Responses of physiological groups of tropical heterotrophic bacteria to temperature and dissolved organic matter additions: food matters more than warming

Abstract

Compared to higher latitudes, tropical heterotrophic bacteria may be less responsive to warming because of strong bottom-up control. In order to separate both drivers, we determined the growth responses of bacterial physiological groups to temperature after adding dissolved organic matter (DOM) from mangroves, seagrasses and glucose to natural seawater from the Great Barrier Reef. Low (LNA) and high (HNA) nucleic acid content, membrane-intact (Live) and membrane-damaged (Dead) plus actively respiring (CTC+) cells were monitored for 4 days. Specific growth rates of the whole community were significantly higher (1.9 day^-1 ) in the mangrove treatment relative to the rest (0.2-0.4 day^-1 ) at in situ temperature and their temperature dependence, estimated as activation energy, was also consistently higher. Strong bottom-up control was suggested in the other treatments. Cell size depended more on DOM than temperature. Mangrove DOM resulted in significantly higher contributions of Live, HNA and CTC+ cells to total abundance, while the seagrass leachate reduced Live cells below 50%. Warming significantly decreased Live and CTC+ cells contributions in most treatments. Our results suggest that only in the presence of highly labile compounds, such as mangroves DOM, can we anticipate increases in heterotrophic bacteria biomass in response to warming in tropical regions.

Figure 1

Dynamics of mean total [sum of low (LNA) and high (HNA) nucleic acid content bacteria] bacterial abundance during the 4 days incubations in the (A) Control, (B) Glucose, (C) Mangrove and (D) Seagrass treatments. Error bars represent standard deviations of 3 replicates. Dashed smooth fitting joins treatment points for clarity. Specific growth rate calculations included at least 3 sampling points (i.e. from day 0 to day 2) except in the Mangrove treatment at in situ and +3°C temperatures. Control at −3°C and in situ and Seagrass at in situ and +3°C temperatures included also day 3 while Seagrass at −3°C included the 4 days.

Figure 2

Mean percent contributions of Live, HNA and CTC+ bacteria in the different DOM and temperature treatments (n = 12) plus at the beginning of the experiments (t0, n = 3). −3, 3°C below in situ temperature; is, in situ temperature; +3, 3°C above in situ temperature. Different letters represent significant differences between DOM treatments at in situ temperature (ANOVA, P < 0.05, Tukey–Kramer HSD). Error bars represent standard deviations.

Figure 3

Dynamics of mean bacterial cell size during the 4 days incubations in the (A) Control, (B) Glucose, (C) Mangrove and (D) Seagrass treatments. Error bars and dashed lines are shown as in Fig. 1. Notice the different Y‐axis scale in C.

Figure 4

Activation energies of the specific growth rates and carrying capacities of the total bacterial community (sum of LNA and HNA cells). Error bars represent standard errors of the estimates. Different letters represent significant differences between DOM treatments (ANOVA, P < 0.05, Tukey–Kramer HSD).

Figure 5

Scatterplot of the mean specific growth rates of the total bacterial community versus BDOC in the different DOM treatments and temperatures. Increasing BDOC values within each treatment match exactly increasing incubation temperatures. Error bars represent standard errors of three replicates.