Photosymbiosis in planktonic foraminifera across the Paleocene–Eocene thermal maximum

Abstract

Under stress, corals and foraminifera may eject or consume their algal symbionts (“bleach”), which can increase mortality. How bleaching relates to species viability over warming events is of great interest given current global warming. We use size-specific isotope analyses and abundance counts to examine photosymbiosis and population dynamics of planktonic foraminifera across the Paleocene–Eocene thermal maximum (PETM, ~56 Ma), the most severe Cenozoic global warming event. We find variable responses of photosymbiotic associations across localities and species. In the NE Atlantic (DSDP Site 401) PETM, photosymbiotic clades (acarininids and morozovellids) exhibit collapsed size-δ13C gradients indicative of reduced photosymbiosis, as also observed in Central Pacific (ODP Site 1209) and Southern Ocean (ODP Site 690) acarininids. In contrast, we find no significant loss of size-δ13C gradients on the New Jersey shelf (Millville) or in Central Pacific morozovellids. Unlike modern bleaching-induced mass mortality, populations of photosymbiont-bearing planktonic foraminifera increased in relative abundance during the PETM. Multigenerational adaptive responses, including flexibility in photosymbiont associations and excursion taxon evolution, may have allowed some photosymbiotic foraminifera to thrive. We conclude that deconvolving the effects of biology on isotope composition on a site-by-site basis is vital for environmental reconstructions.

Adaptive ecological niche migration does not negate extinction susceptibility

“…For D. altispira, migration from the surface mixed layer to the subsurface may be compounded either by a photosymbiont reduction/suppression 18 , or adoption of a facultative symbiotic ecological strategy, recognized to enhance flexibility of nutritional sources through minimal energetic investment 41,42 . This proposed adaptation is suggested over total algal photosymbiont "bleaching" 9,11,18,21,43 , as D. altispira continues to present δ 13 C enrichment higher than values observed in the asymbiotic, subsurface-dwelling taxa analysed in this study 42 , yet similar δ 18 O values (Figs. 1, 3).…”

“…Their calcareous skeletons, or tests, preserve not only their entire life history, but also a biogeochemical expression of the surrounding water column (e.g., 7 ). These features allow for high-resolution species-specific quantification of physiological and ecological adaptation through periods of climate variability (e.g., [8][9][10][11][12][13][14][15][16][17][18][19][20][21] ).…”

Adaptive Ecological Niche Migration does not Negate Extinction Susceptibility

“…For D. altispira, migration from the surface mixed layer to the subsurface may be compounded either by a photosymbiont reduction/suppression 18 , or adoption of a facultative symbiotic ecological strategy, known to enhance flexibility of nutritional sources through minimal energetic investment 42,43 . This proposed adaptation is suggested over total algal photosymbiont "bleaching" 9,11,18,21,44 , as D. altispira continues to present δ C enrichment higher than values observed in the asymbiotic, subsurface-dwelling taxa analysed in this study 43…”

“…Photosymbiont bleaching driven by extreme heat stress has been recorded in extant groups such as corals 48 and larger benthic foraminifera 49,50 , and previously, records of the potential bleaching of algal photosymbionts within fossil planktonic foraminifera have been confined to early Cenozoic hyperthermals (e.g., 9,11,18,21,44 ).…”

“…Their calcareous skeletons, or tests, preserve not only their entire life history, but also a biogeochemical expression of the surrounding water column (e.g., 7 ). These features allow for high-resolution species-specific quantification of physiological and ecological adaptation through periods of climate variability (e.g., [8][9][10][11][12][13][14][15][16][17][18][19][20][21] ).…”