Utilization of DOC from seagrass rhizomes by sediment bacteria: 13C-tracer experiments and modeling

Abstract

Seagrasses are widely recognized as contributing to net ecosystem primary production and to supporting heterotrophy in estuarine systems. We investigated the linkage between seagrass (Thalassia testudinum) rhizosphere carbon exudation and sediment bacteria. In microcosms, we simulated summer conditions and enriched the water column DIC (dissolved inorganic carbon) pool with 13 C, then followed the tracer into the sediment porewater DOC (dissolved organic carbon) and the bacterial biomarkers (phospholipid fatty acids, PLFAs). Subsequently, we developed an inverse analysis of the seagrass microcosm system and calculated the flux of carbon between biological and geochemical compartments. After 18 d, the bacterial pool was enriched by about Δ + 4 ‰, while the DOC pool was enriched by about Δ + 50 to + 60 ‰. We estimate that about 15 to 30% of gross primary production was exuded from the root/rhizome and that this accounts for about 41 to 61% of the carbon required by sediment bacteria. Mineralization of detrital seagrass leaf material or refractory seagrass DOC rhizodeposition may account for the other 38 to 59% of the bacterial carbon demand. We suggest that the sediment bacteria in the seagrass rhizosphere rapidly utilize labile DOC and, consequently, build up 13 C label in the refractory DOC pool. Our results conclusively show a direct linkage between seagrass carbon exudation and sediment biogeochemical processes.

Changes in growth, internode distance and nutrient concentrations of the seagrass Halophila ovalis with exposure to sediment sulphide

“…Dissolved organic carbon (DOC) released by roots has been estimated at 1.6 g m -2 d -1 for the seagrass Cymodocea rotundata (Holmer et al 2001). The labile DOC pool exuded by Thalassia testudinum was preferentially incorporated into bacterial biomass (Kaldy et al 2006). A change in DOC and/or oxygen released by seagrass roots has the potential to shift the balance of oxygen and carbon in the rhizosphere, potentially affecting sediment sulphide concentrations.…”

Organic matter release by the dominant primary producers in a Caribbean reef lagoon: implication for in situ O2 availability

“…Coralderived organic matter is mainly (> 90%) degraded in the sediment (Wild et al 2004a) or by invertebrates in coral cavities (De Goeij & Van Duyl 2007, De Goeij et al 2008 and not in the water column. The primary location for seagrass-derived organic matter is also the sediment rather than the water column (Canuel & Martens 1996, Duarte & Cebrián 1996, Kaldy et al 2006, as a main part of organic carbon is released through roots and rhizomes (Hansen et al 2000), and it is generally more refractory compared to algae exu-dates. Algae exudates, on the contrary, are likely primarily degraded in the water column.…”

Effects of seasonal dynamics in a Zostera noltii meadow on phosphorus and iron cycles in a tidal mudflat (Arcachon Bay, France)

“…Iron oxides precipitated in the root zone thus constitute a huge trap of DIP and a potential reservoir of P available for eelgrass growth (Table 2, Fe asc :P asc = 21 in March). Moreover, oxygen release is accompanied by an export of labile organic matter into the sediment via the root-rhizome web and eelgrass detritus (Kaldy et al 2006, Papadimitriou et al 2006 Mar 21 (± 4) 2 (±1) 20 (± 2) 1 (±1) 12 (± 3) 1.5 (±1) May 32 (± 3) 14 (± 4) 21 (± 4) 1 (±1) 16 (± 2) 4 (± 4) Jul 26 (± 8) 7 (± 7) 20 (±10) 1 (±1) 24 (± 8) 2 (± 2) 10 (± 4) Sep 18 (± 7) 1 (±1) 22 (± 5) 1 (±1) 16 (± 6) 1 (±1) Dec 17 (± 2) 2 (±1) 20 (± 4) 1 (±1) 19 (± 4) 1 (±1) Table 2. Mean ratios (± SD) of ascorbate extractable Fe vs. ascorbate extractable P in the sediments and dissolved reduced iron vs. dissolved inorganic phosphorus (DIP) in pore water from Zostera noltii meadows (Stn Z) and unvegetated sediments (Stn N) in Arcachon Bay.…”

“…Eelgrasses also export via photosynthesis high amounts of organic carbon into the sediments through their root-rhizome system (Kaldy et al 2006), which fuels bacterial benthic respiration. These mechanisms enhance organic matter mineralization, in particular sulphate reduction, which is the dominant carbon-oxidation pathway in coastal marine sediment (Jørgensen 1982, Canfield & Thamdrup 1996.…”