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. 2016 Jul 11:6:29286.
doi: 10.1038/srep29286.

A light-induced shortcut in the planktonic microbial loop

Robert Ptacnik  1   2 , Ana Gomes  3 , Sarah-Jeanne Royer  3 , Stella A Berger  4   5 , Albert Calbet  3 , Jens C Nejstgaard  4   5 , Josep M Gasol  3 , Stamatina Isari  3   6 , Stefanie D Moorthi  1 , Radka Ptacnikova  1   2 , Maren Striebel  1   7 , Andrey F Sazhin  8 , Tatiana M Tsagaraki  4   9   10 , Soultana Zervoudaki  11 , Kristi Altoja  12 , Panagiotis D Dimitriou  10 , Peeter Laas  12 , Ayse Gazihan  13 , Rodrigo A Martínez  3 , Stefanie Schabhüttl  2   14 , Ioulia Santi  10 , Despoina Sousoni  10 , Paraskevi Pitta  9
Affiliations

A light-induced shortcut in the planktonic microbial loop

Robert Ptacnik et al. Sci Rep. .

Abstract

Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.

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Figures

Figure 1
Figure 1. Light regime in the experiment.
The left y-axis gives average (Imix) and range (error bars) of light intensities proportional to incoming light (Iin) as a function of the optical filters used. The right axis gives the corresponding optical depth in the Eastern Mediterranean, assuming an extinction coefficient of 0.05 m−1.
Figure 2
Figure 2
Upper row: time course of heterotrophic bacteria (HB) and Synechococcus. Note that abundances of Synechococcus are multiplied by 5 to scale them with HB. Lower row: Ratio of photosynthetic picoeukaryotes PE:HB and HF:HB, based on cell concentrations, as an indicator of potential grazing control of flagellates on HB (cf. Gasol 1994). The inverse ratio is given on the 2nd y axis. Figure symbols distinguish replicates.
Figure 3
Figure 3. Average abundances of HB & Synechococcus (left) and PE & HF (right) along the light gradient (avg. days 4–8).
Note that abundances of Synechococcus are up-scaled by a factor of five to align them with HB. All regression lines are highly significant. Summary statistics are given in Table S1.
Figure 4
Figure 4. Net growth rates of heterotrophic bacteria (HB) plotted against abundances of PE.
Growth rates were calculated for adjacent time intervals, PE abundances averaged for the corresponding time intervals. Left plot: The colored points give the relationship for the first time interval (day 0–day 1), together with a linear fit (p < 0.001). Right plot: Growth rates for all time intervals. Linear fits calculated separately for each light level. p < 0.001 except for L12 (p = 0.1). Correlation tests for the same data are given in Table 1.
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References

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