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Diatoms are single-celled photosynthetic eukaryotes responsible for CO2 fixation and primary production in aquatic ecosystems. The cosmopolitan marine diatom Coscinodiscus granii can form seasonal blooms in coastal areas and interact with various microorganisms, including the parasitic oomycete Lagenisma coscinodisci. This unicellular eukaryote is mainly present in the northern hemisphere as an obligate parasite of the genus Coscinodiscus. Understanding the interplay of abiotic factors such as temperature and biotic factors like parasitism on algal physiology is crucial as it dictates plankton community composition and is especially relevant during environmental changes and warming events. This study investigates the impact of two temperatures, 13 °C and 25 °C, on Coscinodiscus granii under laboratory conditions. A decreased infection rate of the parasite was observed at the elevated temperature. Comparative metabolomic analysis using UHPLC-HRMS revealed that temperature and parasitism significantly affect the algal cell metabolome. Abundances of metabolites related to sulfur metabolism, including cysteinoleic acid and dimethylsulfoniopropionate, as well as molecules linked to fatty acid metabolism, e.g., carnitine, acetylcarnitine, and eicosapentanoic acid, significantly increase in cells grown at a higher temperature, suggesting the enhanced rate of metabolism of host cells as the temperature rises. Our study reveals how temperature-induced metabolic changes can influence host–parasite dynamics in a changing environment. [ABSTRACT FROM AUTHOR]

Plankton parasites such as viruses, bacteria, fungi, and unicellular eukaryotes are associated symbionts colonizing algal groups in aquatic ecosystems. They occur within a network of microbe–microbe interactions in which they parasitize phytoplankton and seaweeds, i.e., primary producers generating organic carbon and forming the basis of marine food webs. These parasites use algae as a source of nutrients and reproduce at the expense of their host, causing infectious symptoms leading to disease and death. Plankton parasites can reduce the algal populations, and the infection of one specific species may, in turn, favor the development of another, influencing the seasonal succession of algal blooms in oceans, seas, and lakes. Many parasites have free-living stages that zooplankton graze upon, representing a significant trophic link in food webs. The biology and life cycle of plankton parasites are well investigated in marine and freshwater algal hosts. Still, the chemical signaling mediating these microbial interactions and the effect of co-occurring symbionts remain underexplored. This review focuses on the diversity of plankton parasites infecting algae, with a particular emphasis on unicellular eukaryotes. The ecological role of plankton parasites, the mechanisms regulating cellular infection and host resistance, and the interplay of alga-parasite interactions with coexisting microorganisms are discussed. [ABSTRACT FROM AUTHOR]

Microalgae that form phytoplankton live and die in a complex microbial consortium in which they co-exist with bacteria and other microorganisms. The dynamics of species succession in the plankton depends on the interplay of these partners. Bacteria utilize substrates produced by the phototrophic algae, while algal growth can be supported by bacterial exudates. Bacteria might also use chemical mediators with algicidal properties to attack algae. To elucidate whether specific bacteria play universal or context-specific roles in the interaction with phytoplankton, we investigated the effect of cocultured bacteria on the growth of 8 microalgae. An interaction matrix revealed that the function of a given bacterium is highly dependent on the cocultured partner. We observed no universally algicidal or universally growth-promoting bacteria. The activity of bacteria can even change during the aging of an algal culture from inhibitory to stimulatory or vice versa. We further established a synthetic phytoplankton/bacteria community with the centric diatom, Coscinodiscus radiatus, and 4 phylogenetically distinctive bacterial isolates, Mameliella sp., Roseovarius sp., Croceibacter sp., and Marinobacter sp. Supported by a LotkaVolterra model, we show that interactions within the consortium are specific and that the sum of the pairwise interactions can explain algal and bacterial growth in the community. No synergistic effects between bacteria in the presence of the diatom was observed. Our survey documents highly species-specific interactions that are dependent on algal fitness, bacterial metabolism, and community composition. This species specificity may underly the high complexity of the multi-species plankton communities observed in nature. IMPORTANCE The marine food web is fueled by phototrophic phytoplankton. These algae are central primary producers responsible for the fixation of ca. 40% of the global CO2. Phytoplankton always co-occur with a diverse bacterial community in nature. This diversity suggests the existence of ecological niches for the associated bacteria. We show that the interaction between algae and bacteria is highly species-specific. Furthermore, both, the fitness stage of the algae and the community composition are relevant in determining the effect of bacteria on algal growth. We conclude that bacteria should not be sorted into algicidal or growth supporting categories; instead, a context-specific function of the bacteria in the plankton must be considered. This functional diversity of single players within a consortium may underly the observed diversity in the plankton. [ABSTRACT FROM AUTHOR]

Brown macroalgae are an essential component of temperate coastal ecosystems and a growing economic sector. They harbor diverse microbial communities that regulate algal development and health. This algal holobiont is dynamic and achieves equilibrium via a complex network of microbial and host interactions. We now report that bacterial and fungal endophytes associated with four brown algae ( Ascophyllum nodosum , Pelvetia canaliculata , Laminaria digitata , and Saccharina latissima) produce metabolites that interfere with bacterial autoinducer-2 quorum sensing, a signaling system implicated in virulence and host colonization. Additionally, we performed co-culture experiments combined to a metabolomic approach and demonstrated that microbial interactions influence production of metabolites, including metabolites involved in quorum sensing. Collectively, the data highlight autoinducer-2 quorum sensing as a key metabolite in the complex network of interactions within the algal holobiont.

Filamentous fungi asymptomatically colonize the inner tissues of macroalgae, yet their ecological roles remain largely underexplored. Here, we tested if metabolites produced by fungal endophytes might protect their host against a phylogenetically broad spectrum of protistan pathogens. Accordingly, the cultivable fungal endophytes of four brown algal species were isolated and identified based on LSU and SSU sequencing. The fungal metabolomes were tested for their ability to reduce the infection by protistan pathogens in the algal model Ectocarpus siliculosus . The most active metabolomes effective against the oomycetes Eurychasma dicksonii and Anisolpidium ectocarpii , and the phytomixid Maullinia ectocarpii were further characterized chemically. Several pyrenocines isolated from Phaeosphaeria sp. AN596H efficiently inhibited the infection by all abovementioned pathogens. Strikingly, these compounds also inhibited the infection of nori ( Pyropia yezoensis ) against its two most devastating oomycete pathogens, Olpidiopsis pyropiae , and Pythium porphyrae . We thus demonstrate that fungal endophytes associated with brown algae produce bioactive metabolites which might confer protection against pathogen infection. These results highlight the potential of metabolites to finely-tune the outcome of molecular interactions between algae, their endophytes, and protistan pathogens. This also provide proof-of-concept toward the applicability of such metabolites in marine aquaculture to control otherwise untreatable diseases.