Details are outlined in a study led by the ICM-CSIC, which argues that these microorganisms, among the most abundant in the ocean, challenge classical principles of ecology and evolution.
An international team co-led by the Institut de Ciències del Mar (ICM-CSIC) has unveiled new insights into the global distribution and diversity of Picozoa, a little-studied lineage of heterotrophic picoeukaryotes that form a significant part of marine microbial communities. The results, published in the journal Microbiome, contribute to understanding the evolutionary dynamics and ecological strategies of marine protists, which are essential to the functioning of ocean ecosystems.
Picozoa rank among the ten most abundant groups of unicellular eukaryotes in marine environments. However, until now, their study has been limited to general analyses, providing only a narrow view of their ecology and global distribution. This work adopts a comprehensive approach, combining phylogenetic analyses, species distribution modelling, and ecological niche characterisation.
According to the study, Picozoa—classified as part of the picoplankton and measuring just 3 micrometres—inhabit a wide variety of oceanic environments, from polar regions to tropical waters. They are believed to act as predators, feeding on bacteria and playing a key role in marine food webs by regulating other microbial populations.
“As one of the abundant protist groups in the ocean, the ecological importance of Picozoa is undeniable, although their specific role in the food web is yet to be clarified,” explains Ramon Massana (ICM-CSIC), an author of the study.
An evolutionary puzzle
Picozoa are not only abundant and widely distributed but also exhibit a complex phylogeny, encompassing a considerable number of species organised into five groups. This study aimed to address some of the enigmas about their global distribution, their capacity to adapt to diverse environments, and how their ecological strategies differ across species.
“We know that Picozoa are important for the ecological balance of the ocean, and these new findings about their diversity and distribution offer fascinating clues about how these organisms have evolved and adapted to different environmental conditions,” notes Ramiro Logares (ICM-CSIC), another author of the study.
To conduct the research, the scientific team combined phylogenetic analyses with ecological niche models designed to predict species distribution based on environmental data. They used DNA data compiled from the global EukBank database, which includes data on a protist marker gene, alongside environmental variables such as temperature, salinity, and nutrient levels, from thousands of ocean samples. Predictive models allowed the researchers to map the potential distribution of Picozoa across the global ocean, including areas not directly sampled, revealing striking patterns.
The findings confirmed that Picozoa species can be categorised into three main groups based on their distribution: widely distributed species, polar species, and non-polar species. To the surprise of the researchers, phylogenetically related species did not share similar distributions, a phenomenon known as phylogenetic overdispersion, which challenges the classical concept of phylogenetic niche conservation. This principle traditionally posits that closely related species occupy similar ecological niches.
“The study’s results suggest that closely related species can adapt to very different conditions, reflecting diverse ecological strategies,” the team reports.
Implications for science
According to the authors, the results of this research mark an important step toward understanding the evolutionary and ecological dynamics of previously understudied marine microorganisms. Moreover, they emphasise the importance of studying these tiny organisms at both genomic and ecological levels, given their critical role in marine ecosystems.
“The ability of Picozoa species to adapt to different environments suggests complex evolutionary histories that could help us better understand the impacts of climate change and other environmental disruptions on marine biodiversity through the spatiotemporal monitoring of species distribution,” the researchers conclude.
Overall, the study highlights that even the smallest organisms in the ocean can hold key insights into the evolution and functioning of ecosystems on a global scale.