Dissolved Organic Carbon Source Influences Tropical Coastal Heterotrophic Bacterioplankton Response to Experimental Warming

Abstract

Global change impacts on marine biogeochemistry will be partly mediated by heterotrophic bacteria. Besides ocean warming, future environmental changes have been suggested to affect the quantity and quality of organic matter available for bacterial growth. However, it is yet to be determined in what way warming and changing substrate conditions will impact marine heterotrophic bacteria activity. Using short-term (4 days) experiments conducted at three temperatures (-3°C, in situ, +3°C) we assessed the temperature dependence of bacterial cycling of marine surface water used as a control and three different dissolved organic carbon (DOC) substrates (glucose, seagrass, and mangrove) in tropical coastal waters of the Great Barrier Reef, Australia. Our study shows that DOC source had the largest effect on the measured bacterial response, but this response was amplified by increasing temperature. We specifically demonstrate that (1) extracellular enzymatic activity and DOC consumption increased with warming, (2) this enhanced DOC consumption did not result in increased biomass production, since the increases in respiration were larger than for bacterial growth with warming, and (3) different DOC bioavailability affected the magnitude of the microbial community response to warming. We suggest that in coastal tropical waters, the magnitude of heterotrophic bacterial productivity and enzyme activity response to warming will depend partly on the DOC source bioavailability.

Keywords: Great Barrier Reef; dissolved organic carbon; extracellular enzymatic activity; microbial carbon cycling; temperature; tropical coastal waters.

FIGURE 1

Average bioavailable dissolved organic carbon (BDOC; difference between initial and minimum DOC concentration) during the control, glucose, seagrass, and mangrove experiments, performed at –3°C (gray), in situ (white), and +3°C (black). Error bars represent standard deviations.

FIGURE 2

Dynamics of bacterial biomass (BB; A–D), production (BP; E–H), and cell-specific bacterial production (Cell BP; I–L) during the 4 days incubations in the different experiments (control, glucose, seagrass, and mangrove) performed at –3°C (), in situ (), and +3°C (). Values are mean values of three replicates and the Error bars represent standard deviations, where not visible error bars are within the symbol. Please note the difference in Y-axis scaling for the mangrove experiment.

FIGURE 3

Bacterial growth efficiency (BGE in %) during the control, glucose, seagrass, and mangrove experiments performed at –3°C (gray), in situ (white), and +3°C (black). Values are mean values of three replicates and the error bars represent standard deviations.

FIGURE 4

Dynamics of estimated potential enzyme activity rates of alkaline phosphatase (APase A–D), β-glucosidase (BGase; E–H), and leucine aminopeptidase (LAPase; I–L) during the 4 days incubations in the different experiments (control, glucose, seagrass, and mangrove) performed at –3°C (), in situ (), and +3°C (). Values are mean values of three replicates and the Error bars represent standard error, where not visible error bars are within the symbol. Please note the difference in Y-axis scaling for all the enzyme activities in the mangrove experiment and the seagrass LAPase activity.

FIGURE 5

Graphical representation of the bioavailable dissolved organic carbon (BDOC; difference between initial and minimum DOC concentration), bacterial growth and bacterial respiration in all experiments (control, glucose, seagrass, and mangrove), over the range of temperatures (–3°C, in situ, and +3°C). The shaded areas highlight the bacterial respiration, as the difference between bioavailable DOC and bacterial growth. Please note that due to the scale the differences in bacterial growth with temperature are not visible.