Heterotrophic bacteria outcompete diazotrophs for orthophosphate in the Mediterranean Sea

Abstract

Unicellular photoautotrophic diazotrophs such as Crocosphaera spp. are ubiquitous in many oligotrophic and N‐limited oceans, as they can reduce N2 into bioavailable ammonia. The Mediterranean Sea is potentially an ideal environment for photoautotrophic diazotrophic activity, and yet N2‐fixation rates measured in the last two decades are typically very low and no diazotrophic blooms have been recorded in its offshore waters. Previous studies suggest that diazotrophs, as well as nondiazotrophic phytoplankton and heterotrophic bacteria, may be P‐limited, hence their low biomass and activity. Here, we amended surface seawater from six stations across a nutrient gradient in the Mediterranean Sea (east to west transect) with dissolved inorganic phosphorus (DIP), and with seawater‐acclimated inocula of Crocosphaera watsonii, a unicellular cyanobacterial diazotroph, to examine if DIP can stimulate diazotrophy. Our results demonstrate that C. watsonii are poor competitors for DIP relative to native nondiazotrophic heterotrophic microbial populations, especially in the ultraoligotrophic eastern Mediterranean basin, resulting in low N2‐fixation rates. Moreover, the results indicate that when the ambient DIP concentrations are < 35 nmol L−1, unicellular photoautotrophic diazotrophs such as C. watsonii will likely be outcompeted for this macronutrient in the Mediterranean Sea, whereas above 35 nmol L−1 diazotrophy can be stimulated. Our findings support the “bypass theory” stating that photoautotrophs may be outcompeted by heterotrophic bacteria for DIP in nutrient‐poor regions such as the Mediterranean Sea.

Quantification of Aquatic Unicellular Diazotrophs by Immunolabeled Flow Cytometry

“…Corresponding N 2 fixation rates from these samples were between 0.2 to 1.2 nmole N L -1 (Figure 4C), which are in similar ranges to previous reports (Halm et al, 2009; Marcarelli & Wurtsbaugh, 2006, 2009; Rahav et al, 2022). Note, data on N 2 fixation rates in freshwater environments is still limited (Marcarelli et al, 2022).…”

Quantification of Aquatic Unicellular Diazotrophs by Immunolabeled Flow Cytometry

“…Corresponding N 2 xation rates were between 0.2 to 1.2 nmole N L − 1 (Fig. 4C), which are similarly to previous reports (Halm et al, 2009;Marcarelli & Wurtsbaugh, 2006Rahav et al, 2022). Note, data on N 2 xation rates in freshwater environments is highly limited (Marcarelli et al, 2022).…”

Influence of cyclonic and anti-cyclonic eddies on plankton biomass, activity and diversity in the southeastern Mediterranean Sea

“…The differences in cell-specific activity and corresponding doubling time between sites are likely supported by the supplement of limiting nutrients for heterotrophic bacteria. Previous studies showed that P 610 and /or dissolved organic carbon (DOC) are the limiting factors for heterotrophic microbial activity in the SEMS (Pitta et al, 2017;Rahav et al, 2019Rahav et al, , 2021. We hypothesize that in the cyclone heterotrophic bacteria are likely DOC rather than P-limited since the PO4 3+ concentrations at this location were ~3-fold higher than in the other locations (Table 1).…”

“…The equal BP and PP in the background and the cyclone stations demonstrate an imbalanced microbial metabolism, highlighting the importance of heterotrophy in SEMS. Previous studies from the anti-cyclonic Cyprus Eddy and throughout the Mediterranean Sea (Rahav et al, 2021) suggested that heterotrophic bacteria may outcompete phytoplankton or diazotrophs for 630 PO4 3+ . This is in contrast to most oceanic regimes, in which BP:PP<1 at the photic layer (e.g., Lomas et al, 2013).…”