Cell death and aggregate formation in the giant diatom Coscinodiscus wailesii (Gran & Angst, 1931)

Abstract

The demise phase of diatom blooms following nutrient exhaustion is characterised by the formation of aggregates with high sinking rates that facilitate carbon export to the seafloor. However, the nature of the binding substances involved and the physiological status of the phytoplankton during aggregation are not well established. Transparent Exopolymer Particles (TEP), exudated by living cells, have been proposed as a binding agent of aggregates but autolysed cytoplasm released after cell death might also play such a role. To differentiate these processes we studied the response of cultures of the mucilage and TEP-producing giant diatom Coscinodiscus wailesii to nutrient, in particular silicic acid, limitation. Two staining methods were applied: SYTOX® Green to follow cell viability and cell death and Alcian Blue (AB) to quantify the production of TEP. Large aggregates formed exclusively in cultures with high cell densities in which the SYTOX® Green signal increased during the senescence phase. TEP-production under nutrient replete (f/2 treatment) and nitrate reduced growth conditions (f/2-N treatment) with high cell densities was comparatively low, indicating reduced photosynthetic activity in the stationary and senescent phases. In contrast, TEP-production was enhanced in low-density Si-limited cultures (f/2-Si treatment), probably as a means to discharge excess photosynthate, as the cells were densely packed with chloroplasts. The C/N ratios of aggregated and solitary cells did not differ significantly indicating that the binding agent was not mainly polysaccharidic. We propose that aggregate formation in C. wailesii is a consequence of cell lysis after cell death and that autolysed cytoplasm is the binding agent rather than TEP. This would imply that cell lysis plays a substantial role in bloom termination and mucilage formation in C. wailesii, which would in turn influence biogeochemical cycling in regions where this diatom thrives.

High export via small particles before the onset of the North Atlantic spring bloom

“…Long‐term sediment trap data from 3000 m at the Porcupine Abyssal Plain (49°N, 16°W), for example, suggest that prebloom deep flux rates are small (2–4 mg POC m −2 d −1 ) and deep fluxes only increase after surface Chl concentrations have peaked (based on 14 years of data at 3000 m depth [ Lampitt et al ., ]). This is likely because bloom or postbloom export events are often dominated by fecal pellets [ Turner , , and references therein] and larger phytodetritus aggregates [e.g., Lampitt et al ., ], which form during periods of high phytoplankton concentration [e.g., Kiørboe et al ., ; Jackson and Kiørboe , ] or toward the end of a bloom when nutrients are limiting [e.g., Smetacek , ; Armbrecht et al ., ]. These large aggregates sink rapidly (>100 m d −1 ) through the mesopelagic [e.g., Lampitt et al ., ; Martin et al ., ] and can, at times, reach the abyssal plain in large quantities [e.g., Lampitt et al ., ].…”

Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean

“…We assume that the modeled detritus pool can be split into a free external, non-diatom and a diatom frustule-related, void-filling detritus part. We further assume that the external pool is remineralized before the intra-cellular pool of volume V POM and thus neglect cell lysis observed prior to aggregation (Armbrecht et al, 2014) and rather assume mineral protection of detritus (Hedges et al, 2001). If more detritus is remineralized than the frustules void would hold, it is replaced with the respective volume of water V aq of density ρ.…”

Few Layer Graphene sticking by biofilm of freshwater diatom Nitzschia palea as a mitigation to its ecotoxicity

“…Algal viability was determined at 48 h of exposure to FLG suspensions using SYTOX Green ® marking (Molecular Probes, Inc., Eugene, OR) [39] generally used on bacteria [40] but also on diatoms [41]. After scrapping, cells were incubated 10 min in SYTOX Green ® (100 nM) and then observed using a fluorescence microscope (BX-41, Olympus, Center Valley, PA) equipped with an Hg lamp (U-LH100HG, Olympus, Center Valley, PA) using a 470e490 nm/520 nm excitation/emission filter and a 500-nm dichromatic filter (U-MNB2, Olympus, Center Valley, PA).…”