Disease in the marine environment is predicted to increase with anthropogenic stressors and already affects major habitat-formers, such as corals and seaweeds. Solutions to address this issue are urgently needed. The seaweed Delisea pulchra is prone to a bleaching disease, which is caused by opportunistic pathogens and involves bacterial dysbiosis. Bacteria that can inhibit these pathogens and/or counteract dysbiosis are therefore hypothesised to reduce disease. This study aimed to identify such disease-protective bacteria and investigate their protective action. One strain, Phaeobacter sp. BS52, isolated from healthy D. pulchra, was antagonistic towards bleaching pathogens and significantly increased the proportion of healthy individuals when applied before the pathogen challenge (pathogen-only vs. BS52 + pathogen: 41–80%), and to a level similar to the control. However, no significant negative correlations between the relative abundances of pathogens and BS52 on D. pulchra were detected. Instead, inoculation of BS52 mitigated pathogen-induced changes in the epibacterial community. These observations suggest that the protective activity of BS52 was due to its ability to prevent dysbiosis, rather than direct pathogen inhibition. This study demonstrates the feasibility of manipulating bacterial communities in seaweeds to reduce disease and that mitigation of dysbiosis can have positive health outcomes.

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A novel, compact, and broadband optical phase contrast microscopy based on incoherent vortex topological quadrupole is theoretically proposed and experimentally realized. The topological quadrupole, generated in a thin uniaxial crystal, possesses four single‐charge optical vortexes, each of which can act as a two‐dimensional (2D) optical spatial differentiator. The incoherence of light‐emitting diode (LED) light will wash out the optical vortex and the spatial differentiation effect, which can be ingeniously overcome by choosing Kohler illumination and inserting a 1 mm‐thickness uniaxial crystal sandwiched with two polarizers before the camera. By adjusting the polarizers and the tilted angle of crystal to selectively utilize the geometric Berry phase and the angular gradient of Fresnel transmission coefficients of crystal, the 1D, 2D, and second‐order analog spatial differentiations with a spatial resolution better than 0.775 μm can be switched flexibly. A versatile phase contrast microscope is built, which is well compatible with the conventional microscopes. The uniformly incoherent illumination without laser speckle effects results in high‐quality spatial differentiation imaging. Biologically transparent living cells are thus imaged with high contrast.

Despite an increasing awareness of disease impacts on both cultivated and native seaweed populations, the development of marine probiotics has been limited and predominately focused on farmed animals. Bleaching (loss of thallus pigmentation) is one of the most prevalent diseases observed in marine macroalgae. Endemic probiotic bacteria have been characterized to prevent bleaching disease in red macroalgae Agarophyton vermiculophyllum and Delisea pulchra; however, the extent to which probiotic strains provide cross-protection to non-endemic hosts and the influence of native microbiota remain unknown. Using A. vermiculophyllum as a model, we demonstrate that co-inoculation with the pathogen Pseudoalteromonas arctica G-MAN6 and D. pulchra probiotic strain Phaeobacter sp. BS52 or Pseudoalteromonas sp. PB2-1 reduced the disease risks compared to the pathogen only treatment. Moreover, non-endemic probiotics outperformed the endemic probiotic strain Ralstonia sp. G-NY6 in the presence of the host natural microbiota. This study highlights how the native microbiota can impact the effectiveness of marine probiotics and illustrates the potential of harnessing probiotics that can function across different hosts to mitigate the impact of emerging marine diseases.