Resource Colimitation Drives Competition Between Phytoplankton and Bacteria in the Southern Ocean

Abstract

Phytoplankton growth in large areas of the Southern Ocean is regulated by the micronutrient iron (Fe)

Examining the Interaction Between Free‐Living Bacteria and Iron in the Global Ocean

“…These findings are in agreement with the results of Ratnarajah et al. (2021), who used a mechanistic model of FL bacteria‐phytoplankton competition in an idealized setting: Luxury Fe uptake by FL bacteria exacerbates the competition between FL bacteria and phytoplankton. Furthermore, in highly seasonal regimes, such as at mid‐ and high‐latitudes, competition for Fe increases over the course of the productive season as the accumulation of LDOC produced by phytoplankton exudation, sloppy feeding, and zooplankton excretion stimulates the FL bacterial activity (Sturluson et al., 2008).…”

“…Our results are thus consistent with previous measurements in the subarctic Pacific, which reported that FL bacteria account for ∼20%–40% of the total biological Fe uptake (Tortell et al., 1996). In addition, various field and incubation and modeling studies in the Southern Ocean (Boyd et al., 2012; Fourquez et al., 2020; Kuparinen et al., 2011; Ratnarajah et al., 2021) showed that FL bacteria are a significant competitor with phytoplankton for Fe if LDOC is available.…”

“…The degree of competition between bacteria‐phytoplankton is very sensitive to the modeled FL bacterial Fe demand and quotas, which are still poorly constrained at between 1 e − 6 and more than 80 e − 6 mol Fe/mol C by observations (Fourquez et al., 2015; Mazzotta et al., 2020; Tortell et al., 1996, 1999). An elevated FL bacterial demand for Fe, either to meet their metabolic requirements or to build a large Fe internal storage through luxury uptake, enhances their competition with phytoplankton (Bowie et al., 2001; Ratnarajah et al., 2021) and the amount of Fe that is cycled through FL bacteria (Figure 12). However, our model results suggest that most of the Fe used by FL bacteria is recycled in the upper ocean, with very little exported to the ocean interior.…”

“…Despite increasing evidence on the importance of FL bacteria in modulating oceanic Fe cycling, our understanding of the interactions between FL bacteria and Fe is still very limited. For instance, it is still unclear if Fe is the ultimate factor limiting the growth of FL bacteria in the ocean, especially when both labile DOC and Fe concentrations are low (Church et al., 2000; Obernosterer et al., 2015; Ratnarajah et al., 2021). In addition, the limited existing ocean measurements have reported a wide range of values for the Fe contents of FL bacteria, resulting in a large uncertainty in our knowledge of the FL bacterial Fe demand, storage, and uptake capability (Fourquez et al., 2015; Mazzotta et al., 2020; Tortell et al., 1999).…”

Microbiomes of the bloom-forming alga,Phaeocystis globosa, are stable, consistently recruited communities with symbiotic and opportunistic modes

“…For example, phycosphere bacteria can stimulate the growth of specific phytoplankton by producing growth hormones [5] or supplying a limiting nutrient, such as a B-vitamin [6][7][8] or fixed-nitrogen [9]. Conversely, phycosphere bacteria can manipulate phytoplankton communities by secreting algicidal molecules (e.g., roseobacticides) that selectively kill certain phytoplankton [10] or by competing for essential nutrients [11,12]. Even small shifts in phytoplankton communities can alter how much organic carbon is recycled in the surface ocean or exported to deep water [13] since phytoplankton taxa vary in their C:N:P ratio [14], morphology, and sinking rates [15].…”