Adverse Environmental Perturbations May Threaten Kelp Farming Sustainability by Exacerbating Enterobacterales Diseases.

Globally kelp farming is gaining attention to mitigate land-use pressures and achieve carbon neutrality. However, the influence of environmental perturbations on kelp farming remains largely unknown. Recently, a severe disease outbreak caused extensive kelp mortality in Sanggou Bay, China, one of the world's largest high-density kelp farming areas. Here, through in situ investigations and simulation experiments, we find indications that an anomalously dramatic increase in elevated coastal seawater light penetration may have contributed to dysbiosis in the kelp Saccharina japonica's microbiome. This dysbiosis promoted the proliferation of opportunistic pathogenic Enterobacterales, mainly including the genera Colwellia and Pseudoalteromonas. Using transcriptomic analyses, we revealed that high-light conditions likely induced oxidative stress in kelp, potentially facilitating opportunistic bacterial Enterobacterales attack that activates a terrestrial plant-like pattern recognition receptor system in kelp. Furthermore, we uncover crucial genotypic determinants of Enterobacterales dominance and pathogenicity within kelp tissue, including pathogen-associated molecular patterns, potential membrane-damaging toxins, and alginate and mannitol lysis capability. Finally, through analysis of kelp-associated microbiome data sets under the influence of ocean warming and acidification, we conclude that such Enterobacterales favoring microbiome shifts are likely to become more prevalent in future environmental conditions. Our study highlights the need for understanding complex environmental influences on kelp health and associated microbiomes for the sustainable development of seaweed farming.

Phage Infection Benefits Marine Diatom Phaeodactylum tricornutum by Regulating the Associated Bacterial Community.

The interaction between marine phyto- and bacterioplankton is regulated by multiple environmental and biological factors. Among them, phages as the major regulators of bacterial mortality are considered to have important impacts on algae-associated bacteria and algae-bacteria relationship. However, little is currently known about the actual impact of phages from this perspective. Here, we revealed that phage infection improved the maximum quantum efficiency of photosystem II of Phaeodactylum tricornutum by regulating the associated bacterial community. Specifically, phage infection weakened bacterial abundance and eliminated their negative effects on the diatom. Unexpectedly, the structure of the bacterial community co-cultured with the diatom was not significantly affected, likely because the shaping effect of the diatom on the bacterial community structure can far outcompete or mask the impact of phage infection. Our results established a link between algae, bacteria, and phages, suggesting that phage infection benefits the diatom by regulating the associated bacterial community.

Inherent tendency of Synechococcus and heterotrophic bacteria for mutualism on long-term coexistence despite environmental interference.

Mutualism between Synechococcus and heterotrophic bacteria has been found to support their prolonged survival in nutrient-depleted conditions. However, environmental interference on the fate of their mutualism is not understood. Here, we show that exogenous nutrients disrupt their established mutualism. Once the exogenous nutrients were exhausted, Synechococcus and heterotrophic bacteria gradually reestablished their metabolic mutualism during 450 days of culture, which revived unhealthy Synechococcus cells. Using metagenomics, metatranscriptomics, and the 15N tracer method, we reveal that the associated bacterial nitrogen fixation triggered the reestablishment of the mutualism and revival of Synechococcus health. During this process, bacterial community structure and functions underwent tremendous adjustments to achieve the driving effect, and a cogeneration of nitrogen, phosphorus, iron, and vitamin by the heterotrophic bacteria sustained Synechococcus's prolonged healthy growth. Our findings suggest that Synechococcus and heterotrophic bacteria may have an inherent tendency toward mutualism despite environmental interference. This may exhibit their coevolutionary adaptations in nutrient-deficient environments.

A dark-tolerant diatom (Chaetoceros) cultured from the deep sea.

