Fig 1.
(A) Conceptual illustration of the microzone model. The moving particle (POM) creates a comet-shaped eutrophic plume of dissolved organic matter (DOM) with particulate lysate and metabolites of surface-attached bacteria (red). The DOM plume attracts chemotactic bacteria (green), which act as point sinks and reshape the nutrient field. (B) Normalized radial distribution functions (RDF) for the excess bacterial concentration around nutrient-releasing particles, obtained by mining information from published data of in vitro and in silico microscale experiments (RDF parameters in Table 1).
Table 1.
Datasets (1)-(4) are extracted with non-linear regression analysis of published data from microscale and in silico experiments (S1 Appendix), and datasets (5)-(6) are used for hypothesis testing in this work. All datasets are used with the RDF: , and each dataset is normalized by a different average bacterial concentration (
).
Fig 2.
Correlation of bacterial swimming to RDFs.
Impact of the bacterial swimming speed on the accumulation of marine bacteria with run-and-tumble motility around nutrient-exuding algae. (A) The data points originate from individual-based simulations by Bowen et al. (Fig 2A in [25]) and the continuous lines represent optimal fit of our exponential RDF (see section S1.5 in S1 Appendix). The bacterial peak concentration (B) and the chemotactic accumulation length (C) are negatively and positively correlated to the bacterial swimming speed, respectively, for this mode of motility.
Fig 3.
The bacterial microzone model captures in silico observations for the nutrient exposure of free-living bacteria in the undisturbed nutrient field (Da = 0) around a sinking particle. The solid lines correspond to bacterial distributions described by the exponential RDFs used in this work with red color for strong clustering, green color for weak clustering, and blue for a uniform distribution. The shaded areas correspond to results from computer simulations by Desai et al. [37] for chemotactic bacteria with (◊) or without (□) hydrodynamic interactions, and non-chemotactic bacteria with (Δ) or without (○) hydrodynamic interactions.
Fig 4.
Classification of sinking marine particles.
The points represent experimental data for the sinking velocity (SV) and the equivalent sphere diameter (ESD) of individual aggregates. The vertical and horizontal straight lines delimit the boundaries between particle classes. We distinguish three classes with respect to particle size: small (ESD<0.3mm), medium (0.3mm<ESD<0.8mm), and large (ESD>0.8mm); and three classes with respect to sinking velocity: low (SV<20m/d), moderate (20m/d<SV<100m/d), and high (100m/d<SV<300m/d). Source of experimental data: [53] dark green, [54] green, [55] purple, [56] red, [57] blue, [58] magenta, [59] dark cyan, [60] black.
Fig 5.
Plume quenching for marine aggregates.
Predicted impact of the bacterial uptake strength and degree of clustering on the length of the trailing plume behind marine particles. The color represents the length quenching factor, , which is defined as the ratio of the undisturbed plume length,
, at zero-uptake (Da = 0) over the quenched plume length, Lplm. A quenching factor of EL = 2 means that the undisturbed plume is two-times longer than the quenched, and the relative change in the plume length, ΔL ≈ 1 − 1/EL, is 50%. The points represent experimental data for the sinking velocity (SV) and the equivalent sphere diameter (ESD) of individual aggregates. The straight black line corresponds to the timescale condition of Pe/Da = 100 [9]. Computations were performed for small organic solutes, like amino acids and oligo-saccharides, with a diffusivity of
. The contours correspond to selected values of the quenching factor (%relative change): 1.11 (10%), 1.25 (20%), 1.5 (34%), 2 (50%), 3 (67%), 5 (80%), 10 (90%), 15 (93%), 20 (95%), and 30 (97%). Source of experimental data: [53] dark green, [54] green, [55] purple, [56] red, [57] blue, [58] magenta, [59] dark cyan, [60] black.
Fig 6.
Statistics of plume quenching for marine aggregates.
Box chart for the predicted impact of the bacterial uptake strength and clustering on the relative change of the plume length (%ΔL), for the three representative particle classes. Each shaded box defines the interval between the 25th and 75th percentiles, and the middle line is the median of the data. The whiskers of the box define the 10th and 90th percentiles, the square (□) is the average, the crosses (×) define the 1st and 99th percentiles, and the dashes (–) define the min/max. To create this diagram, we calculated the quenching factors for the datasets of {particle size, sinking velocity} from the experimental studies listed in the caption of Fig 5. The quenching data were sorted into bins for small, medium, and large particles in accordance to the classification of the main text. Calculations were carried out for small DOM (), normal and fast uptake (
or 100s), and three levels of clustering (u = no-clustering, w = weak, s = strong; Table 1).
Fig 7.
Bacterial microzones amplify plume quenching.
Predicted impact of the bacterial uptake strength and degree of clustering on plume quenching factors. Similarly to the length quenching factor, , the volume quenching factor,
, is the ratio of the undisturbed plume volume,
, at zero-uptake (Da = 0) over the quenched plume volume, Vplm, at any given conditions. Undisturbed values are given in Fig C of S2 Appendix. The Damköhler is Da = 0.16 for normal uptake and Da = 1.6 for fast uptake. RDF parameters for weak and strong clustering are listed in Table 1. The Péclet number,
, corresponds to a particle radius of
and a solute diffusivity of
[7].
Fig 8.
Three-dimensional plume quenching.
Volumetric representation of the undisturbed nutrient plume in the wake of a slow-sinking particle (Pe = 20, Da = 0) and, also, as reshaped by oligotrophs with uniform distribution and normal uptake (Da = 0.16), mesotrophs with weak clustering and upregulated uptake (Da = 0.8), and copiotrophs with strong clustering and fast uptake (Da = 1.6). Clustering parameters are given in Table 1. Nested isoconcentration surfaces are shown at selected values of nutrient concentration (C = 0.1, 0.2, 0.5 and 0.7). The Péclet number corresponds to an alginate particle of radius , sinking velocity
, and oligo-alginate diffusivity
[7].