acI lineage Actinobacteria
acI lineage (Actinobacteria)
The acI lineage is a group of ubiquitous and dominant freshwater bacteria within the class Actinobacteria, often accounting for up to 50% of the total bacterioplankton in lakes. They are characterized by extremely small genomes and cell sizes, making them typical ultramicrobacteria. These organisms play a crucial role in the carbon cycling of freshwater ecosystems globally.
Details
Identification
Extremely small, rod-shaped cells (ultramicrobacteria); identification is primarily performed via 16S rRNA gene sequencing or FISH (Fluorescence in situ hybridization).
Social behavior
Free-living in the pelagic zone (plankton), does not form colonies.
Diet
Photoheterotrophic; they utilize light energy via actinorhodopsin and preferentially uptake low-molecular-weight organic compounds such as carbohydrates and amino acids.
Overwintering
Persistence in the water column with reduced metabolic rates.
Ecology
Ecological role
Key component of the microbial loop; converts dissolved organic matter into biomass, which is then available to higher trophic levels.
Natural predators
Heterotrophic nanoflagellates (protozoa) and bacteriophages (viruses).
Competitor species
Other pelagic bacterial groups such as the LD12 lineage (Alphaproteobacteria).
Ecosystem service
Water purification through the degradation of organic substances and maintenance of the lacustrine food web.
Threats
Eutrophication of water bodies, which can lead to shifts in the bacterial community composition.
Scientific profile
Profile
Habitat
Nearly ubiquitous in pelagic freshwater ecosystems worldwide (lakes, rivers, reservoirs). They often constitute 20% to over 50% of the total bacterial community in the epilimnion of freshwater lakes.
Ecological role
Central players in the microbial loop of freshwater systems. They process significant amounts of dissolved organic carbon (DOC). Actinorhodopsin allows them to better survive periods of nutrient limitation.