Bioavailable dissolved organic matter and biological hot spots during austral winter in Antarctic waters

Abstract

Primary production and heterotrophic bacterial activity in the Antarctic Ocean are generally low during the austral winter. Organic carbon is considered to be a major factor limiting bacterial metabolism, but few studies have investigated the bioavailability of organic matter during winter. Herein, the chemical composition and bioavailability of dissolved organic matter (DOM) were investigated in surface (5–100 m) and mesopelagic (200–750 m) waters off the northwestern Antarctic Peninsula during August 2012. Concentrations of dissolved organic carbon (DOC) were low (42 ± 4 µmol L−1) and showed no apparent spatial patterns. By contrast, the composition of DOM exhibited significant spatial trends that reflected varying ecosystem productivity and water masses. Surface distributions of chlorophyll‐a and particulate organic carbon depicted a southward decline in primary productivity from open waters (60.0°S–61.5°S) to ice‐covered regions (61.5°S–62.5°S). This trend was evident from concentrations and DOC‐normalized yields of dissolved amino acids in the surface waters, indicating decreasing DOM bioavailability with increasing latitude. A different pattern of DOM bioavailability was observed in the mesopelagic water masses, where amino acids indicated highly altered DOM in the Circumpolar Deep Water and bioavailable DOM in the Transitional Weddell Water. Depth distributions of amino acid yields and compositions revealed hot spots of elevated bioavailable DOM at ∼75 m relative to surrounding waters at most ice‐free stations. Relatively low mole percentages of bacterially derived d‐amino acids in hot spots were consistent with an algal source of bioavailable DOM. Overall, these results reveal the occurrence and spatial heterogeneity of bioavailable substrates in Antarctic waters during winter.

Variability and change in the west Antarctic Peninsula marine system: Research priorities and opportunities

“…Bacterioplankton itself could be an underestimated source of DOM through grazing by zooplankton and mixotrophic algae, or by viral lysis; an important loss factor for Antarctic bacterioplankton (Brum et al 2015;Evans et al 2017;Evans and Brussaard 2012;Vaqué et al 2017). Although studies of DOM origin, speciation and bioavailability are rare in this region, one winter survey revealed that ~35 % of dissolved amino acids, and thus potentially a large proportion of the DOM pool, was of bacterial origin (Shen et al 2017).…”

Pan‐Arctic Distribution of Bioavailable Dissolved Organic Matter and Linkages With Productivity in Ocean Margins

“…Previous seasonal surveys in the west Greenland coastal region have observed low‐moderate levels of phytoplankton biomass and production during the fall (Juul‐Pedersen et al, ). The negative correspondence between DOM bioavailability and sea ice extent has also been observed in surface waters of the Southern Ocean during austral winter (Shen et al, ).…”

“…The broad range of amino acid yields depicted a wide spectrum of DOM bioavailability, ranging from labile to refractory in surface waters of Arctic margins. In comparison, less variation in amino acid yields (approximately threefold) was observed in depth profiles (mostly 0.5–1.5%) of the Pacific and Atlantic Oceans (Kaiser & Benner, ), Japan Sea (Kim et al, ), and Southern Ocean (Shen et al, ; Tremblay et al, ), and in surface waters of the Louisiana margin (mostly 0.7–2.0%) (Shen, Fichot, et al, ). These cross‐system comparisons highlight the dramatic variability in composition and bioavailability of DOM in the Arctic Ocean.…”

Molecular properties are a primary control on the microbial utilization of dissolved organic matter in the ocean

“…Samples were hydrolyzed with a microwave‐assisted vapor phase technique and derivatized with o ‐phthaldialdehyde and N ‐isobutyryl‐ l ‐cysteine. Sample pretreatments and instrument conditions followed the procedures described in Kaiser and Benner () and Shen et al (). A total of 18 amino acids were determined in this study and they were collectively termed total dissolved amino acids (TDAA).…”