Size dependence of metabolism within marine picoplankton populations

Abstract

Cell size is broadly applied as a convenient parameterization of ecosystem models and is widely applicable to constrain the activities of organisms spanning large size ranges. However, the size structure of the majority of the marine picoplankton assemblage is narrow and beneath the lower size limit of the empirical allometric relationships established so far (typically >1 μm). We applied a fine-resolution (0.05 μm increments) size fractionation method to estimate the size dependence of metabolic activities of picoplankton populations in the 0.10-1.00 μm size interval within the surface North Pacific Subtropical Gyre microbial assemblage. Groupspecific carbon retained on each filter was quantified by flow cytometric conversion of light scatter to cellular carbon quotas. Median carbon quotas were 25.7, 22.6, and 5.9 fg C cell −1 for populations of the picocyanobacterium Prochlorococcus, high-scatter heterotrophs, and low-scatter heterotrophs, respectively. Carbon-specific rates of primary production as a function of cell size, using the 14 C method, and phosphate transport, using 33 P radiotracers, resulted in negative power scalings (b) within populations of Prochlorococcus and heterotrophs of b = −1.3 and b = −1.1, respectively. These findings are in contrast to the positive empirical power scaling comprising the broader and larger prokaryote category (b = 0.7) and point to within-population variability in cell physiology and metabolism for these important microbial groups.

Constraining growth rates and the ratio of living to nonliving particulate carbon using beam attenuation and adenosine‐5′‐triphosphate at Station ALOHA

“…Assuming C L : ATP of 400 yielded average C : Chl a ratios of 119 AE 34 g g À1 , C : HBact of ~18 AE 4 fg cell À1 , and PhytoC of ~6 AE 4 mg C m À3 . Although variability is large, these values are all within the reported C : Chl a ratios for the surface ocean at Station ALOHA (Christian and Karl 1994), fg cell À1 of C in HBact from various laboratory studies using local NPSG cultures (Casey et al 2019;White et al 2019), and PhytoC estimated from fieldand satellitebased studies in the NPSG (Graff et al 2015;Arteaga et al 2016), suggesting that the proportion of living particles to the total particulate pool ranging between ~26-42% is reasonable. Indeed, percentages are consistent with estimates indicating that living, viable organisms make up ~30% of living C in oligotrophic regions (e.g., Winn and Karl 1984;Christian and Karl 1994;Graff et al 2015), and is in agreement with previous descriptions of the PC pool in the open ocean as consisting of "living carbon in a soup of nonliving organic matter" (Karl and Dobbs 1998).…”

A Bayesian approach to modeling phytoplankton population dynamics from size distribution time series

“…In the second, cell division still increases monotonically with cell size but is allowed to vary over time. We also considered size dependence in carbon fixation through power-law relationships supported by experimental evidence [ 30 ]. Finally, we implemented a “free” parameterization in which carbon fixation and carbon loss rates are estimated separately for each size class, in order to provide enough flexibility for the model to capture biological processes that are not explicitly accounted for in our models.…”

Trophic interactions with heterotrophic bacteria limit the range of Prochlorococcus

“…First, we note that the smallest heterotrophic bacteria (e.g., SAR11) are smaller than Prochlorococcus . For instance, SAR11 have been found to have cell diameters between about 0.3 and 0.45 m ( 28 ), while Prochlorococcus has been observed with diameters varying between about 0.45 and 0.8 m ( 5 , 9 , 31 , 32 ). Additionally, the carbon cell quota of SAR11 has been shown to be ∼10 times less than for Prochlorococcus ( 33 ).…”