Metabolomics-derived marker metabolites to characterize Phaeocystis pouchetii physiology in natural plankton communities

Osmotrophy of dissolved organic compounds by coccolithophore populations: Fixation into particulate organic and inorganic carbon

Coccolithophores are typically thought of as photoautotrophs, yet a few genera inhabit sub-euphotic environments with insufficient light for photosynthesis, suggesting that other carbon acquisition strategies are likely. Field experiments were performed in the northwest Atlantic (a region with potentially abundant coccolithophores). Phytoplankton populations were incubated with ¹⁴C-labeled dissolved organic carbon (DOC) compounds, acetate, mannitol, and glycerol. Coccolithophores were sorted from these populations 24 hours later using flow cytometry, and DOC uptake was measured. DOC uptake rates were as high as 10^-15 moles cell^-1 day^-1, slow relative to photosynthesis rates (10^-12 moles cell^-1 day^-1). Growth rates on the organic compounds were low, suggesting that osmotrophy plays more of a survival strategy in low-light situations. Assimilated DOC was found in both particulate organic carbon and calcite coccoliths (particulate inorganic carbon), suggesting that osmotrophic uptake of DOC into coccolithophore calcite is a small but notable part of the biological carbon pump and alkalinity pump paradigms.

Phytoplankton metabolite profiles from two Lake Ontario Areas of Concern reveal differences associated with taxonomic community composition

Water quality and phytoplankton community composition are important factors that can indicate freshwater ecosystem health. We combined water quality, phytoplankton community, and metabolomic data from algae and water sampled from two embayments in Lake Ontario, Hamilton Harbour and the Bay of Quinte, over ten weeks from August to October in the year 2020. Metabolomics was performed using liquid chromatography tandem mass spectroscopy (LC-MS/MS) to identify changes in intracellular metabolites within algae communities over time, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was used to characterize putative isomers of extracellular metabolites at sub-ppb mass accuracy. Results from this study indicate that Hamilton Harbour and the Bay of Quinte are two very different ecosystems with respect to water quality, phytoplankton metabolites, and phytoplankton community composition. Community composition is strongly driven by conductivity and nitrates in Hamilton Harbour, while the opposite is true in the Bay of Quinte. Metabolites including α-aminobutyric acid and glycine were found in larger abundance within algal communities at both locations, while taurine was more predominant in algal communities from the Bay of Quinte. These metabolic profiles could reflect the different communities of phytoplankton, and be alternative indicators of algal bloom growth.

Colony formation of Phaeocystis globosa: A case study of evolutionary strategy for competitive adaptation

Some algae possess a multi-morphic life cycle, either in the form of free-living solitary cells or colonies which constantly occur in algal blooms. Though colony formation seems to consume extra energy and materials, many algae tend to outbreak in form of colonies. Here, we hypothesized that colony formation is a selected evolutionary strategy to improve population competitiveness and environmental adaptation. To test the hypothesis, different sizes of colonies and solitary cells in a natural bloom of Phaeocystis globosa were investigated. The large colony showed a relatively low oxidant stress level, a nutrient trap effect, and high nutrient use efficiency. The colonial nitrogen and phosphorus concentrations were about 5-10 times higher than solitary cell phycosphere and cellular nutrient allocation decreased with the enlargement of the colonial diameter following the economies of scale law. These features provide the colony with monopolistic competence and could function as an evolutionary strategy for competitive adaptation.

Dynamic Diatom-Bacteria Consortia in Synthetic Plankton Communities

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 Lotka-Volterra 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.

Temporal and Spatial Signaling Mediating the Balance of the Plankton Microbiome

The annual patterns of plankton succession in the ocean determine ecological and biogeochemical cycles. The temporally fluctuating interplay between photosynthetic eukaryotes and the associated microbiota balances the composition of aquatic planktonic ecosystems. In addition to nutrients and abiotic factors, chemical signaling determines the outcome of interactions between phytoplankton and their associated microbiomes. Chemical mediators control essential processes, such as the development of key morphological, physiological, behavioral, and life-history traits during algal growth. These molecules thus impact species succession and community composition across time and space in processes that are highlighted in this review. We focus on spatial, seasonal, and physiological dynamics that occur during the early association of algae with bacteria, the exponential growth of a bloom, and its decline and recycling. We also discuss how patterns from field data and global surveys might be linked to the actions of metabolic markers in natural phytoplankton assemblages.

Marine Community Metabolomes Carry Fingerprints of Phytoplankton Community Composition

Phytoplankton transform inorganic carbon into thousands of biomolecules that represent an important pool of fixed carbon, nitrogen, and sulfur in the surface ocean. Metabolite production differs between phytoplankton, and the flux of these molecules through the microbial food web depends on compound-specific bioavailability to members of a wider microbial community. Yet relatively little is known about the diversity or concentration of metabolites within marine plankton. Here, we compare 313 polar metabolites in 21 cultured phytoplankton species and in natural planktonic communities across environmental gradients to show that bulk community metabolomes reflect the chemical composition of the phytoplankton community. We also show that groups of compounds have similar patterns across space and taxonomy, suggesting that the concentrations of these compounds in the environment are controlled by similar sources and sinks. We quantify several compounds in the surface ocean that represent substantial understudied pools of labile carbon. For example, the N-containing metabolite homarine was up to 3% of particulate carbon and is produced in high concentrations by cultured Synechococcus, and S-containing gonyol accumulated up to 2.5 nM in surface particles and likely originates from dinoflagellates or haptophytes. Our results show that phytoplankton composition directly shapes the carbon composition of the surface ocean. Our findings suggest that in order to access these pools of bioavailable carbon, the wider microbial community must be adapted to phytoplankton community composition.

IMPORTANCE Microscopic phytoplankton transform 100 million tons of inorganic carbon into thousands of different organic compounds each day. The structure of each chemical is critical to its biological and ecosystem function, yet the diversity of biomolecules produced by marine microbial communities remained mainly unexplored, especially small polar molecules which are often considered the currency of the microbial loop. Here, we explore the abundance and diversity of small biomolecules in planktonic communities across ecological gradients in the North Pacific and within 21 cultured phytoplankton species. Our work demonstrates that phytoplankton diversity is an important determinant of the chemical composition of the highly bioavailable pool of organic carbon in the ocean, and we highlight understudied yet abundant compounds in both the environment and cultured organisms. These findings add to understanding of how the chemical makeup of phytoplankton shapes marine microbial communities where the ability to sense and use biomolecules depends on the chemical structure.