Transport and assimilation of inorganic carbon by Lichina pygmaea under emersed and submersed conditions

Metatranscriptomics reveals diversity of symbiotic interaction and mechanisms of carbon exchange in the marine cyanolichen Lichina pygmaea

Lichens are exemplar symbioses based upon carbon exchange between photobionts and their mycobiont hosts. Historically considered a two-way relationship, some lichen symbioses have been shown to contain multiple photobiont partners; however, the way in which these photobiont communities react to environmental change is poorly understood. Lichina pygmaea is a marine cyanolichen that inhabits rocky seashores where it is submerged in seawater during every tidal cycle. Recent work has indicated that L. pygmaea has a complex photobiont community including the cyanobionts Rivularia and Pleurocapsa. We performed rRNA-based metabarcoding and mRNA metatranscriptomics of the L. pygmaea holobiont at high and low tide to investigate community response to immersion in seawater. Carbon exchange in L. pygmaea is a dynamic process, influenced by both tidal cycle and the biology of the individual symbiotic components. The mycobiont and two cyanobiont partners exhibit distinct transcriptional responses to seawater hydration. Sugar-based compatible solutes produced by Rivularia and Pleurocapsa in response to seawater are a potential source of carbon to the mycobiont. We propose that extracellular processing of photobiont-derived polysaccharides is a fundamental step in carbon acquisition by L. pygmaea and is analogous to uptake of plant-derived carbon in ectomycorrhizal symbioses.

The impact of nitrogen deposition on photobiont-mycobiont balance of epiphytic lichens in subtropical forests of central China

Excessive nitrogen (N) deposition can impact lichen diversity in forest ecosystems, and this is a particular situation in China. Here, we examined the N uptake, assimilation, and the impact of excessive N deposition on the symbiotic balance of dominant epiphytic lichens in the subtropical forests in the Mts. Shennongjia of central China. The results show that lichen species took up, assimilated and utilized more ammonium than nitrate in a species-specific way, following the increase of N availability. The photobiont of the lichens decreased with the increase of N concentration following an initial increase, while the mycobiont response to the N addition was not apparent. Considerable variation in response to excessive N deposition exists among the lichen species. Usnea longissima could regulate its N uptake, resulting in a stable photobiont-mycobiont ratio among N treatments. In contrast, the photobiont-mycobiont ratio of other four lichens increased initially but decreased when N concentration exceeded a certain level, and N stress may have broken the balance between photobiont and mycobiont of these lichens. Our results suggest that most epiphytic lichens in subtropical forest of central China could uptake and assimilate more ammonium than nitrate and that the balance between photobiont and mycobiont of many epiphytic lichens might change with the increasing N deposition load, which could impact the lichen diversity of this forest ecosystem.

Affinities and architecture of Devonian trunks of Prototaxites loganii

Devonian fossil logs of Prototaxites loganii have been considered kelp-like aquatic algae, rolled up carpets of liverworts, enormous saprophytic fungal fruiting bodies or giant lichens. Algae and rolled liverwort models cannot explain the proportions and branching described here of a complete fossil of Prototaxites loganii from the Middle Devonian (386 Ma) Bellvale Sandstone on Schunnemunk Mountain, eastern New York. The "Schunnemunk tree" was 8.83 m long and had six branches, each about 1 m long and 9 cm diam, on the upper 1.2 m of the main axis. The coalified outermost layer of the Schunnemunk trunk and branches have isotopic compositions (δ(13)CPDB) of -25.03 ± 0.13‰ and -26.17 ± 0.69‰, respectively. The outermost part of the trunk has poorly preserved invaginations above cortical nests of coccoid cells embraced by much-branched tubular cells. This histology is unlike algae, liverworts or vascular plants and most like lichen with coccoid chlorophyte phycobionts. Prototaxites has been placed within Basidiomycota but lacks clear dikaryan features. Prototaxites and its extinct order Nematophytales may belong within Mucoromycotina or Glomeromycota.

The role of C4 metabolism in the marine diatom Phaeodactylum tricornutum

Diatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO(2) is much higher than that of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), indicating that a CO(2)-concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C(4), remains elusive. Although (14)C labeling experiments and presence of complete sets of genes for C(4) metabolism in two diatoms supported the presence of C(4), other data and predicted localization of the decarboxylating enzymes, away from Rubisco, makes this unlikely. We used RNA-interference to silence the single gene encoding pyruvate-orthophosphate dikinase (PPDK) in Phaeodactylum tricornutum, essential for C(4) metabolism, and examined the photosynthetic characteristics. The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K(1/2) (CO(2)) was hardly affected, thus clearly indicating that the C(4) route does not serve the purpose of raising the CO(2) concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic V(max) was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation. We propose that the C(4) metabolism does not function in net CO(2) fixation but helps the cells to dissipate excess light energy and in pH homeostasis.

