Marine phytoplankton temperature versus growth responses from polar to tropical waters--outcome of a scientific community-wide study

Abstract

"It takes a village to finish (marine) science these days" Paraphrased from Curtis Huttenhower (the Human Microbiome project) The rapidity and complexity of climate change and its potential effects on ocean biota are challenging how ocean scientists conduct research. One way in which we can begin to better tackle these challenges is to conduct community-wide scientific studies. This study provides physiological datasets fundamental to understanding functional responses of phytoplankton growth rates to temperature. While physiological experiments are not new, our experiments were conducted in many laboratories using agreed upon protocols and 25 strains of eukaryotic and prokaryotic phytoplankton isolated across a wide range of marine environments from polar to tropical, and from nearshore waters to the open ocean. This community-wide approach provides both comprehensive and internally consistent datasets produced over considerably shorter time scales than conventional individual and often uncoordinated lab efforts. Such datasets can be used to parameterise global ocean model projections of environmental change and to provide initial insights into the magnitude of regional biogeographic change in ocean biota in the coming decades. Here, we compare our datasets with a compilation of literature data on phytoplankton growth responses to temperature. A comparison with prior published data suggests that the optimal temperatures of individual species and, to a lesser degree, thermal niches were similar across studies. However, a comparison of the maximum growth rate across studies revealed significant departures between this and previously collected datasets, which may be due to differences in the cultured isolates, temporal changes in the clonal isolates in cultures, and/or differences in culture conditions. Such methodological differences mean that using particular trait measurements from the prior literature might introduce unknown errors and bias into modelling projections. Using our community-wide approach we can reduce such protocol-driven variability in culture studies, and can begin to address more complex issues such as the effect of multiple environmental drivers on ocean biota.

Figure 1. Summary of the locations at which the species/strains were initially isolated.

A) Overlaid (locales denoted by white stars) on a global map of satellite sea surface temperature (°C, from World Ocean Atlas, [91]); B) Projected surface ocean temperature changes for the early and late 21st century relative to the period 1980–1999. The global average surface ocean temperature change is plotted against the relative probabilities of estimated global average warming from several different AOGCM and Earth System Model of Intermediate Complexity. The data are for average projections for the B1, A1B, and A2 SRES scenarios. Plot is from IPCC AR4 .

Figure 2. Thermal reaction norms for multiple strains of Thalassiosira rotula (left panel) Akashiwo sanguinea (central panel) and Thalassiosira pseudonana (right panel) used in our study.

Figure 3. Thermal reaction norms for tropical to polar phytoplankton (single strains) used in our study.

Figure 4. Thermal reaction norms for multiple strains of the tropical a) Trichodesmium erythraeum; b) Crocosphaera watsonii phytoplankton used in our study.

Figure 5. A comparison of the thermal trait, niche width (°C) using box and whisker plots, between previously published studies (using a wide range of experimental protocols, see [43] ) and the species/strains used in the present study.

The black bands denote the median value, the bottom and top of the red/blue boxes represent the 1st and 3rd quartile of the data respectively. The ‘whiskers’ extending from the boxes indicate the positions of the lowest & highest values in the data. If the sample size is small enough, the whiskers may not appear (e.g. if there are only 3 equally spaced points, the value represented as the 1st quartile is the lowest value).

Figure 6. A comparison of the thermal trait, T_opt (°C) (box and whisker plots), between previously published studies (using a wide range of experimental protocols, see [43]) and the species/strains used in the present study.

For details see Figure 5 caption.

Figure 7. A comparison of the maximum specific growth rate (day

⁻¹), using box and whisker plots, between previously published studies (using a wide range of experimental protocols, see [43]) and the species/strains used in the present study.

Figure 8. Thermal reaction norms for the two end-members species from our study compared with predicted ocean warming trends.

Projected warming by 2020–2029 and 2090–2099 red bars (see Figure 1B) and temperature range (arrow) (from [91]), over the annual cycle is overlaid on the two reaction-norms (0.9 to 4.3°C for the polar diatom and 23.7 to 28.3°C for the Crocosphaera strains).