LAUSR

A single diatom cell is microscopic to our eyes, yet to a bacterium, the diatom's cell surface is a rich landscape filled with diverse compounds available for metabolism (Figure 1a). Direct attachment to diatoms is essential for the survival of many heterotrophic bacteria since the close proximity to the cell surface enables bacteria to breakdown complex polysaccharides and other compounds, and in turn, the bacteria can provide diatoms with essential nutrients like vitamin B12 (Amin et al., 2012). This microenvironment surrounding diatoms and other phytoplankton is called the phycosphere, and it is well known that these microscale interactions within the phycosphere influence largescale biogeochemical processes in the ocean (Grossart et al., 2006; Grossart & Ploug, 2001). Yet, many aspects of bacteriaalgae interactions remain mysterious, including how the spatial organization of polysaccharides and associated bacteria changes over the surface of a single phytoplankton cell. Bacterial colonization of diatoms generally depends on the extracellular polymeric substances (EPSs) present on the algal cells and the species of interacting bacteria (Mönnich et al., 2020). Glycoconjugates — glycans covalently linked to lipids, proteins or other compounds — are abundant in diatom EPSs (Bennke et al., 2013). However, identifying specific types of glycoconjugates is tricky: fluorescently labeled lectins that bind to specific glycoconjugates are used for identification, but most commercially available lectins are designed for model organisms or systems, not diatoms. Despite this challenge, a combination of microscopy methods and lectin staining techniques can provide a rare glimpse into the 3D spatial organization of glycoconjugates and bacteria on phytoplankton cells. Using these methods, Tran et al. (2023) investigate diatombacteria interactions and the diversity of glycoconjugates of Thalassiosira rotula, a widely distributed diatom in marine environments. The authors screened 78 commercially available fluorescent lectins and found six lectins that bind to glycoconjugates on T. rotula cells. One of the lectins binds to fucosecontaining glycoconjugates present on the valve surfaces and on tufts of threads coming from younger diatom cells (Figure 1b). The presence of tuftlike structures is surprising since this morphological trait was not previously described for T. rotula, but other closely related diatoms exhibit similar structures. The biological role of the tufts is unknown; however, it may slow the sinking rate of diatom cells as well as promote bacterial colonization by increasing the surface area available for attachment. The authors also explore the spatial organization of bacteria attached to Thalassiosira rotula using four bacterial strains belonging to the Roseobacter group (Dinoroseobacter shibae, Planktotalea frisia, Sulfitobacter sp., Pseudophaeobacter), one Gammaproteobacteria strain (Glaciecola sp.), and one Flavobacteria strain (Gramella forsetii), along with a natural bacterial community from sea water. The bacteria colonize T. rotula using both freeliving and aggregateforming strategies, and three general colonization patterns are apparent: bacterial aggregates restricted to the diatom's tuftlike structures, aggregates DOI: 10.1111/jpy.13319