We report an experiment designed to identify the effect of elevated CO 2 on species of phytoplankton in a simple laboratory system. Major taxa of phytoplankton differ in their ability to take up CO 2 , which might lead to predictable changes in the growth rate of species and thereby shifts in the composition of phytoplankton communities in response to rising CO 2 . Six species of phytoplankton belonging to three major taxa (cyanobacteria, diatoms and chlorophytes) were cultured in atmospheres whose CO 2 concentration was gradually increased from ambient levels to 1000 parts per million over about 100 generations and then maintained for a further 200 generations at elevated CO 2 . The experimental design allowed us to trace a predictive sequence, from physiological features to the growth response of species to elevated CO 2 in pure culture, from the growth response in pure culture to competitive ability in pairwise mixtures and from pairwise competitive ability to shifts in the relative abundance of species in the full community of all six species. CO 2 altered the dynamics of growth in a fashion consistent with known differences among major taxa in their ability to take up and use CO 2 . This pure-culture response was partly successful in predicting the outcome of competition in pairwise mixtures, especially the enhanced competitive ability of chlorophytes relative to cyanobacteria, although generally statistical support was weak. The competitive response in pairwise mixtures was a good predictor of changes in competitive ability in the full community. Hence, there is a potential for forging a logical chain of inferences for predicting how phytoplankton communities will respond to elevated CO 2 . Clearly further extensive experiments will be required to validate this approach in the greater complexity found in diverse communities and environments of natural systems.
The conditions that allow biodiversity to recover following severe environmental degradation are poorly understood. We studied community rescue, the recovery of a viable community through the evolutionary rescue of many populations within an evolving community, in metacommunities of soil microbes adapting to a herbicide. The metacommunities occupied a landscape of crossed spatial gradients of the herbicide (Dalapon) and a resource (glucose), whereas their constituent communities were either isolated or connected by dispersal. The spread of adapted communities across the landscape and the persistence of communities when that landscape was degraded were strongly promoted by dispersal, and the capacity to adapt to lethal stress was also related to community size and initial diversity. After abrupt and lethal stress, community rescue was most frequent in communities that had previously experienced sublethal levels of stress and had been connected by dispersal. Community rescue occurred through the evolutionary rescue of both initially common taxa, which remained common, and of initially rare taxa, which grew to dominate the evolved community. Community rescue may allow productivity and biodiversity to recover from severe environmental degradation.
The concentration of CO 2 in the atmosphere is expected to double by the end of the century. Experiments have shown that this will have important effects on the physiology and ecology of photosynthetic organisms, but it is still unclear if elevated CO 2 will elicit an evolutionary response in primary producers that causes changes in physiological and ecological attributes. In this study, we cultured lines of seven species of freshwater phytoplankton from three major groups at current (approx. 380 ppm CO 2 ) and predicted future conditions (1000 ppm CO 2 ) for over 750 generations. We grew the phytoplankton under three culture regimes: nutrient-replete liquid medium, nutrient-poor liquid medium and solid agar medium. We then performed reciprocal transplant assays to test for specific adaptation to elevated CO 2 in these lines. We found no evidence for evolutionary change. We conclude that the physiology of carbon utilization may be conserved in natural freshwater phytoplankton communities experiencing rising atmospheric CO 2 levels, without substantial evolutionary change.
Nutrients can limit the productivity of ecosystems and control the composition of the communities of organisms that inhabit them. Humans are causing atmospheric CO2 concentrations to reach levels higher than those of the past millions of years while at the same time propagating eutrophication through the addition of nutrients to lakes and rivers. We studied the effect of elevated CO2 concentrations, nutrient addition and their interaction in a series of freshwater mesocosm experiments using a factorial design. Our results highlight the important role of CO2 in shaping phytoplankton communities and their response to nutrient addition. We found that CO2 greatly magnified the increase in phytoplankton growth caused by the increased availability of nutrients. Elevated CO2 also caused changes in phytoplankton community composition. As predicted from physiology and laboratory experiments, the taxonomic group that was most limited by current day CO2 concentrations, chlorophytes, increased in relative frequency at elevated CO2. This predictable change in community composition with changes in CO2 is not altered by changes in the availability of other nutrients.
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