Abstract
Marine protist species have been used for several decades as environmental indicators under the assumption that their ecological requirements have remained more or less stable through time. However, a growing body of evidence suggests that marine protists, including several phytoplankton species, are in fact highly diverse and may quickly respond to changes in the environment. Predicting how future climate will impact phytoplankton populations is important, but this task has been challenged by a lack of time-series of ecophysiological parameters at time-scales relevant for climate studies (i.e. at least decadal). Here, we report on ecophysiological variability in a marine dinoflagellate over a 100-year period of well-documented environmental change, by using the sedimentary archive of living cysts from a Scandinavian fjord (Koljö Fjord, Sweden). During the past century, Koljö Fjord has experienced important changes in salinity linked to the North Atlantic Oscillation (NAO). We revived resting cysts of Pentapharsodinium dalei preserved in the fjord sediments and determined growth rates for 18 strains obtained from 3 sediment core layers at salinity 15 and 30, which represent extreme sea-surface conditions during periods of predominantly negative and positive NAO phases, respectively. Upper pH tolerance limits for growth were also tested. In general, P. dalei grew at a higher rate in salinity 30 than 15 for all layers, but there were significant differences among strains. When accounting for inter-strain variability, cyst age had no effect on growth performance or upper pH tolerance limits for this species, indicating a stable growth response over the 100-year period in spite of environmental fluctuations. Our findings give some support for the use of morphospecies in environmental studies, particularly at decadal to century scales. Furthermore, the high intra-specific variability found down to sediment layers dated as ca. 50 years-old indicates that cyst-beds of P. dalei are repositories of ecophysiological diversity.
Highlights
Our knowledge of past environments and climate change throughout Earth’s history depends on proxy and modelling data, as instrumental recording only began by the end of the 1800 s and for most parameters only within the past few decades
No apparent lag-phases were observed and low variation between replicate flasks indicates that acclimation and balanced growth rates were successfully achieved prior to the start of sampling
Our work represents a novel approach to the study of climateand environmental-driven changes in marine phytoplankton populations
Summary
Our knowledge of past environments and climate change throughout Earth’s history depends on proxy and modelling data, as instrumental recording only began by the end of the 1800 s and for most parameters only within the past few decades. Protists such as foraminifera, coccolithophores, diatoms, and dinoflagellate are widely used as climate proxies, due to their high numbers and rich fossil record They are identified on the basis of their morphology, as morphospecies. Marine protist species have a long evolutionary history, short generation times, huge population sizes, and a large potential to disperse. This has led some authors to argue that protist species are ubiquitous and present little phenotypic variation [1], [2]. The use of morphospecies in environmental research may disregard important diversity and the potential for natural populations to respond rapidly to changing environmental conditions [10]
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