Responses of epiphytic lichens to an experimental whole-tree nitrogen-deposition gradient

Here, we examined the responses of the epiphytic lichens Alectoria sarmentosa and Platismatia glauca to increased atmospheric nitrogen (N) deposition in an old-growth boreal spruce forest, to assess the sensitivity of these species to N and define their critical N load. Nitrogen deposition was simulated by irrigating 15 trees over a 3 yr period with water and isotopically labeled NH(4)NO(3), providing N loads ranging from ambient to 50 kg N ha(-1) yr(-1) . Thallus N concentration increased in both species with increasing N load, and uptake rates of both NH(4)(+) and NO(3)(-) were similar. Photobiont concentration increased linearly with increased N in both species, saturating in A. sarmentosa in the third year at the highest N loads (25 and 50 kg ha(-1) yr(-1)). The simulated N deposition decreased the phosphorus (P) concentration in A. sarmentosa, and increased the N:P ratio in both species. Significant responses in lichen chemistry were detected to inputs of 12.5 kg N ha(-1) yr(-1) or higher, suggesting that resources other than N limit lichens at higher N loads. However, the data also suggest that N saturation may be cumulative over time, even at low N.

Phagotrophy in the origins of photosynthesis in eukaryotes and as a complementary mode of nutrition in phototrophs: relation to Darwin's insectivorous plants

Darwin performed innovative observational and experimental work on the apparently paradoxical occurrence of carnivory in photosynthetic flowering plants. The nutritional use of particulate organic material which also supplies other elements is now known to be widespread in free-living algae as well as in organisms with endosymbiotic algae and with kleptoplastids. In addition to this direct nutritional role, phagotrophy, in the broad sense of internalization of photosynthetic organisms by a eukaryote, is essential for the occurrence of present-day endosymbiotic algae and kleptoplastid-containing protists, and was essential for the origin of plastids themselves. The endosymbiotic phenomena involving photosynthetic organisms clearly played a major role in combining genomes providing different metabolic functions, but, in our opinion, this does not demand a re-appraisal of evolution by natural selection. That the balance of physiological optimization for competition for resources and minimization of losses (e.g. through predation) is a fine one, and thus subject to a complex selective process, is illustrated by the diversity of mixotrophic strategies in extant phytoplankton.

Carbon and nitrogen distribution in the green algal lichens Hypogymnia physodes and Platismatia glauca in relation to nutrient supply

With the aim of understanding how some lichens can survive intensive fertilization we investigated two green algal ( Trebouxia) lichens, Hypogymnia physodes (L.) Nyl. and Platismatia glauca (L.) W. Culb., and compared control (Ctr), and intensively fertilized (F) thalli. We measured total N, proteins and amino acids to assess lichen N status. Chlorophyll a indicated photosynthetic capacity and photobiont mass, ergosterol the metabolic demands of the fungus, and chitin the fungal biomass. For carbon status we measured glucose, the photobiont ( Trebouxia) export product ribitol, and the mycobiont-specific carbohydrates arabitol and mannitol. The F-thalli had 2-3 times higher protein and N concentrations, 5-10 times higher chlorophyll a concentrations, while ergosterol and chitin were doubled. The ribitol concentrations were 4-5 times higher in the F-thalli, while the fungal carbohydrates did not increase to the same extent. The amino acid arginine had increased 60-fold. The F-thalli also had a relatively higher N investment in the photobiont in relation to mycobiont tissue compared to the Ctr-thalli, probably resulting in an increased capacity for carbon assimilation, most possibly required for maintaining the higher nutrient status of the F-thalli. Arginine accumulation possibly avoided toxic effects of accumulated NH4+, albeit binding a significant fraction of assimilated carbon.

Carbonic anhydrase(s) associated with lichens: in vivo activities, possible locations and putative roles

The activity of the enzyme carbonic anhydrase (CA; E.C. 4.2.1.1) was examined in eight species of lichen in vivo by measurements of ¹⁸ O exchange from doubly labelled CO₂ (¹³ C¹⁸ O¹⁸ O) to water, with the aim of determining whether this enzyme is present in lichens and if so to what extent it participates in physiological processes. The lichens were chosen to represent different mycobiont (Heterodera, Leptogium, Lobaria, Peltigera, Pseudocyphellaria, Xantoparmelia) and photobiont (Coccomyxa, Dictyochloropsis, Myrmecia, Nostoc, Trebouxia) genera as well as varying morphology. All lichens were found to possess considerable activity of CA, which varied by a factor of about four between the species when compared on a chlorophyl basis, and by u factor of about 10 when related to the weight. Incubation of lichen tholli with ethoxyzolamide (EZA) and acetazolamide (AZA), which inhibit intracellular and extracellular CA respectively, showed that intracellular CA was present in all of the species. Incubation with EZA also resulted in inhibition of photosynthetic CO₂ , uptake in all species suggesting a role of internal CA in the CO₂ , acquisition process in all the investigated photobiont genera. A part of the CA activity was inhibited by AZA in all except two of the four cyanobacterial Nostoc lichens, indicating the possible presence of extracellular forms of CA. This CA also appeared to be involved in the acquisition of CO₂ , as AZA inhibited photosynthetic CO₂ uptake in these lichens. The CA inhibitors also affected CO₂ efflux in the dark, i.e. fungal respiration, which was inhibited by both AZA and EZA in six of the species, suggesting that a proportion of the CA activity might be located in the mycobiont.