The role of epiphytes in seagrass productivity under ocean acidification

Abstract

Ocean Acidification (OA), due to rising atmospheric CO2, can affect the seagrass holobiont by changing the plant's ecophysiology and the composition and functioning of its epiphytic community. However, our knowledge of the role of epiphytes in the productivity of the seagrass holobiont in response to environmental changes is still very limited. CO2 vents off Ischia Island (Italy) naturally reduce seawater pH, allowing to investigate the adaptation of the seagrass Posidonia oceanica L. (Delile) to OA. Here, we analyzed the percent cover of different epiphytic groups and the epiphytic biomass of P. oceanica leaves, collected inside (pH 6.9–7.9) and outside (pH 8.1–8.2) the CO2 vents. We estimated the contribution of epiphytes to net primary production (NPP) and respiration (R) of leaf sections collected from the vent and ambient pH sites in laboratory incubations. Additionally, we quantified net community production (NCP) and community respiration (CR) of seagrass communities in situ at vent and ambient pH sites using benthic chambers. Leaves at ambient pH sites had a 25% higher total epiphytic cover with encrusting red algae (32%) dominating the community, while leaves at vent pH sites were dominated by hydrozoans (21%). Leaf sections with and without epiphytes from the vent pH site produced and respired significantly more oxygen than leaf sections from the ambient pH site, showing an average increase of 47 ± 21% (mean ± SE) in NPP and 50 ± 4% in R, respectively. Epiphytes contributed little to the increase in R; however, their contribution to NPP was important (56 ± 6% of the total flux). The increase in productivity of seagrass leaves adapted to OA was only marginally reflected by the results from the in situ benthic chambers, underlining the complexity of the seagrass community response to naturally occurring OA conditions.

Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification

“…Even though estimates of ammonification were greater in control areas, P. oceanica carbon fixation did not differ between the control area and the CO 2 vent area. Carbon fixation rates were similar to the rates estimated at the same location previously 8 for blades without epiphytes. The NPP values reported by Berlinghof et al 8 in μmol O 2 convert to ~ 0.7 to 0.9 mg C per g dry mass per hour, assuming a photosynthetic quotient of 1.0 (a 1:1 molar ratio of oxygen release to carbon uptake), a range nearly identical to what we estimated, though they show increased net primary production in proximity to vents.…”

“…Benthic-pelagic coupling and enhancement of pelagic productivity through the host-associated microbes on P. oceanica have implications due to global declines in seagrass cover 37 and the prevalence of seagrass wasting diseases 38 . Marine macrophytes have increasingly been shown to host a species rich microbiome 39 , with diverse functions that include not only ammonification, but also nitrogen fixation 8,35,40 , nitrification 41 and nitrate reduction 14,42 . Seagrasses are important to carbon sequestration due to carbon fixation 43 and the carbon storage in the root phyllosphere 44 and sediment.…”

“…Seagrasses are marine angiosperms and are foundational species in coastal ecosystems that are critical in structuring these ecosystems and modulating the flux of nutrients and energy. They provide habitat for other species 6,7 , are key primary producers 8,9 , and also act as carbon sinks by sequestering carbon 10,11 . Marine angiosperms are some of the most widely distributed plants on the planet 12 , and recent studies suggest they host a diversity of microbial taxa 13,14 , with some functional partnerships demonstrated 15,16 .Among seagrasses, a number of microbial taxa have been identified as important to nutrient cycling, including microbes capable of nitrogen fixation 15 , ammonification 16 and sulfur oxidation 13,14 .…”

The relative contribution of fleshy epiphytic macroalgae to the production of temperate maerl (rhodolith) beds

“…The contribution of epiphytic algae to the productivity of vegetated marine coastal ecosystems has been mainly studied in seagrass beds (Moncreiff & Sullivan 2001, Borowitzka et al 2006), but remains relatively poorly understood in macroalgal habitats despite the potentially high biomass of fast-growing epiphytes that may contribute significantly to the total productivity of these ecosystems. The epiphytic algae of seagrasses are important primary producers that can indeed make up a significant proportion (up to 60%) of the total primary production of seagrass beds (Borowitzka et al 2006, Berlinghof et al 2022. In contrast, epiphytic macroalgae of the kelp species Laminaria hyperborea contribute little to the total productivity of the entire kelp forest, relative to the kelp itself (Pedersen et al 2014).…”

Macroepiphyte biodiversity on Kappaphycus alvarezii surface and its interaction with environment in cultivation centers on Lombok Island, Indonesia

“…Some plants that are commonly used as hosts include Halodule sp. (Papini et al 2011), Pocidonia oceania (Berlinghof et al 2022), Sargassum fusiforme (Xu et al 2022) and Kappahycus alvarezii (Ghazali et al 2021). Most of the macroepiphyte are opportunistic red algae (Rhodophyta), which take advantage of attaching rhizoids to the leaves/thallus of the host.…”