Gesundheit!
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
A friend asked me to write about pollen. He found it fascinating that scientists use such a simple and pervasive botanical miracle to document changes in plant communities over time. My background is in radioactive chemistry specifically using a variety of techniques to assign dates to sediment cores using radioactive elements that decay at known rates. I am somewhat familiar with the microscopic grains of pollen found in peat, ice, and soil cores and decided to give pollen the spotlight it is due.
Through a serendipitous coincidence, the UMass Boston Nantucket Field Station will be hosting the return of Dr. Dorothy Peteet this fall. Dr. Peteet is a scientist in the Paleoecology Laboratory of Lamont-Doherty Earth Observatory (part of Columbia University) and she will give a lecture and continue some of her research investigating pollen grains in our ponds and peat bogs. Dr. Peteet also holds a position with NASA at The Goddard Institute for Space Studies and some of her research can be found at www.giss.nasa.gov/research/briefs/peteet_03/. The study of peat bogs is critical because they store so much carbon; their ability to store and hopefully keep storing carbon is an important part of the carbon cycle.
I was torn about this week's topic because we have a mystery algae occurring in the head of the harbor at the far end of Nantucket Harbor near Wauwinet that is now curling around the outside of the island into Nantucket Sound. This mystery is a small brown algae that a couple of observant fishermen and boat captains have brought to the lab for me to identify, fearing the worst, which in this case could either be the red tide I wrote about last year (http://www.yesterdaysisland.com/2008/features/redtide.php) or the brown tide that decimated scallop populations in the 1970s. At first glance, it does not appear to be either culprit, although we are sending samples out for further analysis. In addition, the two recent storms have brought about concerns about beach erosion on our southern and eastern shores. How are these related? Well, they all involve particles that are controlled by currents (wind and water) and physics. Next week's article will discuss the watery and controversial companions of our wind driven (aeolian) friend, the pollen grain. I am hoping the anticipation will encourage you to delay school and/or work and stay on Nantucket for another week if you are visiting.
Pollen is a fine powder-like substance released by seed plants carrying the male reproductive tissue (sperm) or gamete of a flowering plant. Seed plants first appeared 300 million years ago. Prior to that, the world was dominated by ferns which used a spore method for reproduction that primarily depended upon standing water for fertilization. Pollen is produced in the male organs (anthers) of flowers. Pollination occurs when pollen is transfered from the anthers to the female organs by wind (anemophyly) or by animals (zoophyly). If the female stigma is receptive to a pollen grain, the pollen produces a pollen tube, which grows through the female tissue to the egg, where fertilization takes place by the sperm nucleus. This entire system was not designed exclusively to ruin allergy sufferers' spring and fall, but instead to ensure the genetic diversity and reproduction of thousands of sessile (permanently attached) organisms.
The sperm nucleus is protected from drying out (dessication) or irradiation from the sun by the pollen wall. The tiny (20 - 100 µm) pollen grain is coated with waxes and proteins held in place by something called sculpture elements. There are several components in the amazingly complex pollen wall and they vary a bit between types of plants. The outer pollen wall, or exine, is made up of a two layers, tectum over a foot layer. These are separated by layers of strengthening rods called columella. This construction prevents the wall from collapsing and crushing the genetic material if the pollen grain loses water. The outer wall is constructed with a resistant biopolymer called sporopollenin. A unaltered and relatively delicate cellulose wall (intine) lies within the outer pollen wall. Sporopollenin is waterproof and resistant to almost all chemicals. Escape of the pollen tube through the wall takes places through apertures (germination pores), which further facilitate shrinking and swelling. All of this protection from the elements, along with pollen's natural ability to be carried on the winds, allowed the world-wide dispersal and retention of pollen grains over the centuries. Sporopollenin fossilizes extremely well, preserving the pollen grain characteristics for future generations of curious and patient scientists.
Pollen identification depends on the interpretation of morphological features on the outside of the pollen grain. Exine and aperture patterns are especially varied in the more highly evolved dicots, so that recognition at a family, genus, or even species level may be possible despite the small surface area available on a grain. The exine can have ridges and bumps and spines that distinguish each type of pollen from the next. Since the morphological characters are conservative in the extreme, usually changing very slowly through geologic time, studies of fine detail serve to establish the lineal descent of many plants living today. As difficult as all this sounds, it is not that much different than identifying people from our facial or skeletal features. If we were protected by a waterproof almost indestructible outer coating, it would be even easier.
