11th ICRS Abstract book - Nova Southeastern University

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Evolution Of The Coral Reef Ecosystem Under A Jurassic Perspective: From
Mixotrophy Towards Superoligotrophy
Reinhold LEINFELDER* 1 , Ulrich STRUCK 1
1 Museum of Natural History Berlin, Berlin, Germany
Scleractinian reefs rapidly evolved and expanded during the Jurassic. Many Jurassic
scleractinian calical structures were nearly as complex as in modern star or brain corals
and thus appear modern. However, a much greater variety of coral reef types and coral
reef settings existed than today. During the Jurassic, the adaptation to nutrient restriction
by photosymbiosis was only one of several adaptational strategies, including other
adaptational pathways to terrigeneous and even brackish settings. Many mid and late
Jurassic corals exhibit good indicators for the existence of photosymbionts (integrated
calical growth shapes, depth zonation, typical growth banding with high and low-density
bands and, in some examples, non-linear clustering of C/O stable isotopes). Nevertheless,
even density-banded corals and most pure coral reefs mostly grew under terrigeneous
influence. On the other hand, typical low-nutrient settings such as the 1000 km broad
Arabian shelf or intra-tethydian platforms were dominated by stromatoporoid sponges
with or without scleractinian corals. Taxa distribution shows provincialism of many
stromatoporoids. Jurassic coral taxa distribution data are not yet statistically significant
but also point to differences between near-coast and intra-ocean platforms, indicative of
splitting of the coral reef window into a mesotrophic to mildly oligotrophic subset
dominated by mixotrophic corals and a superoligotrophic subset dominated by
stromatoporoids together with strongly photosymbiontic corals.
The Bahia Almirante coral reefs from Panama may represent a “Jurassic-type” modern
example for mixotrophy of corals. These reefs are adapted to both reduced salinity and to
sewage-related, slightly elevated nutrients. This is shown by the C/O and N stable isotope
pattern, signalling that certain phototrophic coral species consumed considerable
proportions of zooplankton and, possibly, even terrigeneous plant debris.
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Co2-Concentrating Mechanisms, Harmful Blooms, And Late Devonian Reef
Extinction 375 Million Years Ago
Robert RIDING* 1
1 Earth and Planetary Sciences, University of Tennessee, Knoxville, Knoxville, TN
Late Devonian Mass Extinction caused the largest change experienced by reef biotas in the
entire Phanerozoic. Stromatoporoid-coral communities that had dominated metazoan reefs since
the mid-Ordovician disappeared. In the extinction’s immediate aftermath they were replaced by
microbial reefs. By the Early Mississippian rimmed shelves had given way to ramps dominated
by carbonate mud mounds. The Mass Extinction selectively eliminated shallow marine
organisms, including acritarch phytoplankton as well as reef biotas.
Here I suggest induction of CO 2-concentrating mechanisms (CCMs) in marine phytoplankton as
a factor in Late Devonian Mass Extinction. CCMs help maintain photosynthesis when levels of
dissolved inorganic carbon are low. Late Devonian decline in atmospheric CO 2 level was
sufficient to induce CCMs in aquatic algae and cyanobacteria. This is likely to have had several
geologically recognizable effects. Firstly, CCMs promote phytoplankton productivity and bloom
conditions by helping to overcome carbon limitation. Productivity enhances organic matter
burial, and harmful blooms kill shallow-water reefs and pelagic organisms. Secondly, conditions
favouring photosynthetic groups with effective CCMs can promote community restructuring,
resulting in extinction of some phytoplankton groups. Thirdly, cyanobacterial CCMs can
stimulate calcification, both in the water column as ‘whitings’, and in benthic mats as in situ
microbial carbonates.
The Late Devonian marine realm underwent marked changes in addition to Mass Extinction.
Organic carbon-rich sediments, microbial carbonates, and carbonate mud mounds significantly
increased in abundance. These, at first sight disparate, developments all have potential links to
CCM induction in phytoplankton. Harmful blooms could have contributed to extinction of
shallow marine metazoans, and increased phytoplankton productivity would have increased
organic carbon-rich sediments. Phytoplankton community restructuring could have led to
acritarch extinction. Combination of increased microbial productivity and enhanced
cyanobacterial calcification would have promoted both microbial carbonate and carbonate mud
mound development. These possibilities do not exclude the likely involvement of additional
causative factors.
Oral Mini-Symposium 1: Lessons From the Past
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