Technical note: Rapid image-based field methods improve the quantification of termite mound structures and greenhouse-gas fluxes

Abstract

Abstract. Termite mounds (TMs) mediate biogeochemical processes with global relevance, such as turnover of the important greenhouse gas methane (CH4). However, the complex internal and external morphology of TMs impede an accurate quantitative description. Here we present two novel field methods, photogrammetry (PG) and cross-sectional image analysis, to quantify TM external and internal mound structure of 29 TMs of three termite species. Photogrammetry was used to measure epigeal volume (VE), surface area (AE) and mound basal area (AB) by reconstructing 3-D models from digital photographs, and compared against a water-displacement method and the conventional approach of approximating TMs by simple geometric shapes. To describe TM internal structure, we introduce TM macro- and micro-porosity (θM and θμ), the volume fractions of macroscopic chambers, and microscopic pores in the wall material, respectively. Macro-porosity was estimated using image analysis of single TM cross sections, and compared against full X-ray computer tomography (CT) scans of 17 TMs. For these TMs we present complete pore fractions to assess species-specific differences in internal structure. The PG method yielded VE nearly identical to a water-displacement method, while approximation of TMs by simple geometric shapes led to errors of 4–200 %. Likewise, using PG substantially improved the accuracy of CH4 emission estimates by 10–50 %. Comprehensive CT scanning revealed that investigated TMs have species-specific ranges of θM and θμ, but similar total porosity. Image analysis of single TM cross sections produced good estimates of θM for species with thick walls and evenly distributed chambers. The new image-based methods allow rapid and accurate quantitative characterisation of TMs to answer ecological, physiological and biogeochemical questions. The PG method should be applied when measuring greenhouse-gas emissions from TMs to avoid large errors from inadequate shape approximations.

Termite mounds contain distinct methanotroph communities that are kinetically adapted to elevated methane concentrations

“…S2) ; the latter observation is in line with the finding that CH 4 oxidation occurs at low rates in T. pastinator mound material [7]. Consistently, pmoA copy numbers were significantly positively correlated with the porosity of the mound material, with periphery samples showing a stronger dependence (R 2 = 0.49, p = 0.0051) than core (R 2 = 0.26, p = 0.035); this suggests that denser mound material, as found in mound periphery and T. pastinator mounds, limits methanotroph abundance [19]. These differences may also reflect the relatively harsh conditions in the mound periphery, with its strong fluctuations of temperature and water content, compared to the core with termite-engineered homeostasis [39, 40].…”

“…For further investigations following field measurements, we selected 17 termite mounds of an appropriate size for processing in the laboratory (initially 18, but one was damaged during transport and had to be discarded). These mounds were first excavated but kept intact to measure internal structure, volume, densities, and porosities as previously described [19]. They were then deconstructed to (i) sample termites for species identification, (ii) collect mound material for gravimetric water content measurements, and (iii) collect mound material for molecular analyses of methanotrophic community.…”

“…They were then deconstructed to (i) sample termites for species identification, (ii) collect mound material for gravimetric water content measurements, and (iii) collect mound material for molecular analyses of methanotrophic community. For species determination, soldiers were individually picked and stored in pure ethanol for species confirmation as previously described [19]. For gravimetric water content measurements, approximately 200 g of mound material from both core and periphery locations were subsampled and oven dried at 105 °C for >72 hours; subsamples were measured before and after drying and the water content calculated based on mass loss.…”

“…The gas push-pull test was used to estimate in situ activity coefficients as described previously [19, 31]. Michaelis-Menten parameters estimated from in situ methods are integrated measures across a large mass of substrate and are thus better suited to characterise the kinetic potential of whole microbial communities in heterogeneous systems than laboratory microcosms, which suffer from inevitable sampling bias [32].…”

“…Argon concentrations were analysed on a customised gas chromatography system (SRI 8610, SRI Instruments, Torrance, CA) with an external thermal conductivity detector (TCD; VICI Valco Instruments Co., Houston, TX). To improve separation of Ar, oxygen was removed from the sample-gas stream prior to separation with a manually packed Pd-Al catalyst column [19, 33].…”