Although the extreme conditions of the deep sea are typically not suitable for the growth of photosynthetic algae, accumulating evidence indicates that there are diverse healthy phytoplankton living in this environment. However, living phytoplankton from the deep sea have rarely been isolated and cultivated, and so our understanding of where they come from and how they adapt to (or tolerate) the extreme deep-sea environment is limited. Here, under long-term dark stress and subsequent light treatment, we successfully isolated a diatom from a depth of 1,000 m in the Western Pacific Ocean. Morphological observations and molecular phylogenetic analysis revealed that it is affiliated to the genus Chaetoceros, and thus, we tentatively named it Chaetoceros sp. DS1. We observed that the chloroplast genome of this species, is most closely related to that of Chaetoceros simplex. It was shown to have a strong tolerance to darkness in that it maintained its morphological integrity and vitality for up to 3 months in complete darkness at room temperature. We also demonstrated that Chaetoceros sp. DS1 presented a facultative heterotrophic function. Its growth was promoted by many organic carbon sources (e.g., glycerine, ethanol, and sodium acetate) under low light conditions. However, under dark and high light conditions, the growth promotion effect of organic carbon was not obvious. Indeed, Chaetoceros sp. DS1 grew best under low light conditions, indicating that it likely came from the deeper layer of the euphotic zone. The facultative heterotrophic function of this diatom and tolerance to darkness may help it survive in these conditions or enter a dormant period in the deep sea.

Long-Term Survival of Synechococcus and Heterotrophic Bacteria without External Nutrient Supply after Changes in Their Relationship from Antagonism to Mutualism.

Marine phytoplankton and heterotrophic bacteria share a very close but usually changeable relationship. However, the ultimate fate of their unstable relationship on a long-term scale is unclear. Here, the relationship between Synechococcus and heterotrophic bacterial communities underwent a dramatic shift from antagonism to commensalism and eventually to mutualism during long-term cocultivation. The relationship change is attributed to the different (even opposite) effects of diverse bacterial members on Synechococcus and the ratio of beneficial to harmful bacteria. Different bacterial members also interact with each other (e.g., quorum-sensing communication, hostility, or mutual promotion) and drive a dynamic succession in the entire community structure that corresponds exactly to the shift in its relationship with Synechococcus. In the final mutualism stage, a self-sufficient nitrogen cycle, including nitrogen fixation, denitrification, and organic nitrogen degradation, contributed to the healthy survival of Synechococcus for 2 years without an exogenous nutrient supply. This natural selective trait of Synechococcus and heterotrophic bacteria toward mutualism under long-term coexistence provides a novel clue for understanding the ubiquity and competitive advantage of Synechococcus in global oceans. IMPORTANCE Phytoplankton and heterotrophic bacteria have a close but usually changeable relationship. Uncovering the dynamic changes and driving factors of their interrelationships is of great significance for an in-depth understanding of the ecological processes and functions of marine microorganisms. Here, we observed that Synechococcus and heterotrophic bacterial communities underwent a dramatic change in their relationship from antagonism to mutualism during a long-term cocultivation process. We revealed that the interactions between different members of the bacterial community and the combined effects of different bacterial individuals on Synechococcus promoted the dynamic changes of the Synechococcus-bacterium relationship. In the end, a self-sufficient nutrient cycle (especially nitrogen) established by Synechococcus and bacterial communities supported their long-term survival without any external nutrition supply. This study provides novel insight into the interaction between Synechococcus and heterotrophic bacteria in the ocean and provides a novel clue for understanding the ubiquity and competitive advantage of Synechococcus in global oceans.

Marinicaulis aureum sp. nov., isolated from a culture of the green alga Ulva prolifera, and emended description of the genus Marinicaulis.

A Gram-stain negative, facultatively anaerobic, rod-shaped motile bacterium with a single polar flagellum, designed strain HHTR114T, was isolated from a culture of the green alga Ulva prolifera obtained from offshore seawater at Qingdao, China. Optimum growth occurred in the presence of 2-3% (w/v) NaCl, at pH 7.0-8.0 and 30 °C. The major fatty acids (> 10% of total fatty acids) were C16:0 (24.7%), C18:1ω7c 11-methyl (24.3%) and summed feature 3 (C16:1ω6c and/or C16:1ω7c, 19.7%). The major polar lipids were phosphatidylethanolamine, glycolipid and four unidentified polar lipids. The DNA G + C content of strain HHTR114T calculated on the basis of the genome sequence was 58.2% and the genome size is 4.1 Mbp. The predominant isoprenoid quinone was Q-10. The estimated DNA-DNA hybridization values were 21.4% [18.6-24.4%] between strain HHTR114T and Marinicaulis flavus SY-3-19T. On the basis of polyphasic analysis, strain HHTR114T is considered to represent a novel species, for which the name Marinicaulis aureum sp. nov. is proposed. The type strain of the type species is HHTR114T (= KCTC 62394T = MCCC 1K03481T).