cause chemical damage as a result of their metabolism. They directly damage the stone by producing organic acids (pyruvate, glycolic, lactic, acetic, succinic or alpha-ketoglut) as a result of respiration. They can also give rise to biotransference phenomena by producing polysaccharides capable of reacting with metals and leading to salts [10]. However, the main damage caused by microalgae is the formation of carbonate crusts on stone as a result of photosynthetic metabolism. Due to the withdrawal of carbon dioxide from the medium and to other biochemical processes, the pH increases and fosters the precipitation and aggregation of calcium carbonate, forming layers of mineral crusts that can end up covering the original materials. All of the taxonomical groups of microalgae are capable of producing this phenomenon, but it is more apparent in green algae and cyanobacteria [13]. In addition to causing aesthetic damage, these crusts foster physical biodeterioration due to thermal contraction and expansion to a different extent than the substrate, leading to stresses that weaken it, and also due to frost weathering, since they have a porous structure that increases water retention further [14].