Internal carbon recycling by heterotrophic prokaryotes compensates for mismatches between phytoplankton production and heterotrophic consumption.

Molecular observational tools are useful for characterizing the composition and genetic endowment of microbial communities but cannot measure fluxes, which are critical for the understanding of ecosystems. To overcome these limitations, we used a mechanistic inference approach to estimate dissolved organic carbon (DOC) production and consumption by phytoplankton operational taxonomic units and heterotrophic prokaryotic amplicon sequence variants and inferred carbon fluxes between members of this microbial community from Western English Channel time-series data. Our analyses focused on phytoplankton spring and summer blooms, as well as bacteria summer blooms.

Phytoplankton Producer Species and Transformation of Released Compounds over Time Define Bacterial Communities following Phytoplankton Dissolved Organic Matter Pulses.

Phytoplankton-bacterium interactions are mediated, in part, by phytoplankton-released dissolved organic matter (DOMp). Two factors that shape the bacterial community accompanying phytoplankton are (i) the phytoplankton producer species, defining the initial composition of released DOMp, and (ii) the DOMp transformation over time. We added phytoplankton DOMp from the diatom Skeletonema marinoi and the cyanobacterium Prochlorococcus marinus MIT9312 to natural bacterial communities from the eastern Mediterranean and determined the bacterial responses over a time course of 72 h in terms of cell numbers, bacterial production, alkaline phosphatase activity, and changes in active bacterial community composition based on rRNA amplicon sequencing.

Response to Comment on "Models predict planned phosphorus load reduction will make Lake Erie more toxic".

Huisman . claim that our model is poorly supported or contradicted by other studies and the predictions are "seriously flawed." We show their criticism is based on an incomplete selection of evidence, misinterpretation of data, or does not actually refute the model.

Single-cell measurements and modelling reveal substantial organic carbon acquisition by Prochlorococcus.

Marine phytoplankton are responsible for about half of the photosynthesis on Earth. Many are mixotrophs, combining photosynthesis with heterotrophic assimilation of organic carbon, but the relative contribution of these two lifestyles is unclear. Here single-cell measurements reveal that Prochlorococcus at the base of the photic zone in the Eastern Mediterranean Sea obtain only ~20% of carbon required for growth by photosynthesis.

Models predict planned phosphorus load reduction will make Lake Erie more toxic.

Harmful cyanobacteria are a global environmental problem, yet we lack actionable understanding of toxigenic versus nontoxigenic strain ecology and toxin production. We performed a large-scale meta-analysis including 103 papers and used it to develop a mechanistic, agent-based model of growth and microcystin production. Simulations for Lake Erie suggest that the observed toxigenic-to-nontoxigenic strain succession during the 2014 Toledo drinking water crisis was controlled by different cellular oxidative stress mitigation strategies (protection by microcystin versus degradation by enzymes) and the different susceptibility of those mechanisms to nitrogen limitation.

Phytoplankton exudates provide full nutrition to a subset of accompanying heterotrophic bacteria via carbon, nitrogen and phosphorus allocation.

Marine bacteria rely on phytoplankton exudates as carbon sources (DOCp). Yet, it is unclear to what extent phytoplankton exudates also provide nutrients such as phytoplankton-derived N and P (DONp, DOPp). We address these questions by mesocosm exudate addition experiments with spent media from the ubiquitous pico-cyanobacterium Prochlorococcus to bacterial communities in contrasting ecosystems in the Eastern Mediterranean - a coastal and an open-ocean, oligotrophic station with and without on-top additions of inorganic nutrients.

Dynamic carbon flux network of a diverse marine microbial community.

The functioning of microbial ecosystems has important consequences from global climate to human health, but quantitative mechanistic understanding remains elusive. The components of microbial ecosystems can now be observed at high resolution, but interactions still have to be inferred e.g.

Cells Rely on Microbial Interactions Rather than on Chlorotic Resting Stages To Survive Long-Term Nutrient Starvation.

Many microorganisms produce resting cells with very low metabolic activity that allow them to survive phases of prolonged nutrient or energy stress. In cyanobacteria and some eukaryotic phytoplankton, the production of resting stages is accompanied by a loss of photosynthetic pigments, a process termed chlorosis. Here, we show that a chlorosis-like process occurs under multiple stress conditions in axenic laboratory cultures of , the dominant phytoplankton linage in large regions of the oligotrophic ocean and a global key player in ocean biogeochemical cycles.

Distinctive tasks of different cyanobacteria and associated bacteria in carbon as well as nitrogen fixation and cycling in a late stage Baltic Sea bloom.

Cyanobacteria and associated heterotrophic bacteria hold key roles in carbon as well as nitrogen fixation and cycling in the Baltic Sea due to massive cyanobacterial blooms each summer. The species specific activities of different cyanobacterial species as well as the N- and C-exchange of associated heterotrophic bacteria in these processes, however, are widely unknown. Within one time series experiment we tested the cycling in a natural, late stage cyanobacterial bloom by adding 13C bi-carbonate and 15N2, and performed sampling after 10 min, 30 min, 1 h, 6 h and 24 h in order to determine the fixing species as well as the fate of the fixed carbon and nitrogen in the associations.

Mini-review: Phytoplankton-derived polysaccharides in the marine environment and their interactions with heterotrophic bacteria.

Within the wealth of molecules constituting marine dissolved organic matter, carbohydrates make up the largest coherent and quantifiable fraction. Their main sources are from primary producers, which release large amounts of photosynthetic products - mainly polysaccharides - directly into the surrounding water via passive and active exudation. The organic carbon and other nutrients derived from these photosynthates enrich the 'phycosphere' and attract heterotrophic bacteria.

Niche separation of Baltic Sea cyanobacteria during bloom events by species interactions and autecological preferences.

Cyanobacterial blooms regularly occur in the Baltic Sea during the summer months, with filamentous, heterocystous Nodularia spumigena and Dolichospermum sp. and the coccoid picocyanobacterium Synechococcus spp. as important species.

Microcystins do not necessarily lower the sensitivity of Microcystis aeruginosa to tannic acid.

Different phytoplankton strains have been shown to possess varying sensitivities towards macrophyte allelochemicals, yet the reasons for this are largely unknown. To test whether microcystin (MC) is responsible for strain-specific sensitivities of Microcystis aeruginosa to macrophyte allelochemicals, we compared the sensitivity of 12 MC- and non-MC-producing M. aeruginosa strains, including an MC-deficient mutant and its wild type, to the polyphenolic allelochemical tannic acid (TA).

Sensitivity of the green alga Pediastrum duplex Meyen to allelochemicals is strain-specific and not related to co-occurrence with allelopathic macrophytes.

Interspecific differences in the response of microalgae to stress have numerous ecological implications. However, little is known of intraspecific sensitivities and the potential role of local genetic adaptation of populations. We compared the allelochemical sensitivity of 23 Pediastrum duplex Meyen strains, a common component of the freshwater phytoplankton.

Bacterial community composition associated with freshwater algae: species specificity vs. dependency on environmental conditions and source community.

We studied bacterial associations with the green alga Desmodesmus armatus and the diatom Stephanodiscus minutulus under changing environmental conditions and bacterial source communities, to evaluate whether bacteria-algae associations are species-specific or more generalized and determined by external factors. Axenic and xenic algae were incubated in situ with and without allelopathically active macrophytes, and in the laboratory with sterile and nonsterile lake water and an allelochemical, tannic acid (TA). Bacterial community composition (BCC) of algae-associated bacteria was analyzed by denaturing gradient gel electrophoresis (DGGE), nonmetric multidimensional scaling, cluster analyses, and sequencing of DGGE bands.