Sphagnum-associated methanotrophs : a resilient CH4 biofilter in pristine and disturbed peatlands

Boreal peatlands are highly important sinks for carbon (C). This function is enabled largely by one peat-forming plant, the Sphagnum moss. In addition to slowing the decomposition by gradually creating ombrotrophic conditions, it gives a shelter for the organisms mitigating the emissions of methane (CH4) – an effective greenhouse gas formed in submerged, anoxic peat layers. These organisms, methane oxidizing bacteria (methanotrophs, MOB), inhabit the dead, water-filled hyaline cells of the Sphagnum and provide the plant carbon dioxide (CO2) derived from the CH4 oxidation. While several studies have confirmed the presence of Sphagnum-associated methanotrophs (SAM), it is still unclear how dependent they are on the mosses and how environmental conditions affect their community composition and activity. This thesis evaluated SAM dynamics in the different stages of peatland development on both pristine and disturbed areas. Studies were based mainly on molecular methods, targeting the MOB-specific pmoA gene, and laboratory incubations, including stable isotope probing. In the first study, the connection between the SAM and the mosses was assessed by testing whether the SAM will disperse through the water phase. This trait, considered to represent a facultative symbiosis, was demonstrated in two experiments. In the field, mosses inactive in CH4 oxidation were transplanted next to active ones. Within a month, SAM communities of the neighboring mosses become more similar. The water-based colonization was further confirmed by bathing inactive mosses in flark pore water that showed high CH4 oxidation activity. Within just 11 h, activity was induced and the SAM abundance significantly increased in the treated mosses. The other two studies revealed similar SAM community composition patterns on a pristine chronosequence and on a gradient of re-vegetating cutover peatlands. Instead of the Sphagnum species, the general environmental conditions seemed to control the SAM community composition. Different types of SAM seemed to have their preferred environmental niches, with the type Ia MOB present and active especially in the young succession stages and the type II MOB in the older, hydrologically more stable stages. Despite the community differences, the potential CH4 oxidation did not differ along the gradients, suggesting functional redundancy. Only some drier bog samples did not show any detectable CH4 oxidation, demonstrating the regulatory role of the water table level on the SAM activity. The peat layers of the cutover gradient showed similar MOB community patterns but the potential CH4 oxidation increased with succession. The ability to disperse through the water provides a recovery mechanism from disturbances such as droughts, which are predicted to increase with climate warming. In addition, the diversity and functional redundancy of the SAM communities enhance the resilience of this important CH4 biofilter formed by the living Sphagnum mosses. The potential SAM activity in the mosses of the youngest cutover site promotes the Sphagnum transplantation practice as a tool to not only enhance the re-vegetation process, but also to mitigate the CH4 emissions formed in the rewetting and restoration of disturbed peatlands.Pohjoiset suot ovat tärkeitä hiilen nieluja sitoessaan sitä hitaasti kertyviin turvekerroksiin. Samalla ne vapauttavat kasvihuonekaasu metaania (CH4), jota syntyy kasvimateriaalin hajotessa hapettomissa oloissa. Näiden soiden yleisin kasvi, rahkasammal (Sphagnum), on oleellinen paitsi turpeen kertymiselle, myös metaanipäästöjen torjunnalle: Se tarjoaa asuinympäristön metaania hapettaville bakteereille, metanotrofeille, jotka suodattavat osan metaanista rajoittaen sen vapautumista ilmakehään. Samalla metaanin hapetuksessa syntyvä hiilidioksidi (CO2) siirtyy rahkasammalen käyttöön. Rahkasammalien ja metanotrofien muodostama ”metaanisuodin” on havaittu useammassa tutkimuksessa, mutta muun muassa sen herkkyys ympäristön muutoksille tunnetaan huonosti. Tässä väitöstutkimuksessa selvitettiin rahkasammalien ja niissä elävien metanotrofien vuorovaikutuksen tarkempaa luonnetta. Metanotrofien yhteisörakennetta ja aktiivisuutta tarkasteltiin suhteessa suoekosysteemissä tapahtuviin muutoksiin. Vertailun vuoksi tutkittiin myös sammalten alapuolisten turvekerrosten metaaninhapetusta. Metanotrofien molekyylibiologisessa analysoinnissa hyödynnettiin erityisesti metaaninhapetuksen mahdollistavaa pmoA-geeniä sekä siihen perustuvaa koetinsirutekniikkaa. Ensimmäisessä osatyössä metanotrofien ja niiden hapetusaktiivisuuden osoitettiin leviävän rahkasammalesta toiseen veden välityksellä. Työn perusteella nämä metanotrofit kykenevät elämään myös sammalien ulkopuolella eli niiden välillä näyttäisi olevan ns. fakultatiivinen symbioosi. Kahdessa muussa osatyössä rahkasammalten metanotrofien dynamiikan havaittiin olevan hyvin samankaltaista kahdella erilaisella suon kehitysgradientilla: sekä luonnontilaisilla soistuvilla aloilla, että eri-ikäisillä, uudelleen kasvittuneilla turvetuotantoaloilla. Metanotrofien yhteisöt olivat monimuotoisia kaikissa kehitysvaiheissa, mutta niiden valtaryhmät erosivat nuorten, vedenpinnaltaan epävakaiden alojen ja vanhojen, täysin sammalpeitteisten alojen välillä. Huolimatta yhteisöjen eroista metaaninhapetusaktiivisuus ei eronnut merkitsevästi eri kehitysvaiheissa, lukuun ottamatta aivan vanhimpien alojen heikompaa aktiivisuutta. Tämä viittaa ns. toiminnalliseen päällekkäisyyteen, jossa tietystä toiminnosta vastaa useampi, eri olosuhteisiin erikoistunut eliöryhmä. Toisin kuin elävässä sammalkerroksessa, sen alapuolisessa turpeessa hapetuspotentiaali kasvoi turpeentuotantoalojen kasvipeitteen kehityksen mukana. Kyky levitä veden välityksellä sekä toiminnallinen päällekkäisyys viittaavat rahkasammalten metanotrofien pystyvän sekä sopeutumaan erilaisiin olosuhteisiin, että palautumaan suoekosysteemiä kohtaavista häiriöistä, kuten ilmastonmuutoksen mukana mahdollisesti lisääntyvistä kuivista jaksoista. Rahkasammalkerroksessa jo nuorimmalla turvetuotantoalalla havaittu metaaninhapetuspotentiaali tulisi huomioida näitä aloja ennallistettaessa: rahkasammal-istutusten avulla näyttäisi olevan mahdollista paitsi käynnistää hiilensidonta turpeeksi, myös torjua samanaikaisesti lisääntyviä metaanipäästöjä

