Impact of Wood Age on Termite Microbial Assemblages

Abstract

The decomposition of wood and detritus is challenging to most macroscopic organisms due to the recalcitrant nature of lignocellulose. Moreover, woody plants often protect themselves by synthesizing toxic or nocent compounds which infuse their tissues. Termites are essential wood decomposers in warmer terrestrial ecosystems and, as such, they have to cope with high concentrations of plant toxins in wood. In this paper, we evaluated the influence of wood age on the gut microbial (bacterial and fungal) communities associated with the termites Reticulitermes flavipes (Rhinotermitidae) (Kollar, 1837) and Microcerotermes biroi (Termitidae) (Desneux, 1905). We confirmed that the secondary metabolite concentration decreased with wood age. We identified a core microbial consortium maintained in the gut of R. flavipes and M. biroi and found that its diversity and composition were not altered by the wood age. Therefore, the concentration of secondary metabolites had no effect on the termite gut microbiome. We also found that both termite feeding activities and wood age affect the wood microbiome. Whether the increasing relative abundance of microbes with termite activities is beneficial to the termites is unknown and remains to be investigated. IMPORTANCE Termites can feed on wood thanks to their association with their gut microbes. However, the current understanding of termites as holobiont is limited. To our knowledge, no studies comprehensively reveal the influence of wood age on the termite-associated microbial assemblage. The wood of many tree species contains high concentrations of plant toxins that can vary with their age and may influence microbes. Here, we studied the impact of Norway spruce wood of varying ages and terpene concentrations on the microbial communities associated with the termites Reticulitermes flavipes (Rhinotermitidae) and Microcerotermes biroi (Termitidae). We performed a bacterial 16S rRNA and fungal ITS2 metabarcoding study to reveal the microbial communities associated with R. flavipes and M. biroi and their impact on shaping the wood microbiome. We noted that a stable core microbiome in the termites was unaltered by the feeding substrate, while termite activities influenced the wood microbiome, suggesting that plant secondary metabolites have negligible effects on the termite gut microbiome. Hence, our study shed new insights into the termite-associated microbial assemblage under the influence of varying amounts of terpene content in wood and provides a groundwork for future investigations for developing symbiont-mediated termite control measures.

Keywords: Microcerotermes biroi; Reticulitermes flavipes; bacteria; core-microbiome; ectosymbionts; endosymbionts; fungi; plant defenses; terpenoids; wood-feeding termites.

FIG 1

Terpenic compounds content (μg/g DW) (DW, dry weight) present in Norway spruce wood upon storage. FW_C represents freshly cut spruce wood, SW_C denotes commercially available spruce timber felled 3 to 4 years ago, and OW_C represents old wood that was cut circa 120 years ago. Values are the average of two independent analytical determinations of compound concentrations in pooled homogenized (n = 6) samples of wood. Error bars represent the uncertainty (U) of the whole analytical procedure, including extraction and GC-MS (gas chromatography coupled with mass spectrometry) instrumental analysis. U is based on relative standard deviation (RSD) obtained from 9 independent determinations of compounds concentration in one homogenized wood sample. U is calculated using the equation U = 2 × RSD.

FIG 2

Relative abundance of bacterial communities. (a and b) Relative abundance of bacterial phyla (abundance of ≥1% in at least one sample) within termites (Reticulitermes flavipes and Microcerotermes biroi) feeding on different substrates, termite-infested wood, and their controls. “Others” denotes the total relative abundance of other phyla. (c and d) Heatmap illustrating the top 45 bacterial genera with relative abundance of ≥1% in at least one sample documented in Reticulitermes and Microcerotermes before and after feeding on different substrates, termite-infested wood, and their controls. Color gradient from red to green through black represents the relative abundance of bacterial operational taxonomic units (OTUs) present in each sample type. Red color, low abundance; green color, high abundance. FW_C, fresh wood control; SW_C, standard wood control; OW_C, old wood control; FW_R_W, Reticulitermes-infested fresh wood; SW_R_W, Reticulitermes-infested standard wood; OW_R_W, Reticulitermes-infested old wood; R_C_T, Reticulitermes control termite before feeding; FW_R_T, Reticulitermes feeding on fresh wood; SW_R_T, Microcerotermes feeding on standard wood; OW_R_T, Reticulitermes feeding on old wood; Cellu_R_T, Reticulitermes feeding on cellulose; FW_M_W, Microcerotermes-infested fresh wood; SW_M_W, Microcerotermes-infested standard wood; OW_M_W, Microcerotermes-infested old wood; M_C_T, Microcerotermes control termite before feeding; FW_M_T, Microcerotermes feeding on fresh wood; SW_M_T, Microcerotermes feeding on standard wood, OW_M_T, Microcerotermes feeding on old wood; Cellu_M_T, Microcerotermes feeding on cellulose.

FIG 3

Nonmetric multidimensional scaling (NMDS) plot. The variation in the bacterial communities’ present in (a) the two different termites (Reticulitermes flavipes and Microcerotermes biroi) feeding on different substrates and their control. (b) Different wood substrates infested by the termites and control uninfested wood. Samples are denoted by different shapes (square, control; circle, M. biroi; triangle, R. flavipes); colors denote substrate types.

FIG 4

Relative abundance of fungal communities. (a and b) Relative abundance of fungal order with relative abundance of ≥1% in at least one sample within termites (R. flavipes and M. biroi) upon feeding on different substrates, termite-infested wood and their controls. “Others” denotes the total relative abundance of rest of the fungal orders present. (c and d) Heatmap illustrating the top 39 fungal genera documented in Reticulitermes and Microcerotermes before and after feeding on different substrates, termite-infested spruce wood, and control wood. Color gradient from red to green through black represents the relative abundance of fungal OTUs present in each sample type. Red color denotes low abundance, green color signifies high abundance. Group abbreviations are the same as in Fig. 2.

FIG 5

NMDS plot. The variation in the fungal communities present in (a) the two termites feeding on different substrates and their control, and (b) different wood substrates either infested by the termites or control uninfested wood. Sample types are denoted by different symbols as in Fig. 3.

FIG 6

Flower diagram representing the core and unique OTUs. (a and b) Core and unique bacterial OTUs shared among R. flavipes and M. biroi upon feeding on different substrates. (c and d) Common and unique fungal OTUs shared among Reticulitermes and Microcerotermes upon feeding on different substrates. Group abbreviations are the same as in Fig. 2.