The effect of elevated CO2 on growth and competition in experimental phytoplankton communities

Abstract

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.

Long-term culture at elevated atmospheric CO2fails to evoke specific adaptation in seven freshwater phytoplankton species

“…We used modified Bold's Basal Medium (BBM) with pH adjusted to 7 using HCl , supplemented with silicate (0.58 g l 21 Na 2 SiO 3 ) and vitamins (2 ml l 21 of the vitamin mix from F/2 Medium; for further information on the medium and references see [22]). We allowed the medium to equilibrate with chamber air for 5 days before inoculation.…”

The biological pump in a high CO<sub>2 world

“…Shifts in phytoplankton composition due to ocean acidification (Tortell et al 2002, Low-Décarie et al 2011, Meakin & Wyman 2011 or increased temperatures (Hare et al 2007) are commonly observed experimentally. The impact of ocean acidification on the calcifying coccoli thophores has received special attention.…”

How competition affects evolutionary rescue

“…Ecological and evolutionary responses to changing environments are contingent on the community in which the change occurs [6][7][8][9][10]. A population's ability to adapt and persist in changing environments will therefore also hinge on the surrounding community [11] (see also [12]).…”

“…But competition can also increase selective pressure [23], speed niche expansion [24][25][26] and increase rates of evolution [27], possibly allowing populations to adapt to new conditions faster. These potentially contrasting effects may account for the unanticipated population dynamics and patterns of persistence in competitive communities [6] (but see [10]). …”