Restriction of plant roots in boreal forest organic soils affects the microbial community but does not change the dominance from ectomycorrhizal to saprotrophic fungi

Boreal forest soils store significant amounts of carbon and are cohabited by saprotrophic and ectomycorrhizal fungi (ECM). The 'Gadgil effect' implies antagonistic interactions between saprotrophic fungi and ECM. Plant photosynthates support the competitive fitness of the ECM, and may also shape the soil bacterial communities. Many 'Gadgil effect' experiments have focused on litter layer (O-L) or have litter and root-fragments present, and thus possibly favor the saprotrophs. We compared how the restriction of plant roots and exudates affect soil microbial community structures in organic soil (mixed O-F and O-H). For this, we established a 3-yr field experiment with 3 different mesh treatments affecting the penetration of plant roots and external fungal hyphae. Exclusion of plant photosynthates induced modest changes in both fungal and bacterial community structures, but not to potential functionality of the microbial community. The microbial community was resilient towards rather short-term disturbances. Contrary to the 'Gadgil effect', mesh treatments restricting the entrance of plant roots and external fungal hyphae did not favor saprotrophs that originally inhabited the soil. Thus, we propose that different substrate preferences (fresh litter vs. fermented or humified soil), rather than antagonism, maintain the spatial separation of saprotrophs and mycorrhizal fungi in boreal forest soils.Peer reviewe

Above- and belowground fluxes of CH4 from boreal shrubs and Scots pine

201

Above- and belowground fluxes of methane from boreal dwarf shrubs and Pinus sylvestris seedlings

