Effect of humic substance photodegradation on bacterial growth and respiration in lake water

Abstract

This study addresses how humic substance (HS) chemical composition and photoreactivity affect bacterial growth, respiration, and growth efficiency (BGE) in lake water. Aqueous solutions of HSs from diverse aquatic environments representing different dissolved organic matter sources (autochthonous and allochthonous) were exposed to artificial solar UV radiation. These solutions were added to lake water passed through a 0.7-microm-pore-size filter (containing grazer-free lake bacteria) followed by dark incubation for 5, 43, and 65 h. For the 5-h incubation, several irradiated HSs inhibited bacterial carbon production (BCP) and this inhibition was highly correlated with H2O2 photoproduction. The H2O2 decayed in the dark, and after 43 h, nearly all irradiated HSs enhanced BCP (average 39% increase relative to nonirradiated controls, standard error = 7.5%, n = 16). UV exposure of HSs also increased bacterial respiration (by approximately 18%, standard error = 5%, n = 4), but less than BCP, resulting in an average increase in BGE of 32% (standard error = 10%, n = 4). Photoenhancement of BCP did not correlate to HS bulk properties (i.e., elemental and chemical composition). However, when the photoenhancement of BCP was normalized to absorbance, several trends with HS origin and extraction method emerged. Absorbance-normalized hydrophilic acid and humic acid samples showed greater enhancement of BCP than hydrophobic acid and fulvic acid samples. Furthermore, absorbance-normalized autochthonous samples showed approximately 10-fold greater enhancement of BCP than allochthonous-dominated samples, indicating that the former are more efficient photoproducers of biological substrates.

FIG. 1.

Bacterial carbon production response {i.e., [(BCP_UV − BCP_dark)/BCP_dark] × 100} after 5 h and 43 to 65 h of the addition of irradiated or nonirradiated humic substances. Results are given as the means ± standard errors (number of samples = 4). *, statistically significant differences between the dark and UV treatments (t test, P < 0.05). See Table 1 for humic substance number.

FIG. 2.

Change in bacterial carbon production (i.e., percentage of increment in BCP from the addition of irradiated HS relative to nonirradiated control) 5 h after the humic samples were added to unfiltered lake water plotted as a function of the concentration of the hydrogen peroxide produced during humic substance irradiation (y = −0.27x + 0.14, r² = 0.90).

FIG. 3.

(a) Bacterial abundance and (b) bacterial production response on a per cell basis {as percentages, i.e., [(BCP_UV − BCP_dark)/BCP_dark] × 100}, after the addition of irradiated or nonirradiated humic substances to lake water. Results are given as means (43 to 65 h) ± standard errors (number of samples = 4). *, statistically significant differences between dark and UV treatments (t test, P < 0.05).

FIG. 4.

(a) Comparison of absorbance-normalized BCP (i.e., increment in BCP from the addition of irradiated HS relative to nonirradiated control, divided by the absorbance coefficient at 300 nm) after the 43-h incubation for different humic substance extraction types. (b) Comparison of the average absorbance-coefficient-normalized BCP for the 43-h incubation, among the different HS sample sources [i.e., (HPOA + HPIA)/2 and (HA + FA)/2]. Abbreviations: Allocht (predominantly allochthonous), Mix (mixed source), Autocht (predominantly autochthonous). See Table 1 for more details about the HSs.