Effect of temperature on the production and degradation of bromoform and other brominated methanes by marine microorganisms

Abstract

Brominated methanes such as bromoform (CHBr₃) are known to be important carriers of bromine from the ocean to the atmosphere. Bromine released from brominated methanes by photolysis has been shown to catalyze ozone depletion. Marine phytoplankton has been reported as a source of CHBr₃ and marine bacteria as a sink for CHBr₃. The effects of temperature on both CHBr₃ production by phytoplankton and CHBr₃ degradation by bacteria have yet to be investigated. We investigated the effects of temperature on CHBr₃ production and CHBr₃ degradation by marine microorganisms. The marine diatom Ditylum brightwellii (CCMP358) was cultured at 15 °C, 20 °C, 24 °C, and 30 °C. The maximum CHBr₃ production rate at 24 °C was 1.57–2.39 pmol (μg chlorophyll a)⁻¹ d⁻¹, several times higher than that at 15 °C (0.25–0.41 pmol (μg chlorophyll a)⁻¹ d⁻¹). Higher rates of CHBr₃, CHBr₂Cl, and CHBrCl₂ production were observed in the late exponential phase (and stationary phase) than in the early exponential phase at each temperature. These results suggest that temperature affects the rate of CHBr₃ production during plankton growth. We then cultured the marine α-proteobacterium Phaeobacter gallaeciensis (JCM 21319) and the γ-proteobacterium Pseudomonas sp. HKF-4 at 10 °C, 15 °C, 20 °C, and 25 °C for up to 15 days to analyze temperature effects on spiked ¹³CHBr₃ degradation. The degradation rate of ¹³CHBr₃ by P. gallaeciensis increased with increasing temperature from 10 °C to 25 °C. The half-life of ¹³CHBr₃ at 25 °C was about 1.1 d, which is about 6 times shorter than the half-life at 10 °C (about 6.9 d). On the other hand, the change in the half-life of the degradation of ¹³CHBr₃ by HKF-1 was relatively small as the temperature increased from 10 °C (half-life: about 5.5 d) to 25 °C (half-life: about 1.8 d). Considering the rate of CHBr₃ production and degradation at each temperature, we estimated how much of the CHBr₃ produced by D. brightwellii for 7 days was degraded by the coexisting bacteria and how much remained after 7 days at each temperature. When coexisting with P. gallaeciensis, the residual CHBr₃ concentration in the culture was relatively higher at 20–25 °C. Similarly, when coexisting with HKF-4, it was relatively higher at 20–25 °C. To estimate the impact of future warming on CHBr₃ concentrations in the oceans, we assume a 5 °C increase in sea surface temperature, with two sea surface temperatures, 15 °C and 20 °C, changing to 20 °C and 25 °C, respectively. Under this assumption, the residual concentration of CHBr₃ produced by D. brightwellii in seawater would be “increased” (or, less likely, “no change”) when P. gallaeciensis coexisted in these areas. Similarly, the total amount of CHBr₃ residuals produced by D. brightwellii in the seawater areas where HKF-4 coexisted increased as the temperature increased by 5 °C. If, as the present results suggest, there is a temperature effect on microbial CHBr₃ production and degradation in the oceans, then future increases in surface seawater temperature could result in an upward trend in CHBr₃ concentrations in the open oceans, although such effects would be greatly influenced by both the phytoplankton species and coexisting bacterial species.