Distributions of hydrogen peroxide and superoxide in seawater east of New Zealand

Abstract

During four seasonally disparate voyages in 2005 and 2006, hydrogen peroxide (H₂O₂) concentrations were measured in surface seawater east of New Zealand on a transect from 46° 38′ S to 41° 10′ S along the 178° 30′ E meridian. Concentrations of H₂O₂ along the transect ranged from 8.5 nM to 138.5 nM, and varied seasonally with the highest mean concentrations measured in summer and the lowest in winter. Differences in the intensities of photosynthetically-active radiation between seasons explained much of this seasonal variation in H₂O₂ concentrations (r² = 0.997, p < 0.01). These data, as well as results from formation and decay studies, strongly support the present hypothesis that a photochemically-initiated formation mechanism causes seasonal variation in open ocean sea surface H₂O₂ concentrations east of New Zealand.

Steady-state concentrations of superoxide (HO₂/O₂⁻) measured in depth-profile samples during a voyage in February 2009 ranged from below the detection limit (218 pM) to a maximum of 32.7 nM. Superoxide maxima were coincident with distinct deep chlorophyll maxima at two sampling sites in the Subtropical Front (STF) east of New Zealand. The data presented in the present study suggest that biologically-mediated production mechanisms can sometimes lead to steady-state concentrations of superoxide in seawater in the nanomolar range. The present study also confirms that solar irradiance alone is capable of explaining much of the natural temporal variation in H₂O₂ concentrations in surface seawater.

Introduction

In seawater, hydrogen peroxide (H₂O₂) is a transient chemical species resulting mainly from the protonation of the superoxide radical anion (O₂⁻), and disproportionation of the resulting hydroperoxyl radical (HO₂) (Bielski and Allen, 1977, Zafiriou, 1990, Micinski et al., 1993). The superoxide radical anion (O₂⁻) is the product of one-electron reduction of diatomic oxygen (Bielski and Allen, 1977), which can be initiated photochemically when an excited-state substrate transfers an electron to oxygen (Blough and Zepp, 1995).

substrate + hv → substrate*

substrate* + O₂ → substrate⁺ + O₂^.−

O₂⁻ + H⁺ ↔ HO₂

HO₂ + HO₂ → H₂O₂ + O₂

HO₂ ^. + O₂^−. → HO₂ ⁻+ O₂ + H₂O → H₂O₂ + O₂ + OH⁻

Thus, the main sources of H₂O₂ in seawater are expected to involve photochemically-initiated processes (e.g. Cooper and Zika, 1983, Palenik et al., 1987, Cooper et al., 1988, Gerringa et al., 2004). In contrast, the main sinks for H₂O₂ in seawater have been shown to involve biologically-mediated processes mediated by the enzymes catalase and peroxidase (Zepp et al., 1987, Cooper and Zepp, 1990, Moffett and Zafiriou, 1990). Additionally, aqueous redox processes involving oxygen, transition metals, and natural organic matter act as both sources and sinks of H₂O₂ and can influence concentrations of H₂O₂ in seawater over daily and seasonal timescales (Zika et al., 1985a, Zika et al., 1985b, Moffett and Zika, 1987, Zuo and Holgne, 1992, Hanson et al., 2001). Because H₂O₂ can be formed photochemically, and subsequently plays important roles related to nutrient acquisition and metabolism in biological systems (Kutska et al., 2005, Rose et al., 2005, Garg et al., 2007), information about the energetic relationships between large-scale physicochemical processes and smaller-scale biological processes in the upper ocean can be determined from spatial and temporal distributions of H₂O₂ (Steigenberger and Croot, 2008). The usefulness of H₂O₂ as an energy carrier between the physicochemical and biological systems stems from oxygen's efficacy as an electron carrier for biologically-mediated redox reactions (Rose et al., 2005, Falkowski, 2006), and the occurrence of hydrogen peroxide as a relatively stable intermediate in the O₂/H₂O redox cycle.

New Zealand is located near the boundaries of several major oceanic water masses, and therefore it provides “a natural laboratory” for measurement of chemical parameters and study of biogeochemical processes in a variety of oceanic conditions (e.g. Nodder et al., 2005, Tian et al., 2006). The Subtropical Front (STF) is a circum-global oceanographic feature, part of which is located east of the South Island of New Zealand (Sutton, 2001) (Fig. 1). The STF is an area of dynamic chemical and biological activity where macronutrient-poor Subtropical (ST) surface water from the north mixes with macronutrient-rich Subantarctic (SA) surface water from the south, (Boyd et al., 1999, Tian et al., 2006).

Compared to ST water, SA water is cooler, less saline, and contains higher concentrations of the macronutrients nitrate, silicate, and phosphate (Nodder et al., 2005, Tian et al., 2006). Due to the perennially high concentrations of nitrate and low concentrations of chlorophyll that are caused by iron-limitation of phytoplankton growth, SA water is described as high-nitrate low-chlorophyll (HNLC) (Banse and English, 1997, Boyd et al., 1999, Boyd et al., 2000). In contrast, ST water, north of the STF, is characterized by warmer temperatures, higher salinities, and lower concentrations of macronutrients, relative to SA water (Nodder et al., 2005, Tian et al., 2006). Annual mean concentrations of chlorophyll-a are at least twofold higher in ST waters, relative to SA waters, and primary production rates are generally higher in ST waters on seasonal time-scales (Bradford-Grieve et al., 1997, Nodder et al., 2005). Previous studies have found the phytoplankton assemblages in the STF region to be dominated by diatoms, with the genus Lauderia being most common (Chang and Gall, 1998). Other common genera in the STF include Stephanopyxis, Thalassiosira, Chaetoceros, Nitszchia/Pseudonitzschia, and Hemiaulus, all of which are diatoms (Chang and Gall, 1998). Dinoflagellates, especially Ceratium fusus, sometimes dominate the ST water, particularly in winter, whereas nanoflagellates, particularly Cryptomonas spp., are reportedly dominant in SA water (Chang and Gall, 1998). Some aspects of the biology and chemistry in this region are well-defined; however, prior to the present study, very little has been known about the processes and feedbacks that influence concentrations of H₂O₂ and superoxide in marine waters of the STF region.

Due to the dramatic differences in the chemical compositions, nutrient regimes, and levels of biological primary productivity between SA and ST waters, and the role that reactive oxygen species play in influencing the speciation and bioavailability of nutrients including iron (e.g. Croot et al., 2004, Croot et al., 2005, Kutska et al., 2005, Rose et al., 2005), significant spatial variation in H₂O₂ concentrations were expected along a transect spanning the water masses. Specifically, it was hypothesized that concentrations of H₂O₂ are higher in the SA surface water mass, where primary productivity is limited by low levels of bioavailable iron, silica, and light (Boyd et al., 1999), with a converse hypothesis for ST waters. Temporal variations in concentrations of H₂O₂ over seasonal timescales were also expected in the study area, and it was hypothesized that seasonal variations in H₂O₂ concentrations would be related to variations in photosynthetically-active irradiance (PAR) causing H₂O₂ concentrations in seawater to be higher in summer than in winter in both ST and SA surface waters.