Radiolarians decreased silicification as an evolutionary response to reduced Cenozoic ocean silica availability

Abstract

It has been hypothesized that increased water column stratification has been an abiotic ''universal driver'' affecting average cell size in Cenozoic marine plankton. Gradually decreasing Cenozoic radiolarian shell weight, by contrast, suggests that competition for dissolved silica, a shared nutrient, resulted in biologic coevolution between radiolaria and marine diatoms, which expanded dramatically in the Cenozoic. We present data on the 2 components of shell weight change-size and silicification-of Cenozoic radiolarians. In low latitudes, increasing Cenozoic export of silica to deep waters by diatoms and decreasing nutrient upwelling from increased water column stratification have created modern silicapoor surface waters. Here, radiolarian silicification decreases significantly (r ‫؍‬ 0.91, P < 0.001), from Ϸ0.18 (shell volume fraction) in the basal Cenozoic to modern values of Ϸ0.06. A third of the total change occurred rapidly at 35 Ma, in correlation to major increases in water column stratification and abundance of diatoms. In high southern latitudes, Southern Ocean circulation, present since the late Eocene, maintains significant surface water silica availability. Here, radiolarian silicification decreased insignificantly (r ‫؍‬ 0.58, P ‫؍‬ 0.1), from Ϸ0.13 at 35 Ma to 0.11 today. Trends in shell size in both time series are statistically insignificant and are not correlated with each other. We conclude that there is no universal driver changing cell size in Cenozoic marine plankton. Furthermore, biologic and physical factors have, in concert, by reducing silica availability in surface waters, forced macroevolutionary changes in Cenozoic low-latitude radiolarians.evolution ͉ microfossils ͉ micropaleontology ͉ morphometrics ͉ Ocean Drilling Program T he evolution of ocean plankton has played an important role in the development of the earth's climate system, and changes in ocean plankton may affect future changes in climate (1). The deep-sea microfossil record of protist plankton provides an unusual opportunity to understand how plankton evolution and environmental change mutually affect each other. Recently, it has been proposed that Cenozoic changes in upper ocean water column stratification have influenced the evolution of cell size in a variety of marine protist plankton groups, including planktonic foraminifera (2), diatoms (3), and dinoflagellates (4), and it has been suggested that these patterns are indicative of a ''universal driver'' of size change in Cenozoic plankton (4). The polycystine radiolarians are an important marine protist zooplankton group abundant as fossils in Cenozoic and Mesozoic deep-sea sediments. Their general size, feeding ecology, and distribution patterns are similar to those of the better-known planktonic foraminifera (5, 6), although radiolarians are more diverse and, at least since the Oligocene (7), possess distinct, diverse endemic high-latitude faunas. Radiolarians are most diverse in low latitudes and most abundant in near-surface waters, although some species inhabit ...

Experimental silicon demand by the sponge Hymeniacidon perlevis reveals chronic limitation in field populations

“…The results also support the view that such a chronic limitation probably arises from the persistence in modern sponges of ancestral uptake systems that evolved in ancient oceans characterized by DSi concentrations being at least an order of magnitude higher than the maxima available in Recent oceans . It also reinforces the notion that the ''current'' DSi limitation is not a modern ecological process, since it probably started with the ecological expansion of diatoms during the Early Tertiary (Harper & Knoll, 1975;Maldonado et al, 1999;Lazarus et al, 2009). This process has been and it is still operating as an important environmental pressure.…”

Continental erosion and the Cenozoic rise of marine diatoms

“…S1). Their fossil record indicates a secular trend of decreasing test weight throughout the Cenozoic (7,8) (Fig. S2).…”

“…Fossil evidence strongly suggests that the rise of marine diatom diversity over the latter half of the Cenozoic era is mirrored by a simultaneous decrease in the weight of radiolarian tests (7)(8)(9), suggesting that an increasingly larger proportion of the ocean silica reservoir has been appropriated by diatoms. The most pronounced decrease in radiolarian test weight is coeval with a pulse of diatom diversity across the Eocene−Oligocene (E/O) transition (∼38-32 Ma), supporting the hypothesis that competition for H 4 SiO 4 played a role in the rise of diatoms (7,8). This evolutionary model assumes a constant (steady-state) input of H 4 SiO 4 to the ocean.…”

Morphospaces and Databases: Diatom Diversification through Time

“…This view plays a central role in a number of evolutionary narratives involving the diatoms, including their coevolution with grasses (Falkowski et al 2004) and whales (Marx and Uhen 2010), their role in reshaping the silica cycle, and its effect on radiolarians (Lazarus et al 2009). Although widely accepted, this view has recently been challenged (Rabosky and Sorhannus 2009).…”

“…For example, changes in the degree of silicification of diatom frustules are not well captured by the morphological characters in this study, since these are not necessarily visible in those characters that can be coded cohesively at the genus level. If predictions from the fossil record of radiolarians (Lazarus et al 2009) and semi-quantitative observations of the diatom fossil record (Finkel and Kotrc 2010) …”