Types and Distribution of Bioactive Polyunsaturated Aldehydes in a Gradient from Mesotrophic to Oligotrophic Waters in the Alborán Sea (Western Mediterranean)

Abstract

Polyunsaturated aldehydes (PUAs) are bioactive molecules suggested as chemical defenses and infochemicals. In marine coastal habitats, diatoms reach high PUA production levels during bloom episodes. Two fractions of PUA can usually be analyzed: pPUA obtained via artificial breakage of collected phytoplankton cells and dissolved PUA already released to the environment (dPUA). In nature, resource supply arises as a main environmental controlling factor of PUA production. In this work, we monitored the vertical distribution and daily variation of pPUA associated with large-size phytoplankton and dPUA, at three sites located in the Alborán Sea from mesotrophic to oligotrophic waters. The results corroborate the presence of large-size PUA producers in oligotrophic and mesotrophic waters with a significant (58%-85%) diatom biomass. In addition to diatoms, significant correlations between pPUA production and dinoflagellate and silicoflagellate abundance were observed. 2E,4E/Z-Heptadienal was the most abundant aldehyde at the three sites with higher values (17.1 fg·cell^-1) at the most oligotrophic site. 2E,4E/Z-Decadienal was the least abundant aldehyde, decreasing toward the oligotrophic site. For the first time, we describe the daily fluctuation of pPUA attributable to cellular physiological state and not exclusively to taxonomical composition. Our results demonstrate the persistence of threshold levels of dPUA deep in the water column, as well as the different chromatographic profiles of dPUA compared with pPUA. We propose different isomerization processes that alter the chemical structure of the released PUAs with unknown effects on their stability, biological function, and potential bioactivity.

Keywords: Alborán Sea; diatoms; oligotrophy; oxylipins; polyunsaturated aldehydes, PUA.

Figure 1

Map showing the location of the different sampling sites named Coast, Jet, and Gyre sites, with overlaid mean chlorophyll a concentration (mg·m⁻³) for the sampling period (from 7–9 October 2015). Chlorophyll a data were downloaded from the Copernicus Marine Environmental Monitoring Service (CMEMS; http://marine.copernicus.eu/).

Figure 2

Vertical distribution of nutrients concentrations along the sampling day (μM) (NO₃⁻, (A–C); PO₄³⁻, (D–F); SiO₄, (G–I)) from surface to 200 m at the Coast, Jet, and Gyre sites.

Figure 3

Different fractions of polyunsaturated aldehydes (PUAs) at the three sites: (A) averaged particulate PUA (pPUA of large-size phytoplankton) expressed as pmol from cells in 1 L at 5 m and deep chlorophyll maximum (DCM). (B) pPUA normalized by large-size phytoplankton cell abundance (fg PUA·cell⁻¹) at 5 m and DCM; (C) averaged vertical dPUA (pM) at the three sites. C7: 2E,4E/Z-heptadienal; C8: 2E,4E/Z-octadienal; C10: 2E,4E/Z-decadienal; TPUA: total PUA.

Figure 4

Daily vertical distribution of total dissolved PUA (dTPUA) at the three sites. Please note that depth scales are different for each site for a better observation of dPUA patterns. The black dots indicate sampling depths.

Figure 5

Vertical distribution scaled from 0 to 200 m of in situ fluorescence at the Coast, Jet, and Gyre sites.

Figure 6

Box plot representing ranges of dissolved total PUA (dTPUA) and rates of dissipation of turbulent kinetic energy (ε) at the different sites.

Figure 7

Combined illustration showing the temporal variation along the day of TChla (mg·m⁻³), pPUA (pg·cell⁻¹), and large phytoplankton abundance at the DCM of the different sites. Upper pie charts show the percentage of TpPUA of pC7 (2E,4E/Z-heptadienal), pC8 (2E,4E/Z-octadienal), and pC10 (2E,4E/Z-decadienal). Lower pie charts show the percentage of abundance of large-size phytoplankton fraction categories. Gray areas indicate the night period.