a b s t r a c tIn most terrestrial environments, our knowledge of the elemental composition and stoichiometry of microorganisms stems from indirect whole community analyses. In contrast, we have little direct knowledge of the elemental composition of specific microorganisms and the variation between and within Fungi and Bacteria. To address this issue, we isolated and identified the elemental content of 87 strains of Fungi and Bacteria isolated from grassland leaf litter. The isolated strains were affiliated with a broad range of diversity including Ascomycota and Basidiomycota for Fungi, and Proteobacteria, Bacteroidetes, and Actinobacteria for Bacteria. The C:P and C:N but not N:P ratios were significantly higher in Fungi than in Bacteria. Extensive strain variation in elemental composition was partly linked to phylogeny and growth rate. Across all strains, the geometric mean C:N:P was 88:15:1. This overall ratio was significantly higher than reported for other leaf litter and terrestrial whole communities but closer to the canonical Redfield ratio characterizing marine microorganisms. This result warrants further investigation into the discrepancy between whole community and isolated strain elemental ratios.

The globally abundant marine Cyanobacteria Prochlorococcus and Synechococcus share many physiological traits but presumably have different evolutionary histories and associated phylogeography. In Prochlorococcus, there is a clear phylogenetic hierarchy of ecotypes, whereas multiple Synechococcus clades have overlapping physiologies and environmental distributions. However, microbial traits are associated with different phylogenetic depths. Using this principle, we reclassified diversity at different phylogenetic levels and compared the phylogeography. We sequenced the genetic diversity of Prochlorococcus and Synechococcus from 339 samples across the tropical Pacific Ocean and North Atlantic Ocean using a highly variable phylogenetic marker gene (rpoC1). We observed clear parallel niche distributions of ecotypes leading to high Pianka's Index values driven by distinct shifts at two transition points. The first transition point at 6°N distinguished ecotypes adapted to warm waters but separated by macronutrient content. At 39°N, ecotypes adapted to warm, low macronutrient vs. colder, high macronutrient waters shifted. Finally, we detected parallel vertical and regional single-nucleotide polymorphism microdiversity within clades from both Prochlorococcus and Synechococcus, suggesting uniquely adapted populations at very specific depths, as well as between the Atlantic and Pacific Oceans. Overall, this study demonstrates that Prochlorococcus and Synechococcus have shared phylogenetic organization of traits and associated phylogeography.

CitationBiogeochemical interactions control a temporal succession in the elemental composition of marine communities 2016, 61 (2):531 Limnology and Oceanography

In the North Atlantic Ocean, we found that natural populations of Prochlorococcus adhered to Redfield ratio dimensions when comparing cell quotas of carbon to nitrogen, but had flexible composition under nutrient and light stress, allowing for a broad range of cellular carbon- and nitrogen-to-phosphorus ratios. Synechococcus populations also exhibited a wide range of elemental stoichiometry, including carbon-to-nitrogen ratios and increased their carbon-to-phosphorus ratios in response to low dissolved phosphorus availability. Small eukaryotic populations tended to have lower carbon-to-phosphorus ratios than single cell cyanobacterial groups, with the exception of one group of samples, which highlights the importance of community composition when determining how biological diversity influences bulk particle stoichiometry. The ratio of dissolved nitrogen:phosphorus fluxes into the euphotic zone was not correlated to nitrogen:phosphorus cellular quotas. The lack of a homeostatic relationship implies that other mechanisms, such as species-specific adaptation to oligotrophic phosphorus concentrations, control elemental particle ratios.

Variability in plankton elemental requirements can be important for global ocean biogeochemistry but we currently have a limited understanding of how ocean temperature influences the plankton C/N/P ratio. Multiple studies have put forward a ‘translation-compensation’ hypothesis to describe the positive relationship between temperature and plankton N/P or C/P as cells should have lower demand for P-rich ribosomes and associated depressed QP when growing at higher temperature. However, temperature affects many cellular processes beyond translation with unknown outcomes on cellular elemental composition. In addition, the impact of temperature on growth and elemental composition of phytoplankton is likely modulated by the life history and growth rate of the organism. To test the direct and indirect (via growth rate changes) effect of temperature, we here analyzed the elemental composition and ratios in six strains affiliated with the globally abundant marine Cyanobacteria Prochlorococcus. We found that temperature had a significant positive effect on the carbon and nitrogen cell quota, whereas no clear trend was observed for the phosphorus cell quota. The effect on N/P and C/P were marginally significantly positive across Prochlorococcus. The elemental composition and ratios of individual strains were also affected but we found complex interactions between the strain identity, temperature, and growth rate in controlling the individual elemental ratios in Prochlorococcus and no common trends emerged. Thus, the observations presented here does not support the ‘translation-compensation’ theory and instead suggest unique cellular elemental effects as a result of rising temperature among closely related phytoplankton lineages. Thus, the biodiversity context should be considered when predicting future elemental ratios and how cycles of carbon, nitrogen, and phosphorus may change in a future ocean.