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The cell size of picophytoplankton populations affects their ecology and biogeochemical role, but how different environmental drivers control its variability is still not well understood. To gain insight into the role of temperature and nutrient availability as determinants of picophytoplankton population mean cell size, we carried out five microcosm experiments across the Atlantic Ocean (45°N-27°S) in which surface plankton assemblages were incubated under all combinations of three temperatures (in situ, 3 °C cooling and 3 °C warming) and two nutrient levels (unamended and addition of nitrogen and phosphorus). The overall range of variability in cell volume was 5-fold for Prochlorococcus, 8-fold for Synechococcus and 6-fold for the picoeukaryotes. We observed, in all the treatments and in the control, a consistent trend toward larger mean cell sizes over time for both Prochlorococcus and Synechococcus, which was likely the result of sample confinement. Changes in temperature and nutrient status alone did not cause clear changes in cell size, relative to the control, but the combination of warming and nutrient addition resulted in an increase in Prochlorococcus and Synechococcus cell size. The largest increases in cell volume were associated with slow or negative population net growth rates. Our results emphasize the importance of considering changes in biovolume to obtain accurate estimates of picophytoplankton biomass and suggest that the inverse relationship between growth rate and population mean cell size may be a general pattern in marine phytoplankton., Competing Interests: Declarations Competing interests The authors declare no competing interests., (© 2024. The Author(s).)

Depth-resolved flow cytometric observations have been used to determine the size distribution and refractive index (RI) of picoplankton throughout the Atlantic Ocean. Prochlorococcus frequently showed double size distribution peaks centered on ${0.75 \pm 0.25}$0.75±0.25 and ${1.75 \pm 0.25}\,\,{\rm \unicode{x00B5}{\rm m}}$1.75±0.25µm; the smallest peak diameters were ${\le}{0.65}\,\,{\rm \unicode{x00B5}{\rm m}}$≤0.65µm in the equatorial upwelling with larger cells (${\sim}{0.95}\,\,{\rm \unicode{x00B5}{\rm m}}$∼0.95µm) in the surface layers of the tropical gyres. Synechococcus was strongly monodispersed: the smallest (${\sim}{1.5}\,\,{\rm \unicode{x00B5}{\rm m}}$∼1.5µm) and largest cells (${\sim}{2.25{-}2.50}\,\,{\rm \unicode{x00B5}{\rm m}}$∼2.25-2.50µm) were encountered in the lowest and highest abundance regions, respectively. Typical RI for Prochlorococcus was found to be ${\sim}{1.06}$∼1.06, whereas for Synechococcus surface RI varied between 1.04-1.08 at high and low abundances, respectively.

The Cyanobacteria Prochlorococcus and Synechococcus account for a substantial fraction of marine primary production. Here, we present quantitative niche models for these lineages that assess present and future global abundances and distributions. These niche models are the result of neural network, nonparametric, and parametric analyses, and they rely on >35,000 discrete observations from all major ocean regions. The models assess cell abundance based on temperature and photosynthetically active radiation, but the individual responses to these environmental variables differ for each lineage. The models estimate global biogeographic patterns and seasonal variability of cell abundance, with maxima in the warm oligotrophic gyres of the Indian and the western Pacific Oceans and minima at higher latitudes. The annual mean global abundances of Prochlorococcus and Synechococcus are 2.9 ± 0.1 × 10(27) and 7.0 ± 0.3 × 10(26) cells, respectively. Using projections of sea surface temperature as a result of increased concentration of greenhouse gases at the end of the 21st century, our niche models projected increases in cell numbers of 29% and 14% for Prochlorococcus and Synechococcus, respectively. The changes are geographically uneven but include an increase in area. Thus, our global niche models suggest that oceanic microbial communities will experience complex changes as a result of projected future climate conditions. Because of the high abundances and contributions to primary production of Prochlorococcus and Synechococcus, these changes may have large impacts on ocean ecosystems and biogeochemical cycles.

To better understand the temporal and spatial dynamics of Prochlorococcus populations, and how these populations co-vary with the physical environment, we followed monthly changes in the abundance of five ecotypes-two high-light adapted and three low-light adapted-over a 5-year period in coordination with the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time-series (HOT) programs. Ecotype abundance displayed weak seasonal fluctuations at HOT and strong seasonal fluctuations at BATS. Furthermore, stable 'layered' depth distributions, where different Prochlorococcus ecotypes reached maximum abundance at different depths, were maintained consistently for 5 years at HOT. Layered distributions were also observed at BATS, although winter deep mixing events disrupted these patterns each year and produced large variations in ecotype abundance. Interestingly, the layered ecotype distributions were regularly reestablished each year after deep mixing subsided at BATS. In addition, Prochlorococcus ecotypes each responded differently to the strong seasonal changes in light, temperature and mixing at BATS, resulting in a reproducible annual succession of ecotype blooms. Patterns of ecotype abundance, in combination with physiological assays of cultured isolates, confirmed that the low-light adapted eNATL could be distinguished from other low-light adapted ecotypes based on its ability to withstand temporary exposure to high-intensity light, a characteristic stress of the surface mixed layer. Finally, total Prochlorococcus and Synechococcus dynamics were compared with similar time series data collected a decade earlier at each location. The two data sets were remarkably similar-testimony to the resilience of these complex dynamic systems on decadal time scales.

By using data collected during a continuous circumnavigation of the Southern Hemisphere, we observed clear patterns in the population-genetic structure of Prochlorococcus, the most abundant photosynthetic organism on Earth, between and within the three Southern Subtropical Gyres. The same mechanisms that were previously invoked to account for the vertical distribution of ecotypes at local scales accounted for the global (horizontal) patterns we observed. Basin-scale and seasonal variations in the structure and strength of vertical stratification provide a basis for understanding large-scale horizontal distribution in genetic and physiological traits of Prochlorococcus, and perhaps of marine microbial communities in general.