**1. Introduction**

In recent years, the excessive growth of phytoplanktonic organisms in reservoirs, lagoons, and in general, in lentic, low-tide water bodies with a high concentration of phosphate and nitrogenous nutrients, which are easily assimilated, has generated worldwide concern [1, 2]. Physical factors such as temperature, solar radiation, wind, rain, water column stratification, water flow, or biological interactions with other organisms, among others, play an important role in the so-called cyanobacterial harmful blooms (CyanoHAB) [3, 4].

CyanoHABs can be defined as events in which visually noticeable turbidity of the water occurs due to a rapid accumulation of cyanobacterial cells, often at the water surface, but sometimes deeper in the water column [3, 5]. These blooms have the potential to generate a variety of adverse effects due to their ability to produce toxins [6, 7] that, in turn, cause negative impacts on animals, including humans, aquatic ecosystems, the economy, drinking water supply, real property values, and recreational activities, including swimming and commercial and recreational fishing [8, 9].

CyanoHAB-forming cyanobacteria are often accompanied by a variety of lesserknown taxa that contribute greatly to the total cyanoHAB biomass, such as nanocyanobacteria and picocyanobacteria [10]; e.g., Vardaka et al. [11] have described blooms composed of multiple species.

Picocyanobacteria are bacteria that play a key role in primary production and dominate phytoplankton biomass in both oligotrophic and eutrophic waters [12, 13]. They are the smallest cell-sized, most numerous, and ubiquitous cyanobacteria in freshwater, marine, and even in environments with high salt concentrations [14]. Among the cyanobacteria are the so-called planktonic picocyanobacteria; microorganisms that are part of the smallest aquatic plankton and are often associated with various species [15]. In freshwater, the main representatives are the genera *Synechococcus*, *Cyanobium,* and *Synechocystis*, and in brackish water, *Synechococcus* and *Prochlorococcus* predominate [13, 16].

Although this group of microorganisms is ubiquitous and causes environmental concerns, it is still understudied [13]. Much research continues to use microscopy techniques that require long processing times and can produce erroneous results [17] since picocyanobacteria are difficult to observe and most of the time are found forming groups or present diverse biological forms ranging from single cells to microcolonies [18]; besides, their physiological or epigenetic plasticity means that cyanobacteria with the same genotype can appear very different due to the external factors to which they are influenced [19]. This is determined by the growth conditions, adaptations, and expansion of the cells in response to the stay in complex communities and fluctuating environments [20].

Recently, attempts have been made to study picocyanobacteria through molecular techniques by amplification and sequencing of the 16S rRNA gene or next-generation sequencing (NGS), which allows obtaining results quickly, with high sensitivity and high detection efficiency [21]. However, research aimed at describing picocyanobacteria present in surface waters is atomized, moreover, it is limited and there are no current review articles focused on this. Therefore, this study aims to summarize the results of original research articles on the predominant picocyanobacteria in surface water bodies collected from indexed journal articles and gray literature involving the molecular identification of picocyanobacteria. It also provides an understanding of the factors that influence the predominance of picocyanobacteria in these environments, such as trophic status and the method of molecular identification, as well as research trends and the countries that contribute most to this field of research.

### **2. Materials and methods**

#### **2.1 Data collection**

This research was conducted as described in the PRISMA Declaration [22]. Thus, for the development of this study, a systematic literature search was carried out in


#### **Table 1.**

*Searches applied for the selection of articles in the three databases used.*

three bibliographic databases: Scopus, ScienceDirect, and Scielo, which articles are part of publications in indexed journals [23]; in addition, the Google Scholar search engine was used, focused and specialized in the search for scientific-academic content and bibliography that includes gray literature defined by Garousi et al. [24] as: "literature that is not formally published in sources such as books or journal articles". This allowed an exhaustive search, broadening the information to be analyzed.

Keywords were defined using free terms and the Agrovoc and DeCS thesauri to increase the sensitivity of the search: picocyanobacteria, freshwater, sweetwater, small cyanobacteria, reservoirs, dams, lakes, lagoons, and small blue-green algae. At the same time, Boolean operators (AND and OR) were used to logically connect concepts or groups of terms and to quickly broaden, specify, limit, and define the search (**Table 1**) see annex.

We then proceeded to eliminate duplicate articles using the free tool Zotero-5.0.93. Three investigators independently applied the inclusion and exclusion criteria presented in **Table 2** (see annex) to the resulting articles to avoid bias and ensure reproducibility of the selection.

#### **2.2 Data analysis**

The statistical program R Studio® (V 3.6.1) was used to perform the descriptive analysis of the collected data. A database was created using Microsoft Excel where


**Table 2.** *Inclusion and exclusion criteria established and applied to each article for eligibility.* certain attributes of the research were recorded, such as year of publication, the country where the article was published, journal, picocyanobacteria species, water body, trophic state, and picocyanobacteria molecular identification method. Particularly, using this database, the relative and absolute frequencies of the number of publications of picocyanobacteria per year, the number of publications per country, and the number and species of picocyanobacteria most frequently found were determined, as well as some factors related to the prevalence of picocyanobacteria in surface water bodies.

The free software VOSviewer (V 1.6.14) was used to analyze the data on the frequencies of index and author keywords to determine the most frequent keywords researched in the articles included in the systematic review and thus identify trends in research on the topic.