I find it enchanting that someone thought to look for remnants of flower pollination in sediment and ice cores in order to learn more about our atmosphere and earth over the past thousand or million years.
Palynology is the study of organic microfossils in the size range of 5-500 microns which include some small insect parts and the remnants of plant spores and pollens. Palynology is used in oil and gas exploration and for interpreting and evaluating climatic change. The identification of pollen grains in conjunction with accurate dating methods using a variety of tools can determine how quickly has climate changed in the past and whether climate changes occurred simultaneously around the world. In marine science, a type of ameoboid protist (single celled organisms; 275,000 species are recognized) called Foraminifera is widely studied in sediment and ice cores.
According to Wikepedia (http://en.wikipedia.org/wiki/Palynology), the term palynology was introduced by Hyde and Williams in 1944, following correspondence with the Swedish geologist Antevs, in the pages of the Pollen Analysis Circular (readership of at least three). Hyde and Williams chose palynology on the basis of the Greek words paluno meaning “to sprinkle” and pale meaning “dust.” This followed the lineage of the Latin word pollen which originally meant "mill dust, fine flour" and was related to polenta, "peeled barley", and pulvis, "dust" and was appropriated by our nomenclature buddy Linneus in 1751.
Pollen's sporopollenin outer sheath affords it some resistance to the rigors of the fossilization process that destroy weaker objects; it is also produced in huge quantities. As such, there is an extensive fossil record of pollen grains, often disassociated from their parent plant due to the vagaries of the wind. The discipline of palynology is important for not only determining biostratigraphy (correlation of the rock record using fossils) but to also gain information about the abundance and variety of plants alive which can itself yield important information about paleoclimates. Pollen is first found in the fossil record in the late Devonian period and increases in abundance until the present day. The study of palynology has been used in freshwater and marine environments and can document use of plants over time in different civilizations, for instance what plants were carried by nomadic tribes. Recently, palynology is even used in forensic science to determine where crimes occurred.
The usual suspects for the best location for obtaining terrestrial soil cores is in peat bogs or small ponds or lakes. Seeds, needles, pollen and other plant parts are very well preserved in lake muds because of the lack of oxygen in the sediments. Nantucket would represent some challenges because the island is relatively young and our ponds fill in very quickly. Land use changes which effect sediment load can also change how quickly a pond fills in with soil. Pollen research on island was conducted by Peter W. Dunwiddie in the 1980s and 1990s. He identified a series of pine and non-pine pollens in a core taken at No Bottom Pond (a kettle hole pond located approximately 1 km west from “Nantucket Town” proper) and was able to evaluate changes in the vegetation of the island going back approximately 13,790 years. The evolution of the island's plants starting with tundra type vegetation through a series of pines to a more boreal composition of birch and oaks with the emergence of our heathlands and grasslands is well documented in this core.
He also documents the introduction of pitch pine to the island in the mid 1800's by Josiah Sturgis. Peter Dunwiddie's chapter (Using Historical data in Ecological Restoration: A Case Study from Nantucket can be found in The Historical Ecology Handbook: a Restorationist's Guide to Reference Ecosystems by Dave Egan and Evelyn A. Howell. This is a fascinating account of the history of botanical changes on the island. In addition to investigating No Bottom Pond, he also examined pollen from two sphagnum bogs (Taupawshas and Donut Pond). Unfortunately, the harvesting of peat in Taupawshas removed about 3500 years worth of data! The history of fire use by the Wampanoags and the early settlers can also be traced in these cores.
Dr. Dorothy Peteet participated in some of the pollen research conducted by Dunwiddie, and she plans to come back to Nantucket and see what more can be learned about vegetation changes on the island and how they might relate to pollens found along the East Coast. She has traveled from Siberia to Easter Island in search of paleoecological and climatic truths. Dr. Peteet’s work is described at www.giss.nasa.gov/staff/dpeteet.html and also at www.ldeo.columbia.edu/res/pi/paleoecology.
