Photosynthetic energy conversion efficiency in the West Antarctic Peninsula

Abstract

The West Antarctic Peninsula (WAP) is a highly productive polar ecosystem where phytoplankton dynamics are regulated by intense bottom-up control from light and iron availability. Rapid climate change along the WAP is driving shifts in the mixed layer depth and iron availability. Elucidating the relative role of each of these controls and their interactions is crucial for understanding of how primary productivity will change in coming decades. Using a combination of ultra-high-resolution variable chlorophyll fluorescence together with fluorescence lifetime analyses on the 2017 Palmer Long Term Ecological Research cruise, we mapped the temporal and spatial variability in phytoplankton photophysiology across the WAP. Highest photosynthetic energy conversion efficiencies and lowest fluorescence quantum yields were observed in iron replete coastal regions. Photosynthetic energy conversion efficiencies decreased by~60% with a proportional increase in quantum yields of thermal dissipation and fluorescence on the outer continental shelf and slope. The combined analysis of variable fluorescence and lifetimes revealed that, in addition to the decrease in the fraction of inactive reaction centers, up to 20% of light harvesting chlorophyll-protein antenna complexes were energetically uncoupled from photosystem II reaction centers in iron-limited phytoplankton. These biophysical signatures strongly suggest severe iron limitation of photosynthesis in the surface waters along the continental slope of the WAP.

Acutely Rising Temperature Reduces Photosynthetic Capacity of Phytoplankton Assemblages in Tropical Oceans: A Large-Scale Investigation

“…Chla biomass in the surface showed a markedly spatial variation, as well as pico-cells abundance (Figure 2). Generally, the growth and primary productivity of phytoplankton are regulated by macro-(e.g., nitrogen and phosphate) and/or micronutrients (e.g., dissolved iron) (Li et al, 2012a;Chinni et al, 2019;Sherman et al, 2020). In the surveyed areas, the surface N and the P levels were <1.0 and 0.10 µM (data not shown), with the N:P ratio being lower than 16, indicating a nitrogen limitation (Li et al, 2012a), especially in the southern Indian Ocean (Wilcoxon test, p < 0.05), where Chla biomass and picocells abundance were lower.…”

“…Major pigments of Synechococcus to constitute the lightharvesting complexes are phycobilisomes (Sliwińska-Wilczewska et al, 2020), while that of Prochlorococcus are divinyl derivatives Chla/b (Ralf and Repeta, 1992); such a pigment difference may vary their light-harvesting abilities, thus leading to the differential photosynthetic responses to the acutely increased temperature between the surface and DCM layers. Larger σ PSII prevailed in the southern part of the surveyed areas where phytoplankton may have adaptively improved its light-harvesting abilities to sustain growth (Zhu et al, 2017;Sherman et al, 2020), because of the low nutrient status therein (data not shown). The larger σ PSII has also been suggested to be attributed to light-adapted genotypic feature due to niche partition (Jin et al, 2016), evidenced by the higher σ PSII values of phytoplankton assemblages from deep waters.…”

“…In the surveyed areas, the surface N and the P levels were <1.0 and 0.10 µM (data not shown), with the N:P ratio being lower than 16, indicating a nitrogen limitation (Li et al, 2012a), especially in the southern Indian Ocean (Wilcoxon test, p < 0.05), where Chla biomass and picocells abundance were lower. Such a biomass variation could also be attributed by the spatial changes of trace mental iron (Chinni et al, 2019;Twining et al, 2019) that is often believed to regulate phytoplankton growth and has been observed to covary with Chla (Chinni et al, 2019;Sherman et al, 2020). On the other hand, the spatial variation in Chla was less in the DCM layer; it could be explained by the high nutrients that may be enough to support the growth of phytoplankton.…”

Chlorophyll fluorescence-based estimates of photosynthetic electron transport in Arctic phytoplankton assemblages

“…Given these sources of variability, NPQ relaxation times needed for robust F v / F m measurements are expected to differ significantly across ocean regimes. In cold waters, such as those encountered along our ship track, NPQ relaxation is slower [61], and this may have contributed to the longer-lived quenching observed in our low-light samples. As the spatial and temporal resolution of F v / F m and ETR a measurements are constrained by such acclimation periods, it is recommended that future field deployments of FRRf conduct experiments using natural assemblages to determine the regional minimum relaxation period necessary to achieve steady-state dark-acclimation.…”

Local‐ and Large‐Scale Drivers of Variability in the Coastal Freshwater Budget of the Western Antarctic Peninsula

“…Shallow sediments in the coastal zone can be influential in this supply, as can glaciers that scour underlying rock and sediments prior to supplying freshwater to the coastal seas (Annett et al, 2017;Boyd & Ellwood, 2010;Raiswell, 2011;Sherrell et al, 2018). Stronger glacial discharge to the ocean has the potential to deliver increased micronutrients (Hawkings et al, 2014), and could impact phytoplankton assemblage composition and biomass (Moline et al, 2004;Sherman et al, 2020). For example, Dierssen et al (2002) noted that >70% of the phytoplankton blooms observed offshore of Anvers Island (Figure 1) occurred soon after meltwater pulses from local glaciers.…”