Microorganisms play a crucial role in supporting biodiversity, maintaining marine and terrestrial ecosystems at the crux of the nutrient cycle. They are the most diverse and abundant of all living creatures, yet little is understood about their distribution or their intimate relationship with the environment. Antarctic ecosystems are among the most simple on Earth; with basic trophic structuring and the absence of many taxonomic groups, they are also isolated geographically with small patchy areas of nutrient inputs. In this instance, Antarctica becomes a pristine laboratory to examine the ecological paradigms already applied to macro-organisms, to determine if common biological laws govern the distribution of biology globally. The decline of biodiversity with increasing latitude is one such observation in the distribution of macro-organisms. In this study, soil microbial community samples were retrieved over a latitude of 56 to 72 °S across the Antarctic Peninsula region. This is a region of special interest due to a rapidly warming climate with mean temperatures increasing at several times the rate of mean global warming. Sites were biologically and environmentally profiled and data used in a variety of multivariate analysis in order to identify spatial trends and infer mechanisms that may be driving Antarctic terrestrial food webs; or where this was not possible, the areas where focus was needed to increase the information profile to allow this. Results indicate a lack of linear latitudinal gradient in microbial diversity, but do show a correlation with environmental heterogeneity; analysis of site diversity identified a gradient between warmer wetter areas, and areas synonymous with cold desert environment at 66⁰S, supported by both phylum composition and indicative soil chemistry. This was confirmed through principal co-ordinates of neighbours’ matrices analysis (PCNM), with distinct regions of community composition being identified when viewed with respect to environmental variables. Considering an overview of diversity with respect to environmental variables provided additional structure to test hypotheses about nutrient webs through structural equation modelling (SEM), and inferred that areas of patchy nutrient input exist and by means of ornithogenic guano additions promote higher C and N availability, increasing microbial abundance and richness.

Laboratory experiments on the physiological response of members of the nanophytoplankton to temperature and light limitation and nutrient saturation were conducted in order to investigate if nanophytoplankton conforms to Plankton Functional Types (PFTs) for modelling purposes. This thesis concluded that nanophytoplankton does not follow all of the assumed physiological traits. The Q10 estimates for members of the nanophytoplankton are considerably lower than Eppley, and since nanophytoplankton does not follow the Eppley curve at warmer temperatures, the results suggest that the Eppley assumptions cannot be used to describe nanophytoplankton. μmax0 is used as a temperature physiological modelling parameter (as well as Q10) which are components of the exponential and linear fits. However, nanophytoplankton best fits to an optimum function which uses μopt, Topt and dT as model parameters. These results are in contrast to the Eppley assumptions. Using a dynamic photosynthesis model five phytophysiological parameters were derived including the maximum photosynthesis rate (Pcm,), respiration rate (resp), the initial slope of the line (achl), light inhibition (βchl) and the maximum chlorophyll to carbon ratio (θmax). These parameters were estimated using an acclimated model which used the instantaneous rates of photosynthesis to estimate the other parameters. The acclimated model gave the best fit (AIC = -3.75 vs. = -0.95). These results are in contrast to those used for PFT modelling purposes. Parameters are comparable for Pcm, resp and θmax but showed significant differences for αchl and βchl the latter of which was underrepresented in the dynamic model, and the former of which is used as a model parameter for PFT parameterization. Chlorophytes had stronger light inhibition (mean βchl= 0.72 g C m2 (mol photons g Chl α)-1) than haptophytes (mean βchl = 0.34 g C m2 (mol photons g Chl α)-1). βchl is significantly lower for haptophytes (P = 0.002). Members of the nanophytoplankton showed relatively high μmax (0.81 d-1 from the acclimated model fit) and mean photosynthesis rates 1.8 Pcm (d-1) mean cell volume 37 μm3). Maximum growth rates increased with increasing cell volume for all of the species. Members of the nanophytoplankton alter their elemental stoichiometry and assimilated nutrients in excess of their requirements but as a PFT, there were no statistically significant deviations from Redfield. Under nutrient replete conditions Chl α:C increased linearly with increasing temperature and increased linearly with decreasing light. Overall, these results suggest that further physiological data is required in order to parameterize models to estimate nanophytoplankton physiological responses to climate change.

Les bactéries et champignons des sols sont impliqués dans différentes fonctions des écosystèmes terrestres. Ils sont notamment investis dans la formation des sols, la stabilité des agrégats et les successions végétales. La Nouvelle- Calédonie est un archipel subtropical, classé comme hotspot de biodiversité et dont un tiers de la surface est recouvert par les substrats ultramafiques. Ces milieux sont caractérisés par de faibles concentrations en nutriments (N, K, P) et de fortes concentrations en métaux lourds (Ni, Co, Cr, Mn). Les écosystèmes présents sur ces substrats sont originaux et diversifiés. Ils sont aussi fortement menacés par l’activité minière. Cependant, pour pouvoir correctement restaurer ces milieux et relancer les dynamiques végétales, il est important de connaître les communautés microbiennes associées à ces écosystèmes ainsi que les facteurs les structurant. Ce travail de recherche a permis d’améliorer nos connaissances sur les communautés microbiennes issues de différents écosystèmes des sols ultramafiques néo-calédoniens, ainsi que sur les interactions existantes entre ces microorganismes et les facteurs biotiques et abiotiques. Les résultats obtenus ont montré que chaque formation végétale et chaque site possèdent une communauté microbienne qui lui est propre, d’où l’intérêt de conserver et protéger les écosystèmes calédoniens. De plus, ces travaux ont aussi montré la capacité des communautés bactériennes et fongiques de servir de bio-indicateurs, et plus particulièrement les communautés fongiques qui sont plus sensibles aux perturbations et variations de la couverture végétale. Il a aussi été démontré qu’en maquis ou forêts monospécifiques, les communautésectomycorhiziennes possèdent des fonctions similaires dans la production d’enzymes de dégradation de la matière organique. Ces travaux ont permis une meilleure connaissance des communautés microbiennes associées auxformations végétales des substrats ultramalfiques ainsi que des facteurs les structurant. Cela devrait améliorer la mise en place des futurs chantiers de restauration de ces écosystèmes. / Soil bacteria and fungi play different functions in terrestrial ecosystems. They are implicated in soil formations, aggregate stability, and plant succession. New Caledonia is a subtropical archipelago, classified as a biodiverse hotspot and a third of its surface is covered by ultramafic soils. These soils are characterised by low concentrations of nutrients (N, K, P) and high concentrations of heavy metals (Ni, Co, Cr, Mn). Ecosystems present in these soils are origina and diversified but strongly threatened by mining activity. It is a necessity to restore these ecosystems after ore exploitation. However, to correctly restorethese environments and relaunch plant dynamics, it is important to identify the microbial communities associated with these ecosystems as well as the structuring factors.This research enabled us to improve our knowledge of microbial communities from different ecosystems on New Caledonian ultramafic substrates, as well as the interactions which exist between these microorganisms and biotic and abiotic factors. Results obtained showed that each plant formation and each site possessed its own microbial community,hence the interest in conserving and protecting New Caledonian ecosystems. Moreover, these works also showed the capacity of bacterial and fungal communities to be used as bioindicators, and more particularly fungal communities which are more sensitive to disturbance and plant cover variations. It has also been demonstrated that in monospecific maquis and rainforests, ectomycorrhizal communities have similar functions in the production of degradative enzymes of organic matter. This research improved understanding of microbial communities associated with plant formations on ultramafic substrates as well as structuring factors. This should improve the implementation of future restoration projectson these ecosystems.