Phytoplankton life history and its relevance for sea surface pCO2
Contact: Elisa Schaum, Inga Hense
Phytoplankton are the small but mighty foundation of aquatic ecosystem and contribute greatly through carbon cycling. Yet, we are only beginning to understand how their fundamental biology is connected to how fast, and how much, they will react to a changing environment through evolution. As a result, many models still assume that phytoplankton responses to warming are ‘fixed’ and do not change throughout time. We show how responses vary across physiological, seasonal, and micro-evolutionary time-scales, and how biodiversity is directly linked to metabolism and environmental predictability in coastal regions (Zhong et al., 2020). We collected data on primary producer carbon metabolism on a fine-grained temporal and spatial scale in the Baltic Sea across several seasons. Even communities with the same or similar species composition from biogeographically related regions differed drastically in the temperature-sensitivity of traits relevant for carbon fluxes (e.g. photosynthesis rates, Fig. 3 left panel, and biomass production, Fig. 3 right panel), and the vulnerability of these traits to species loss. Communities from the overall warmer, less predictable region had overall higher photosynthesis and biomass production rates, whereas samples from the overall cooler, more predictable region had lower photosynthesis and biomass production rates. In communities from the cooler region, functional traits were also more vulnerable to losses in community diversity.
Fig. 3: Sampling locations: Kiel Bight (KB, orange squares), Bornholm Basin (BB, blue squares). Steepness of slopes for traits across dilution gradients indicates functional redundancy, with shallow slopes indicating high redundancy for net photosynthesis (left) and biomass production rates (right).
In another study, we examined to which degree phytoplankton isolates from the same two regions of the Baltic Sea can sustain growth using organic carbon (Listmann et al 2021), since our data repeatedly showed that photosynthesis rates were too low to allow for the growth rates measured in the laboratory and estimated on board. To sustain (exponential) growth, photosynthesis rates need to be higher than respiration rates. However, we found that this was rarely the case, with high respiration at the beginning of the growth phase, and photosynthesis never exceeding respiration even during exponential growth. This was the case regardless of the evolutionary history (sampling region) or physiology (assay temperature) of the samples. When we grew samples on organic carbon sources, organic carbon uptake was highest when photosynthesis was lowest relative to respiration, especially under warm assay temperatures, and in samples with a history of warming and environmental unpredictability. This indicates how evolutionary history may select for different mechanism explaining differences in carbon metabolism in phytoplankton.
Moreover, we investigated biological origins of seasonal changes in sea surface carbon dioxide partial pressure (pCO2) in the central Baltic Sea by using a life cycle model (LCM) that is coupled to a water column model (GOTM). Accounting for the carbonate system in the model, we evaluate the effects of primary production and cyanobacteria life cycle events on pCO2. The simulated Baltic Sea spring and summer blooms capture the observed two pCO2 minima. Our results reveal that mainly diatoms are responsible for the undersaturation of CO2 in spring and N2–fixing cyanobacteria in summer. Including the life cycle of cyanobacteria and thus the timing of the bloom significantly improves the temporal patterns of pCO2 (Fig. 4).
Fig. 4: Model comparison with and without life cycle; without the life cycle the model is unable to capture the observed summer pCO2 minima.
References:
Listmann, L., Kerl, F., Martens, N., Schaum, C. E. (2021). Differences in Carbon Acquisition Could Explain Adaptive Responses in a Baltic Sea Pico-Phytoplankton. Front Mar Sci, 8, 740763, doi: 10.3389/fmars.2021.740763
Zhong, D., Listmann, L., Santelia, M. E., Schaum, C. E. (2020). Functional redundancy in natural pico-phytoplankton communities depends on temperature and biogeography. Biology Letters, 16, 20200330, doi: 10.1098/rsbl.2020.0330