The contribution of boreal forest plants to the methane (CH4) cycle is still uncertain. We studied the above and belowground CH4 fluxes of common boreal plants, and assessed the possible contribution of CH4 producing and oxidizing microbes (methanogens and methanotrophs, respectively) to the fluxes. We measured the CH4 fluxes and the amounts of methanogens and methanotrophs in the above- and belowground parts of Vaccinium myrtillus, Vaccinium vitis-idaea, Calluna vulgaris and Pinus sylvestris seedlings and in non-planted soil in a microcosm experiment. The shoots of C. vulgaris and P. sylvestris showed on average emissions of CH4, while the shoots of the Vaccinium species indicated small CH4 uptake. All the root-soil-compartments consumed CH4, however, the non-rooted soils showed on average small CH4 emission. We found methanotrophs from all the rooted and non-rooted soils. Methanogens were not detected in the plant or soil materials. The presence of plant roots seem to increase the amount of methanotrophs and thus CH4 uptake in the soil. The CH4 emissions from the shoots of C. vulgaris and P. sylvestris demonstrate that the plants have an important contribution to the CH4 exchange dynamics in the plant-soil systems.Peer reviewe

Metaanin hapettajat – suon hiilen ja typen kierron kaksoisagentit

Tieteen tor

Soil-tree-atmosphere CH4 flux dynamics of boreal birch and spruce trees during spring leaf-out

Peer reviewe

Integrating Decomposers, Methane-Cycling Microbes and Ecosystem Carbon Fluxes Along a Peatland Successional Gradient in a Land Uplift Region

Peatlands are carbon dioxide (CO2) sinks that, in parallel, release methane (CH4). The peatland carbon (C) balance depends on the interplay of decomposer and CH4-cycling microbes, vegetation, and environmental conditions. These interactions are susceptible to the changes that occur along a successional gradient from vascular plant-dominated systems to Sphagnum moss-dominated systems. Changes similar to this succession are predicted to occur from climate change. Here, we investigated how microbial and plant communities are interlinked with each other and with ecosystem C cycling along a successional gradient on a boreal land uplift coast. The gradient ranged from shoreline to meadows and fens, and further to bogs. Potential microbial activity (aerobic CO2 production; CH4 production and oxidation) and biomass were greatest in the early successional meadows, although their communities of aerobic decomposers (fungi, actinobacteria), methanogens, and methanotrophs did not differ from the older fens. Instead, the functional microbial communities shifted at the fen-bog transition concurrent with a sudden decrease in C fluxes. The successional patterns of decomposer versus CH4-cycling communities diverged at the bog stage, indicating strong but distinct microbial responses to Sphagnum dominance and acidity. We highlight young meadows as dynamic sites with the greatest microbial potential for C release. These hot spots of C turnover with dense sedge cover may represent a sensitive bottleneck in succession, which is necessary for eventual long-term peat accumulation. The distinctive microbes in bogs could serve as indicators of the C sink function in restoration measures that aim to stabilize the C in the peat.Peer reviewe

New insight to the role of microbes in the methane exchange in trees : evidence from metagenomic sequencing

Methane (CH4) exchange in tree stems and canopies and the processes involved are among the least understood components of the global CH4 cycle. Recent studies have focused on quantifying tree stems as sources of CH4 and understanding abiotic CH4 emissions in plant canopies, with the role of microbial in situ CH4 formation receiving less attention. Moreover, despite initial reports revealing CH4 consumption, studies have not adequately evaluated the potential of microbial CH4 oxidation within trees. In this paper, we discuss the current level of understanding on these processes. Further, we demonstrate the potential of novel metagenomic tools in revealing the involvement of microbes in the CH4 exchange of plants, and particularly in boreal trees. We detected CH4-producing methanogens and novel monooxygenases, potentially involved in CH4 consumption, in coniferous plants. In addition, our field flux measurements from Norway spruce (Picea abies) canopies demonstrate both net CH4 emissions and uptake, giving further evidence that both production and consumption are relevant to the net CH4 exchange. Our findings, together with the emerging diversity of novel CH4-producing microbial groups, strongly suggest microbial analyses should be integrated in the studies aiming to reveal the processes and drivers behind plant CH4 exchange.Peer reviewe

Methane (CH4) emissions from trees in boreal upland and drained peatland forests

201

Importance of methanotrophy in the C and N cycle

Abstrakti O9201