Phytoplankton are the primary producers in marine and freshwaters and key components of ecological food webs. Their growth is often shaped by the presence of important resources, such as light, nutrients (trace metals and macronutrients such as nitrogen and silica) and an organism’s optimal growth temperature. In certain regions of the ocean, phytoplankton productivity is low, despite high macronutrient concentrations. Typically, the macronutrient nitrogen tends to play a role in limiting phytoplankton growth. However, in high-nutrient, low-chlorophyll (HNLC) regions nitrogen is never significantly depleted due to insufficient iron supply (Martin et al., 1990a, Martin et al., 1990b). The Southern Ocean is the largest HNLC region, and it accounts for around 40% of the oceanic uptake of carbon dioxide (CO2) through biological and physical processes. The capacity of marine phytoplankton to remove CO2 from surface waters potentially increases carbon drawdown from the atmosphere into the deep ocean (Boyd et al., 2010), making the Southern Ocean crucial to understanding how atmospheric CO2 concentrations will change in the future. Therefore, determining how different levels of light, iron, and temperature affect diatom photosynthesis in the Southern Ocean allows us to better understand how atmospheric CO2 concentrations will change in the future.
Figure 1. Black and white image of an Antarctic mixed diatom assemblage, including Coscinodiscus, Proboscia, Fragilaria, Phaeocystis, Chaetoceros and Pseudo-nitzschia species (Photo credit: Carly Moreno)
Phytoplankton blooms in the Southern Ocean are usually dominated by diatoms (Figure 1) or the haptophyte Phaeocystis antarctica, and these taxa have evolved to grow at lower iron concentrations due to the low iron inputs from continental sources. Iron plays an essential role in photosynthesis, which accounts for most of the cells’ iron requirements. Southern Ocean diatoms have been forced to adapt to the low iron supply in a variety of ways. These strategies differ between species and include, but are not limited to, the reorganization of proteins in photosynthesis (Strzepek and Harrison, 2004), or substituting iron-containing proteins for metal-free proteins or proteins that utilize other metals (La Roche et al., 1996). Usually when temperate diatoms respond to low light, they increase cellular iron concentrations to optimize photosynthesis by increasing the number of proteins required to process light (Figure 2B). However, this does not happen in Southern Ocean diatoms, which inhabit the low light and low iron conditions of the Southern Ocean. Southern Ocean diatoms have overcome the problematic iron and light relationship by increasing the size of their photosynthetic antennae, increasing the amount of light captured, and does not require extra iron-containing proteins (Figure 2A; Strzepek et al., 2012, Strzepek et al., 2019).
Figure 2. Southern Ocean diatoms (A) are surprisingly efficient at fixing carbon per mol of iron (Fe) compared to temperate diatoms (B). Southern Ocean diatoms (A) have larger chlorophyll antennae (σ) that transfer light energy to the iron containing photosystem (PS), in the first step of photosynthesis were water is split into protons, electrons and oxygen. In contrast, temperate diatoms (B) optimize photosynthesis by increasing the number of iron containing photosystems, which are associated with smaller chlorophyll antennae, therefore increasing cellular iron requirements in temperate diatoms. (Strzepek et al., 2019)
However, as global temperatures climb due to increased atmospheric CO2 concentrations, an increase in temperature may also increase carbon fixation by increasing enzyme reaction rates. As iron-rich proteins are essential in photosynthesis, warming may increase cellular iron concentrations by increasing the rate at which the cells photosynthesize. Recently, consequences for warming and iron demand in the Southern Ocean have been discussed on the basis of individual diatom culture responses to a range of temperatures (Andrew et al., 2019, Boyd, 2019). Perhaps not surprisingly, it was found that iron availability plays a role in thermal tolerance in diatoms (Figure 3). Both studies also discuss the ecological impacts for Southern Ocean diatom communities, showing that despite the evidence for highly specialized polar species, thermal generalists also currently persist under the low light and low iron conditions in the Southern Ocean. This contrasts with the genetic and physiological evidence that phytoplankton need unique specializations to photosynthesize optimally in the cold, low light and low iron Antarctic environment. Subsequently the long-held belief that these specialist Southern Ocean species are biologically isolated from generalist species from lower latitudes is up for discussion (Fraser et al., 2018). Understanding diatom diversity within the Southern Ocean and community resilience to global change is an important step toward understanding future climate and ecosystem impacts.
Figure 3. Surface seawater temperature increase will select for diatoms with higher optimum growth temperatures (Topt = Temperature optimum)
Sarah Andrew is a postdoctoral research associate in the Department of Marine Sciences, University of North Carolina at Chapel Hill, North Carolina, USA
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