Iron Deficiency in Seagrasses and Macroalgae in the Red Sea Is Unrelated to Latitude and Physiological Performance

Abstract

Iron can limit primary production in shallow marine systems, especially in tropical waters characterized by carbonated sediments, where iron is largely trapped in a non-available form. The Red Sea, an oligotrophic ecosystem characterized by a strong N-S latitudinal nutrient gradient, is a suitable setting to explore patterns in situ of iron limitation in macrophytes and their physiological performance under different iron regimes. We assessed the interactions between environmental gradients and physiological parameters of poorly-studied Red Sea macrophytes. Iron concentration, chlorophyll a concentration, blade thickness, and productivity of 17 species of macrophytes, including seven species of seagrasses and 10 species of macroalgae, were measured at 21 locations, spanning 10 latitude degrees, along the Saudi Arabian coast. Almost 90% of macrophyte species had iron concentrations below the levels indicative of iron sufficiency and more than 40% had critically low iron concentrations, suggesting that iron is a limiting factor of primary production throughout the Red Sea. We did not identify relationships between tissue iron concentration, chlorophyll a concentration and physiological performance of the 17 species of seagrass and macroalgae. There was also no latitudinal pattern in any of the parameters studied, indicating that the South to North oligotrophication of the Red Sea is not reflected in iron concentration, chlorophyll a concentration or productivity of Red Sea macrophytes.

Carbon and Nitrogen Concentrations, Stocks, and Isotopic Compositions in Red Sea Seagrass and Mangrove Sediments

“…Despite the scarcity of data on Blue Carbon ecosystems from arid regions, recent studies have reported that sediments of seagrass and mangrove ecosystems in the Central Red Sea and the Arabian Gulf are depleted in C org Cusack et al, 2018;Serrano et al, 2018), but contain high carbonate loads . These observations are consistent with the lack of riverine inputs and the oligotrophic nature of the Red Sea, that lead to stunted mangroves (Almahasheer et al, 2016b) and irondepleted leaves of mangroves and seagrasses (Almahasheer et al, 2016a;Anton et al, 2018), and its warm and salty nature, that favors carbonate deposition. However, these previous assessments of sediment C org stocks in Red Sea seagrasses and mangroves are limited to the Central Red Sea Serrano et al, 2018), and no information exists on the associated nitrogen (N) stocks.…”

“…We sampled 43 stations along the Saudi coast of the Red Sea, including 27 seagrass meadows and 16 mangrove stands ( (for further details on cruises and survey methods see Garcias-Bonet and Duarte, 2017;Anton et al, 2018;Duarte et al, 2018;Sea et al, 2018). In addition, stations located in the Central Red Sea were sampled using outboard motor boats on November 2014, February 2016, January 2017, March 2017, and July 2017.…”

Characterization of the CO2 System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea

“…We do not have information regarding the balance between sulfate reduction‐sulfite oxidation at our mangrove forest and seagrass meadow sites. However, there are arguments in the literature against net sulfate reduction in carbonate sediments containing low iron (Burdige et al, ; Holmer et al, ; Hu & Burdige, ; Krumins et al, ), such as in our seagrass meadow and mangrove forest sites (Almahasheer, Duarte, & Irigoien, ; Anton et al, ; Saderne et al, ).…”

Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation

“…Sediment organic N content in 10-11 mg of the sediment samples was also assessed using Organic Elemental Analyser Flash 2000 (Thermo Fisher Scientific, MA, United States). For assessing the P and the Fe concentrations of the leaves and the sediments, 50-100 mg of powderized sample material was digested with 5 ml of HNO 3 (69%) and 1 ml of H 2 O 2 (30%) using a two-step temperature heating protocol [see Anton et al (2018) for further details]. The samples were analyzed using an inductively coupled plasma-optical emission spectrometer (Varian Inc. model 720-ES, CA, United States).…”

“…The samples were broken using beads on a TissueLyser II (Qiagen, Germany), and chl a was extracted by placing the sample overnight in 10 ml of 80% acetone in the dark. The pigment concentration was calculated spectrophotometrically by measuring the absorbance at 646-, 663-, and 710-nm wavelengths and calculating following Wellburn (1994) [see Anton et al (2018) for further details].…”